ANTH 322 UM Theory of Evolution by Natural Selection Questions

Book: Low, Bobbi S. 2014. Why Sex Matters: A Darwinian Look at Human Behavior,Revised Edition

Test 1: 35 multiple choice questions, each worth one point.
THIS IS AN ONLINE TEST.
The test will be available on Canvas from Monday December 27- Sunday, January 2. The
test is a CLOSED-BOOK test — that means that you are NOT ALLOWED to use your book,
notes, posted lectures, or any other material while taking the test. The test will be TIMED
— you will have 75 minutes to complete the test. IMPORTANT: Once you open the test,
you will NOT be able to pause, save, restart, or retake the test. Be sure that you are
ready to take the test before accessing it on Canvas. The test will automatically submit
after 75 minutes, or when you click “submit.” (Obviously if you have a legitimate technical
issue, we will figure something out. Send me a message on REMIND immediately if you
encounter such a problem. If you can, provide a screenshot of the problem.) The test will
CLOSE at 11:59 PM on SUNDAY, JANUARY 2. To make sure you have the full time to take
the test, be sure to access the test no later than 10:45 PM on SUNDAY. If you start the test
at 11:30 PM, you will have only 29 minutes to take it.
Materials Covered:
Lectures:
1-6
Videos:
Prosanta Chakrabarty, Four Billion Years of Evolution in Six Minutes
Marlene Zuk, What We Learn from Insects’ Sex Lives
Discovery Channel, The Science of Sex Appeal
Readings:
Low, Chapters 1-5 (plus preface)
Bock & Johnson, 2004
Usual caveats apply: This is not a comprehensive list nor a guarantee of what questions
will or will not appear on the test, but is a guideline for studying.
1. Theory of Evolution by Natural Selection (Lec 1-2; Low Chapts 1-2; Chakrabarty)
a. Define fitness, genotype, and phenotype
b. List and Explain Darwin’s Three Postulates (the three simple ideas on which
the theory of evolution by natural selection is based)
c. Darwin’s finches
i. Identify how observations of the finches on Daphne Major illustrate
Darwin’s Three Postulates
ii. Understand how the drought in 1976-78 affected the population of
finches, and how this event illustrates the process of evolution by
natural selection
d. Distinguish between stabilizing selection and directional selection; what
causes each kind of selection?
e. Define and explain the different levels of explanations (or different kinds of
questions), with particular emphasis on ultimate vs. proximate levels
f. Define and be able to give an example of the naturalistic fallacy
g. Generating complexity
i.
ii.
iii.
Define categorical vs. continuous traits
Understand the role of multiple genes in producing continuous
characters
Explain the idea that variation is random, but selection is systematic
(Lec 2)
2. The Phenotypic Gambit (Lec 1-2; Low, Chpts 1-2)
a. Identify different kinds of phenotypic traits (e.g. physical traits, chemical
traits, behavioral traits, cognitive traits, etc.)
b. Define the phenotypic gambit, and describe how it is used to generate
hypotheses about phenotypic traits (see examples in lectures; Lec 1 for
physical trait and Lec 2 for behavioral trait)
c. Understand how genes affect behavior, and how this is different from the
“biological determinism” view
d. Define and explain the concept of “reaction norms”
e. Define and explain the “hypothetico-deductive method” (also known more
simply as the “scientific method,” and how we use the phenotypic gambit
3. Adaptive strategies and sexual selection (Lec 3; Low, Chpt 3; Zuk)
a. Competition (hint: this is essentially another way of thinking about Darwin’s
Three Postulates)
i. Who is competing with whom?
ii. What are the competing for?
iii. Why is competition necessary?
iv.
Define/explain the concept of the selfish gene
v.
Recognize that selfish genes can produce cooperation, generosity, and
kindness (see Lec 2; only basic understanding here, we’ll be talking
about this in more depth in coming weeks)
b. Recognize how the term “strategies” is used in behavioral ecology, including
“reproductive strategies”
c. Understand the “specialization” explanation of why there are two sexes
d. Sexual Selection
i. Define and describe intersexual selection (mate choice) and
intrasexual selection (direct competition for mates)
ii. Recognize the kinds of traits that are typically produced by
intersexual selection (typically ornaments) and intrasexual selection
(typically weapons) and explain why
e. Kin Selection (also referred to as “inclusive fitness”)
i. Explain what is meant by the term inclusive fitness
ii. Define “Hamilton’s Rule” – technically (i.e., rb>c) and conceptually
(what is the significance of the idea?)
f. Define the EEA
g. Identify the assumptions on which the field of behavioral ecology rests
4. Reproductive Ecology
a. Mating vs. Parenting effort
i.
ii.
iii.
Identify the specific aims of engaging in mating or parenting effort
Identify the specific costs entailed in mating vs.parenting effort
Identify and explain sex differences in emphasizing mating or
parenting effort
iv.
Recognize the expected payoff structure for focusing on mating vs.
parenting effort
b. Define obligate parental investment and understand how it varies among
species, sexes, and ecological contexts
i. In particular, identify sex differences of obligate parental
investment in humans
c. Understand Trivers’ Parental Investment Theory
i. Recognize that the difference between the sexes in obligate parental
investment is more important than the overall level of obligate
investment for understanding mating systems (and other sex
differences)
ii. Describe what kinds of behaviors are predicted based on differences
in obligate parental investment
d. Reproductive Success and Reproductive Variance
i. Define reproductive success and reproductive variance (they are
NOT the same thing!!)
ii. Identify the primary constraints on reproductive success for male
and female mammals
iii. Explain how reproductive variance is related to obligate parental
investment (hint: sex differences in obligate investment produce sex
differences in reproductive variance – how??)
iv.
Identify which sex in humans has greater reproductive success
(neither; men and women on average have the same reproductive
success) and which has greater reproductive variance (men) and
explain why
5. Mating Systems in Apes and Humans (Lec 5; Low, Chapter 4)
a. Know the major mating system definitions – polygyny, polyandry,
monogamy, polygynandry, including:
i. Difference between mating system and mating strategy
ii. Difference between the social monogamy and sexual monogamy
iii. Difference between perennial monogamy and serial monogamy
iv.
Difference between “attributed mating system” and “effective mating
system” (this is related to all of the above)
b. How does the ecology affect the mating system of a species or population?
i. In broad terms: the ecology describes the pattern of needs, resources,
and risks that can impact reproductive success; females will distribute
themselves to according to these factors (optimizing resources,
minimizing risks), and males will distribute themselves according to
female distribution (maximizing reproductive opportunities,
ii.
iii.
iv.
minimizing reproductive competition).
You should be able to broadly make predictions about the
mating system given information about the ecology and vice
versa
Ecologically‐imposed monogamy or polygyny –under what patterns of
male investing and variation in male resource holding would each of
these mating systems be most beneficial to females?
Socially‐imposed monogamy or polygyny – when males impose a
mating system that is less beneficial to female reproductive success.
Think about the costs to females – e.g., under what circumstances
would females benefit from polygyny and why would males impose
monogamy; under what circumstances would females benefit from
monogamy and why would males impose
6. Mate choice (Video: Science of Sex Appeal; Low, Chapt 5; Lec 6)
a. The video highlights research investigating variation in men’s and women’s
mate preferences, that is, what men and women prefer in potential mates, and
why. For each pattern of preferences, you should be able to explain why that
preference evolved, that is, how is the preference related to men’s or
women’s specific reproductive challenges, and how does it help individuals
meet those challenges?
b. Universal preferences (shared by men and women)
i. Both men and women prefer more symmetrical faces – why?
ii. Both men and women prefer gender consistent body shape and body
motion cues (high vs. low waist-to-hip ratio, hip sway vs. shoulder
swagger) – why?
iii. Both men and women prefer the scent of unrelated individuals
(women more so at high fertility) – why?
c. Divergent preferences
i. Voice: women tend to prefer lower pitched voices; men tend to prefer
higher pitched voices – what does voice pitch tell us?
ii. Scent: women are typically turned off by male scents (except when
ovulating); men are rendered incapable of determining physical
attractiveness when exposed to vaginal scents
iii. Effect of status and resources: information about a man’s status and
resources has a significant impact on women’s perception’s of men’s
physical attractiveness, but has no such effect on men’s perceptions of
women’s physical attractiveness – why?
d. Error management
i. Men tend to overperceive sexual or romantic interest on the part of
women (think they’re interested when they are not), but women tend
to be more accurate in assessing men’s interest – why?
Women tend to underperceive a man’s commitment to a relationship
(worry he’s not really committed even if he is), but men tend to be
more accurate in assessing women’s relationship commitment – why?
e. Ovulatory effects – the film reviewed many findings regarding changes in
women’s appearance, voice, preferences, and other traits at high fertility
(during ovulation) compared to other times in the menstrual cycle
i. These really reflect two sets of adaptations:
1. Women become more attractive to men when they are at high
fertility (women’s voices, faces, scents, and body movements
are rated as more attractive when the woman is fertile
compared to other times in the menstrual cycle) – these reflect
adaptations in men to increase male reproductive success,
that is, men are sensitive to cues that women are fertile (able
to conceive) and find those women to be particularly
attractive, serving to make mate search and mating effort more
efficient
2. Women’s own preferences shift at high fertility, focusing on
men who are more masculine, more symmetrical, and not
related to them, and backgrounding information about
resources and relationship qualities – these reflect
adaptations in women to increase female reproductive
success, that is, when women are most likely to conceive, they
are particularly sensitive to cues to a man’s genetic quality.
ii. Three neural systems of love
1. Brain imaging studies support the idea that lust (sexual drive),
passionate love (crush), and companionate love (attachment)
are separate evolved systems and serve distinct functions
ii.
Introduction to Behavioral Ecology
• What is a “Darwinian
Approach’ to human
behavior?
• Darwin’s theory of evolution
• The adaptationist approach
• The phenotypic gambit
• Levels of explanation
Behavioral ecology
✤ The goal of a behavioral ecology approach is to try to predict the kind
of behavior we are likely to see given a specific set of environmental
conditions
✤ “Environment” includes the subsistence ecology; predator/prey
ecology; social context; one’s own status (e.g. age); etc.
✤ Some factors in the environment change very slowly or not at all:
e.g.
gravity has functioned in the same way throughout evolutionary
history
✤ Some factors change very rapidly: e.g. a female’s fertility status changes
monthly or seasonally; testosterone levels change throughout the day
Environments of Evolutionary
Adaptedness (EEAs)
✤ The EEA is the environment for which a given trait
was selected
✤ A trait is in equilibrium if the current environment is
that for which the trait evolved; i.e. the trait is
functioning as it should
✤ A trait is in disequilibrium if the current environment
is different from that for which the trait evolved; i.e.
the trait is no longer adaptive
Behavioral ecology rests upon several
assumptions
✤ Organisms are typically well-suited to the environments in which
they live (the phenotypic gambit)
✤ The primary mechanism for genes frequencies to change (i.e. for
biological adaptation to take place) is through reproduction
✤ Organisms that are more efficient in getting resources in any
environment will survive and reproduce better than others
✤ Because we cannot observe genes directly, we rely on the
“proximate correlates” of reproductive success — e.g. resources,
status — to measure efficiency
Behavioral ecology rests upon several
assumptions (con’t)
✤ Organisms do not evolve adaptations to pursue the spread of genes
(which we cannot perceive), but to pursue the proximate correlates
of reproductive success (i.e. to pursue resources, status, sex, etc)
✤ Species in novel environments may find that the proximate
correlates are no longer appropriate; thus the phenotypic gambit
will be false (the behavior observed is not well-suited to the
environment even though it evolved through natural selection)
✤ Humans are not qualitatively different from other animals in terms
of what we “evolved” to do: i.e. obtain and use resources to spread
our genes into the next generation
Two-minute biography of Charles Darwin
• Son of upper-class doctor, medical
school dropout, divinity student
• Avid naturalist and collector; worked
as ship’s naturalist on The Beagle,
1832-36
• Made extensive collections and kept
detailed journal of research and
ideas
• Upon return to London, began going
through his collections, established
network of scientific colleagues
Charles Darwin, 1809-1882
1859: On the Origin of Species
Three Simple Ideas
1. Populations are constrained
by the environment’s ability to
support them
2. Individuals within
populations vary in ways that
affect their ability to survive
and reproduce
3. Variations are transmitted
from parents to offspring
★This is the Theory of Evolution by Natural Selection
Definition
• Fitness (or, Evolutionary Fitness)
‣ n. The probability that the line of descent from an individual with a specific
trait will not die out
‣ Biologists use the word fitness to describe how good a particular genotype
is at leaving offspring in the next generation relative to how good other
genotypes are at it.
‣ An organism’s fitness depends on the environment in which it lives. The
fittest genotype during an ice age, for example, is probably not the fittest
genotype once the ice age is over.
Birds of the Galapagos show us how
natural selection works
Daphne Major
“Darwin’s Finches”
The finches’ beaks demonstrate all
three of Darwin’s postulates
Variation
Affects fitness
Heritability
An example: a drought in 1976-78
changed the environment
Due to drought
conditions,
available seeds
were larger and
harder than in
previous years
Smaller-beaked birds were more likely to
die of starvation during the drought
• The drought is an example of a “selection event,” i.e. when
something occurs in the environment that making individuals
with some characteristics more likely to survive and reproduce
Change in
population
phenotype after
selection event
Characteristics of the population
changed over time
• Large beaked adults
survived better
• Large beaked birds
had large beaked
offspring
• Mean beak size
increased in population
★ This is called Directional Selection
In the absence of a specific selection event,
stabilizing selection maintains the
phenotype
• Birds with very small beaks
can’t find enough food
• Birds with very large beaks
have higher juvenile mortality
• So, selection favors
intermediate beak size
• At equilibrium (when there is
no selection event), selection
will maintain stasis (no
change)
If there is no selection,
characteristics will
“drift” randomly
Definitions
• Phenotype: Any observable
trait of an organism, including
‣ Morphology
‣ Development
‣ Physiological/biochemical
properties
‣ Behavior / products of
behavior
• Genotype: The inherited
instructions carried in the genetic
code
‣ The specific alleles found at
any given DNA locus
Complex traits are affected by genes at
many loci
• If only one gene affected
beak depth
‣ + + have largest beaks
‣ – – have smallest beaks
‣ + – have intermediate
beaks
• Distribution is not smooth
Complex traits are affected by genes at
many loci
• If only one gene affected
beak depth
‣ + + have largest beaks
‣ – – have smallest beaks
‣ + – have intermediate
beaks
• If you add a second locus,
there are more possible
combinations
• Variation is slightly
smoother
Complex traits are affected by genes at
many loci
• If only one gene affected
beak depth
‣ + + have largest beaks
‣ – – have smallest beaks
‣ + – have intermediate
beaks
• If you add a third locus, the
distribution gets even
smoother
• Imagine if there were 10s or
hundreds of loci
Environmental variation smoothes out
the distribution
• Environment affects
expression of genes
• When effect of each
locus is small,
environmental
variation will blur
genetic differences
• Variation can remain
“hidden”
★ Selection acts directly on the phenotype, and only
indirectly on the genotype
But… isn’t it just a theory?
According to the United States National Academy of Sciences:
Some scientific explanations are so well established that no new evidence is likely to alter
them. The explanation then becomes a scientific theory. In everyday language a theory
means a hunch or speculation. Not so in science. In science, the word theory refers to a
comprehensive explanation of an important feature of nature supported by facts gathered
over time. Theories also allow scientists to make predictions about as yet unobserved
phenomena
According to the American Association for the Advancement of Science:
A scientific theory is a well-substantiated explanation of some aspect of the natural world,
based on a body of facts that have been repeatedly confirmed through observation and
experiment. Such fact-supported theories are not “guesses” but reliable accounts of the real
world. The theory of biological evolution is more than “just a theory.” It is as factual an
explanation of the universe as the atomic theory of matter or the germ theory of disease.
Our understanding of gravity is still a work in progress. But the phenomenon of gravity,
If it’s so great, why isn’t it a Law?
• A scientific law can generally be reduced to a mathematical
statement, such as E = mc²; it’s a specific statement based on
empirical data, and its truth is generally confined to a certain
set of conditions. For example, in the case of E = mc², c refers
to the speed of light in a vacuum.
• FYI: The universal law of gravitation is represented by the following equation:
F = G × [(m1m2)/r²], where F is the gravitational force between the two objects, measured in
Newtons. M1 and m2 are the masses of the two objects, while r is the distance between
them. G is the gravitational constant, a number currently calculated to be 6.672 × 10-11 N
m² kg-2
★ Scientific laws describe specific features or events;
Scientific theories provide an explanatory
framework for a wide range of features or events.
Some other things that are “just theories”
• Heliocentrism (Copernicus, 1543): The
heliocentric model is a theory that places the
Sun as the center of the universe, and the
planets orbiting around it.
• Plate Tectonics (Wegener, 1912; Wilson,
1960s): The theory that the outer rigid layer
of the earth (the lithosphere) is divided into
a couple of dozen “plates” that move around
across the earth’s surface relative to each
other, like slabs of ice on a lake.
• Oxygen Theory of Combustion (Lavoisier,
1770s): Replaced the theory of phlogiston,
that every material contained an element of
fire, with one that identified the key role of
oxygen.
• Germ Theory (Pasteur, 1860s): Germ
theory states that many diseases are
caused by the presence and actions of
specific micro-organisms within the body.
So, what is an “adaptationist approach”?
• An adaptationist starts with the
assumption that all organisms are
the result of a selection process that
has favored some phenotypes over
others
• The common phenotype of a
population should therefore be
made up of traits that have, on
average, provided a fitness benefit
to those who had them
• One does not need to know the
actual genetics of the trait to apply
an adaptationist perspective
• The questions we are asking are
“Why does the trait look like it
does?” “What was the adaptive
benefit provided by that trait?”
“What alternative traits would have
provided less fitness benefit?”
★ From an evolutionary perspective, physical,
psychological, and behavioral traits can all be
understood as strategies.
An example: Why do these moths have the
coloring that they do?
An adaptationist answer
✤
Because the moth’s coloring
is the product of natural
selection, we assume that it
must confer some fitness
advantage
✤
That is, in previous
generations, those moths who
had the typical coloring were
more likely to survive and
reproduce than those with
other patterns or colors
“The phenotypic
gambit”
✤
Note that we do not need to
know which genes code for
coloration, or how many
genetic variants there are, to
make this assumption
Using the phenotypic gambit
Some possible hypotheses:
1.We assume that there is a
fitness benefit to the typical
behavior
• Coloration affects their
ability to attract mates
• Coloration affects their
ability to absorb Vitamin D
2.We make hypotheses about
what the benefit may be
✓ A hypothesis is a statement
about the likely relationship
between two variables
• Coloration affects their
ability to find food
• Coloration affects their
ability to avoid predators
(camouflage)
There are two moths in each
of these photographs…
can you see them?
Text
Ecological change produced a change in the
typical phenotype
✤
There is natural variation in the
coloring of peppered moths
✤
Originally, the light-colored
moths were far more common;
they hid on lichen-covered oak
trees
✤
During the industrial
revolution in England,
pollution killed much of the
lichen, and turned the trees
dark with soot
✤
The rare dark moths were now
at an advantage, and became
the more common type in the
environment
✤
With environmental
improvement, the light-colored
moth is again becoming
common
★ This seems to support the
hypothesis that coloration
affects the moths’ ability to
avoid predators
Levels of Explanation
There are different ways to answer the
question “Why do these moths have the
coloring that they do?
1. Ultimate Explanation: The way this phenomenon
affects the survival and/or reproduction of the organism;
why it was favored by natural selection.
2. Proximate Explanation: The immediate way the
phenomenon comes about; e.g. hormones,
environmental triggers
3. Ontogenetic Explanation: How it develops across the lifespan
4. Phylogenetic Explanation: How it developed in the species
We use the same approach to generate hypotheses
about the evolution of human physical traits
For example:
• Bipedality
• Cranial shape
• Dental pattern
• Encephalization (big brains)
• Genital morphology
• Gracile form
• Hairlessness
• etc.
Australopithecus afarensis
Approx. 3.9 – 3 mya
But what about behavior?
• Behavior is much more
complicated than physical
traits because it is
constantly changing in
response to infinite
possible stimuli in the
world
• Can we use the same
adaptationist approach to
try to understand
behavior, including
human behavior??
Using an evolutionary approach to explain
human behavior may seem controversial
• There is a popular conception of natural selection as “selfish,”
“competitive,” “red in tooth and claw”
• There are concerns about the moral and political implications
of evolutionary ideas
• There is skepticism about the power of the process of
evolution to produce complexity
• There is a perception that much of human behavior seems
incompatible with evolutionary theory
• Many people assume that an evolutionary explanation or
approach implies that behavior is genetically predetermined
• There is concern that evolutionary approaches don’t account
for the role of culture and social learning in human life
Next Lecture
• Applying the phenotypic gambit
to behavior
• Addressing popular
controversies and
misconceptions
• Explaining complexity with
natural selection
• Constraints on adaptation
The Phenotypic Gambit
✤
Applying the phenotypic
gambit to behavior (reaction
norms)
✤
Addressing popular
controversies and
misconceptions
✤
Explaining complexity through
natural selection
✤
Constraints on adaptation
Geisha women from Japan
Men from the Wodaabe Tribe
Can we apply the phenotypic gambit to
behavior?
✤
Assume that the common
phenotype has, on
average, conferred some
fitness benefit relative to
other phenotypes
✤
An easy task when we’re
talking about a physical
trait, such as camouflaging
colors
✤
Perhaps also easy when
we’re talking about
“instinctive” behaviors,
e.g. reflexes
[Almost] everyone one in this picture is exhibiting a
behavioral adaptation (flinching)
What about more complex behaviors?
✤
Complex social behaviors
include:
‣ Status striving
‣ Responses to authority
‣ Flirting and seduction
‣ Sharing and friendship
‣ Taking care of others
‣ Competing with others
‣ Taking advantage of others
‣ Trade and commerce
‣ Etc., etc., etc……
Behavior is
genetic
✤
Behavior is produced by biological
structures: the central and
peripheral nervous systems,
muscles, neurochemicals,
hormones, etc.
✤
The central nervous system (a
biological structure) also produces
the goals and desires that motivate
behavior (hunger, pain, sexual
desire, etc.)
✤
Biological structures are built by
proteins with instructions encoded
in DNA.
Does this mean we are controlled by
our genes??
✤
NO
✤
Biological Determinism is wrong
‣ It is the idea that our behaviors,
beliefs, and desires are fixed by
genetics
‣ That is, they are like the cringe
reflex when something comes flying
at us; we can’t help it
‣ An example: Men cannot help but
sexually pursue an attractive
woman … WRONG
Genes (usually) do not code directly for
behavior
Biological
determinism
There is a gene
that dictates that
I pick up and
hold my child.
Genes, along with environment, shape the
way our cognitive machine reacts
Biological
foundation of
behavior
✤
Genes shape how our senses work
✤
They shape our sensitivity to social cues
✤
They shape our emotional responses to various inputs
✤
All of these things combine in complicated ways to produce behavior
Reaction norms
✤
Often, it is not the specific phenotype itself (morphological,
physiological, or behavioral) that is encoded in the genes, but the
ability to respond correctly to a variety of contexts (strategy)
✤
Examples in physiology: increased metabolic rate in cold
environments; increased red blood cell concentration, greater lung
size, and smaller body size at high altitudes
‣ Between population differences not due to genetic differences
‣ Environmental exposure initiates specific phenotypic response
✤
Behavior in particular is almost always characterized by a reaction
norm. By definition, behavior is a rapid phenotypic change in
response to the environment.
Let’s look at an example
✤
Soapberry bugs are known for
their sexual stamina
✤
Copulations may last up to 11
days (!), with a minimum duration
of about 3 hours
✤
There are behavioral differences
between soapberry bug
populations:
‣
In some populations, copulation times
are highly variable (sometimes quick,
sometimes prolonged)
‣
In other populations, there is little
variation in copulation time
Variable copulation
duration
Stable copulation
duration
This presents two questions
1. What is the purpose of extended copulatory bouts (what is the
fitness benefit relative to other strategies)?
2. Why is variability favored in one population, while standardized
behavior is favored in the other?
** Note that when assessing the potential fitness benefits
that a particular strategy may confer, you must also
consider the costs of that strategy
Using the phenotypic gambit to
answer Question 1
Some possible hypotheses:
✤
✤
We assume that there is a
fitness benefit to the typical
behavior
• Longer copulation results in
greater fertilization and more
offspring
‣
• Longer copulation reduces the
female’s likelihood of re-mating
afterwards
Extending copulation time
We make hypotheses about
what the benefit may be
• Longer copulation increases the
female’s likelihood of retaining
that male’s sperm
• Longer copulation is a form of
“mate-guarding,” keeping other
males away from the female
A valid hypothesis must provide testable
predictions
Hypotheses
Predictions
•Longer copulation results in greater
fertilization and more offspring
•Females mated to males who engage
in longer copulations have more
offspring
•Longer copulation reduces the female’s
likelihood of re-mating
•Once copulation has ended, females
who engaged in longer copulation are
less likely to mate again
•Longer copulation increases the
female’s likelihood of retaining that
male’s sperm
•Males who copulate longer sire a
greater proportion of a female’s
offspring
•Longer copulation is a form of “mateguarding,” keeping other males away
from the female
•Males will copulate longer when there
is more competition for females
A valid hypothesis must provide testable
predictions
Hypotheses
Predictions
•Longer copulation results in greater
fertilization and more offspring
•Females mated to males who engage
in longer copulations have more
offspring
•Longer copulation reduces the female’s
likelihood of re-mating
•Once copulation has ended, females
who engaged in longer copulation are
less likely to mate again
•Longer copulation increases the
female’s likelihood of retaining that
male’s sperm
•Males who copulate longer sire a
greater proportion of a female’s
offspring
•Longer copulation is a form of “mateguarding,” keeping other males away
from the female
•Males will copulate longer when there
is more competition for females
Deriving novel predictions
✤
If we think the mate-guarding
explanation is correct, we can come
up with novel predictions
✤
E.g. If there is no need for mate
guarding, there will be no evidence
of extended copulatory bouts
✤
E.g. If the degree of mate guarding
needed is constant, there will be no
variation in copulatory bouts
✤
This can help differentiate between
competing hypotheses and help
refine existing hypotheses
Hypothetico-deductive
method
Using the phenotypic gambit to
answer Question 2
One possible hypothesis:
✤
We assume that there is a
fitness benefit to the typical
behavior
‣
✤
Varying copulation time vs.
Not varying copulation time
We make hypotheses about
what the benefit may be
• If there is variation in
competition, it is beneficial to
assess the current environment
and adjust copulation time to
maximize fitness (maximize
reproductive events vs. maximize
reproductive assurance)
• If there is no variation in
competition, energy spent on
assessing the environment and
adjusting copulation time is
wasted; therefore it is more
beneficial to have a fixed
copulation time
Variable sex ratio;
Variable copulation
duration
Stable sex ratio;
Stable copulation
duration
✤
Variable annual rainfall in Oklahoma means there is a lot of variation in sex ratio from
year to year (sex ratio impacts competition: more males = more competition)
✤
Stable annual rainfall in Florida means there is little variation in sex ratio from year to
year, therefore no benefit to being able to adjust copulation time
ype that responds sensitively to varia
We use the same method for thinking about
human behavior
✤
There are some characteristics
that are generally considered to
be sexually attractive:
‣ Secondary sexual characteristics
(e.g. muscularity in men)
‣ Symmetry of features
‣ Clear skin, healthy hair
✤
What fitness benefit did
individuals who had that
preference gain?
This makes sense, so tell me again why
it’s controversial?
✤
There is a popular conception of natural selection as “selfish,”
“competitive,” “red in tooth and claw”
✤
There are concerns about the moral and political implications of
evolutionary ideas
✤
There is skepticism about the power of the process of evolution to
produce complexity
✤
There is a perception that much of human behavior seems
incompatible with evolutionary theory
✤
Many people assume that an evolutionary explanation or approach
implies that behavior is genetically predetermined
✤
There is concern that evolutionary approaches don’t account for the
role of culture and social learning in human life
This makes sense, so tell me again why
it’s controversial?
✤
There is a popular conception of natural selection as “selfish,”
“competitive,” “red in tooth and claw”
✤
There are concerns about the moral and political implications of
evolutionary ideas
✤
There is skepticism about the power of the process of evolution to
produce complexity
✤
There is a perception that much of human behavior seems
incompatible with evolutionary theory
✤
Many people assume that an evolutionary explanation or approach
implies that behavior is
genetically
Selection
canpredetermined
favor behavioral variability
✤
There is concern that evolutionary approaches don’t account for the
role of culture and social learning in human life
Aversion to popular conception of natural
selection
✓ “Nature, red in tooth and claw”
✓ “Competition”
✓ “Survival of the fittest”
✤
The engine that runs evolution
is selection, that is, some types
will be more likely to survive
and reproduce than others
✤
Direct competition is only one
outcome of selection
✤
Natural selection can also favor
cooperative or altruistic traits
This makes sense, so tell me again why
it’s controversial?
✤
There is a popular conception
of can
natural
selection
as “selfish,”
Selection
favor
generosity
and sharing
“competitive,” “red in tooth and claw”
✤
There are concerns about the moral and political implications of
evolutionary ideas
✤
There is skepticism about the power of the process of evolution to
produce complexity
✤
There is a perception that much of human behavior seems
incompatible with evolutionary theory
✤
Many people assume that an evolutionary explanation or approach
implies that behavior is
genetically
Selection
canpredetermined
favor behavioral variability
✤
There is concern that evolutionary approaches don’t account for the
role of culture and social learning in human life
Concerns about the moral and political
implications of evolutionary ideas
Some people interpret adaptationist
explanations as justifications for
certain behaviors
✤ This is called the NATURALISTIC
FALLACY
✤
✓ The false assumption that “nature”
and “morality” are linked
✓ The false belief that if something is
evolutionarily adaptive, it is
morally justified
✓ The false belief that if something is
morally bad, it could not have
evolved by natural selection
This makes sense, so tell me again why
it’s controversial?
✤
There is a popular conception
of can
natural
selection
as “selfish,”
Selection
favor
generosity
and sharing
“competitive,” “red in tooth and claw”
✤
There are concerns about
theismoral
and political
implications
of
This
the Naturalistic
Fallacy
– it’s wrong
evolutionary ideas
✤
There is skepticism about the power of the process of evolution to
produce complexity
✤
There is a perception that much of human behavior seems
incompatible with evolutionary theory
✤
Many people assume that an evolutionary explanation or approach
implies that behavior is
genetically
Selection
canpredetermined
favor behavioral variability
✤
There is concern that evolutionary approaches don’t account for the
role of culture and social learning in human life
Can this simple process generate
complex adaptations?
• Critiques argue the genetic mutations necessary to change an amoeba
into a tapeworm are as unlikely as a monkey typing out the
soliloquies of Shakespeare.
• How about just the phrase, “Methinks it is like a weasel” (Hamlet)?
• Imagine a keyboard with letters A…Z and
space key = 27 characters
• Chance of typing first letter of phrase
correctly = 1/27
• Chance of typing first and second letters
correctly = 1/27 x 1/27
• Chance of typing all 28 letters correctly =
(1/27)28 = 10-40
(BTW, this is not a monkey!)
This is an infinitesimally small chance!
But selection is not random! (Variation
is!)
• Dawkins’ WEASEL experiment
• The “monkey” is replaced by a
computer, changing letters at
random
• Correct letters are “selected”
and retained
1st try
wdlmnlt dtjbkwirzrezlmqco p
10th try
mdldmnls itjiswhrzrhez mecs p
20th try
meldinls it iswprke z wecsle
30th try
methings it iswlike b wecsel
40th try
methinks it is like i weasel
Cumulative Selection
• Each incremental step must be favored by natural selection
• It must confer a fitness benefit, making the organism with that trait
more likely to survive or reproduce than those without it
• “Macroevolution” (big changes) are rare, and almost never beneficial
Limpet:
Directional
information
• Small changes are much more likely to
be adaptive.
• Complex adaptations arise through
many small steps.
Beyrich Split Shell:
Better
directional
information
• Each step must be favored by selection
Spiny Murex:
Better
image
Abalone:
Crude image
Evolution can produce rapid change
• Wolves were domesticated ≈ 15,000 yrs ago in Asia
But where does all that variation
come from?
The Chihuahua problem:
• How is it that some dogs are
smaller than the smallest
wolves if all dogs are
descended from wolves?
One assumption is that evolutionary change is the
product of random mutations in the genetic code
• Mutations are “mistakes” in DNA replication that
alter DNA message; they arise spontaneously
• Rates of mutation are very low
• Mutations are usually deleterious (bad)
Hidden variation resolves the
Chihuahua problem
• Normal sized wolves carry some alleles
for small body size (–) and many +
alleles
• As big wolves die (or people prefer
small ones), frequency of – allelles
increases
• Variation is shuffled, some new
combinations arise
• As – alleles become more common,
more – alleles likely to be combined in a
single individual
• New combinations with more – alleles
will be outside initial range of variation
This makes sense, so tell me again why
it’s controversial?
✤
There is a popular conception
of can
natural
selection
as “selfish,”
Selection
favor
generosity
and sharing
“competitive,” “red in tooth and claw”
✤
There are concerns about
theismoral
and political
implications
of
This
the Naturalistic
Fallacy
– it’s wrong
evolutionary ideas
✤
There is skepticism about the power of the process of evolution to
produce complexity Selection can produce rapid, complex change
✤
There is a perception that much of human behavior seems
incompatible with evolutionary theory
✤
Many people assume that an evolutionary explanation or approach
implies that behavior is
genetically
Selection
canpredetermined
favor behavioral variability
✤
There is concern that evolutionary approaches don’t account for the
role of culture and social learning in human life
We’ll deal with the last two points in
the coming weeks….
Implications from today’s lecture
✤
Although our behavior is not
biologically determined, our
evolutionary history does make
some patterns of behavior unlikely
and rarely seen
✤
E.g., lactation is energetically
expensive and time-consuming,
and until very recently in human
history, was the only option for
feeding infant
✤
How does that affect the costs and
benefits of pursuing different
behaviors, such as foraging
activities?
Lecture 3
The Selfish Gene
01
Competition is the key concept in
evolution
✤
Natural selection works though competition
✤
Competition occurs at all levels: from the gene to societies
✤
Genes compete for locations on the chromosome
✤
Groups of genes (i.e. those that produce a certain trait or characteristic)
compete with other groups of genes (other traits)
✤
Collections of traits are bundled into complex organisms that compete
against other organisms
✤
Those genes that get copied into more individuals are those that persist
through time; they are favored by natural selection
What are genes competing for?
✤ The competition is for the resources required to reproduce,
including mates
✤ It is one’s conspecifics (others of the same species) that are most in
need of the same resources, and who are most likely to face the
same challenges (e.g. species-specific pathogens or parasites)
✤ The competition is relative, not absolute
✤ The characteristics or traits that allow one individual to obtain more
resources and produce more offspring than his/her conspecifics are
the characteristics that will become common in the population
Evolutionary competition
in a nutshell
Two men are sleeping in a campsite
when an angry bear crashes through
the bushes. They both leap up, but as
one starts running, the other pauses to
lace up his sneakers.
“Are you crazy?” yells the first guy.
“You’ll never outrun the bear if you
stop to do that!”
“I don’t have to outrun the bear,”
replies the second guy as he lopes
past the first. “I just have to outrun
YOU.”
01
Genetically selfish behaviors are
efficient
✤ Time and energy are finite resources; what is spent on one activity
cannot be spent on another
✦
For example, somatic energy expended healing injuries is energy that cannot be spent seeking
mates
✤ Genetically selfish behaviors are efficient, that is, they enable the
individual to reap the greatest benefits at the lowest cost
✤ Sometimes the most efficient behavior is to just take what you need,
trampling whoever is in the way
✤ Often, however, the costs or potential costs of such reckless behaviors
make them inefficient instead
Reproductive Strategies
✤ The behaviors designed to reproduce one’s genes into the next generation
are referred to collectively as “reproductive strategies”
✤ The various patterns of behaviors that are employed to get one’s genes
into the next generation are referred to as “reproductive strategies”
✤ In evolutionary terms, the term “strategies” need not imply a conscious
plan of action
✤
Age at maturation is part of a reproductive strategy, though it is not under conscious
control
✤ Strategies may be fixed (invariant) or facultative (dependent on features
of the local environment)
Strategies do not always
appear to be efficient
✤ Bloody battles and outrageous
adornment appear to be
inefficient strategies
✤ They require lots of energy
(which requires resources)
✤ They expose the individual to
risk, e.g. of predation
✤ They often seem not to serve
any purpose
01
1871: “The Descent of
Man, and Selection in
Relation to Sex”
✤
Darwin described a mechanism
distinct from “natural selection,” and
much more powerful: sexual
selection
✤
Can be used to explain extravagant
secondary sexual characteristics, esp
in males
✤ Identified two primary forms
✤ Intersexual selection (mate choice;
selection imposed by one sex on the
other)
✤
Intrasexual selection (competition
for mates; selection imposed by one
sex on others of the same sex)
Charles Darwin, 1874
01
Different kinds of selection
produce different kinds of
traits
✤ Intersexual selection (mate
choice) typically produces
adornments
Bright feathers, songs,
markings, etc.
✤ Intrasexual selection (direct
competition) typically produces
weapons
✤ QUESTION:
What kind of
selection do you think
produced human creativity? Is
it a weapon? Or an adornment?
Teeth, antlers, claws, large
body size, etc.
01
Genetic selfishness ≠ behavioral
selfishness
✤
In fact, genetically selfish behavior can actually appear to be
costly to the individual, while benefitting someone else
(altruistic)
✤
Reproductive success is measured at the level of the genes
✤
You share your genes with your kin
✤
“Degree of relatedness” (r) refers to the amount of genes in
common with a particular class of relative
–
For example, because you get half your genes from your mother and
half from your father, the degree of relatedness with a parent or
offspring is .5
Hamilton’s rule of kin selection
✤ If you benefit a relative, even at a cost to your own
reproduction, you will be benefitting some of your
genes (genes that also appear in your kin)
Hamilton’s Rule:
rb>c
relatedness*benefit to kin > cost to self
Individuals will incur greater
costs to benefit closer kin
✤ r = .5 for full sibs, parents
✤ r = .25 for half sibs
✤ r = .125 for first cousins
✤ The more distant the relative,
the higher the threshold for a
cost to be “worth it.”
✤ “I would give my life for 1
brother or 4 cousins” (J.S.
Haldane)
01
Behavioral ecology
✤ The goal of a behavioral ecology approach is to try to predict the kind
of behavior we are likely to see given a specific set of environmental
conditions
✤ “Environment” includes the subsistence ecology; predator/prey
ecology; social context; one’s own status (e.g. age); etc.
✤ Some factors in the environment change very slowly or not at all:
e.g.
gravity has functioned in the same way throughout evolutionary
history
✤ Some factors change very rapidly: e.g. a female’s fertility status changes
monthly or seasonally; testosterone levels change throughout the day
Environments of Evolutionary
Adaptedness (EEAs)
✤ The EEA is the environment for which a given trait
was selected
✤ A trait is in equilibrium if the current environment is
that for which the trait evolved; i.e. the trait is
functioning as it should
✤ A trait is in disequilibrium if the current environment
is different from that for which the trait evolved; i.e.
the trait is no longer adaptive
Behavioral ecology rests upon several
assumptions
✤ Organisms are typically well-suited to the environments in which they
live (the phenotypic gambit)
✤ The only mechanism for genes frequencies to change (i.e. for biological
adaptation to take place) is through reproduction or kin selection
✤ Organisms that are more efficient in getting resources in any
environment will survive and reproduce better than others
✤ Because we cannot observe genes directly, we rely on the “proximate
correlates” of reproductive success — e.g. resources, status — to
measure efficiency
Behavioral ecology rests upon several
assumptions (con’t)
✤ Organisms do not evolve adaptations to pursue the spread of genes
(which we cannot perceive), but to pursue the proximate correlates
of reproductive success (i.e. to pursue resources, status, sex, etc)
✤ Species in novel environments may find that the proximate
correlates are no longer appropriate; thus the phenotypic gambit
will be false (the behavior observed is not well-suited to the
environment even though it evolved through natural selection)
✤ Humans are not qualitatively different from other animals in terms
of what we “evolved” to do: i.e. obtain and use resources to spread
our genes into the next generation
Reproductive
Ecology
Why two sexes?
✤ Strategies of the sexes:
✤
‣
Mating effort
‣
Parenting effort
✤
Parental investment
theory
✤
Reproductive variance
✤
The most common reproductive
pattern in nature is to have two
sexes, each with a specialized role
✤
Sex differences begin with
differences in gametes (sex cells)
1. Giant, costly-to-produce eggs, rich in
genetic and somatic material
2. Tiny, much cheaper-to-produce, and
genetically and somatically
streamlined sperm
✤
The two gametes derive from
strategic specialization
3. Be prepared and wait for others to
find you
4. Travel light and go far
The biological definition of
sex is based on gamete size:
within a species, the morph
(type) that has the larger
gamete is denoted “female”
✤
In order to get copies of one’s genes into the next
generation, all members of sexually-reproducing
species must:
‣ Combine one’s own genes with those of someone else (find an
appropriate mate, convince that individual to have sex with
you, and have sex that results in conception)
‣ Ensure the new combination of genes will survive to
reproduce in its turn (either invest in that offspring, be sure
that someone else will invest in that offspring, or produce
enough offspring that odds are in your favor that some will
survive)
✤
Individuals will pursue different combinations of
strategies to accomplish this
Mating effort
✤
Mating effort = effort
expended on obtaining
sexual opportunities
‣ Control mates
‣ Control resources
‣ Display to mates
✤
Characterized by high “start up” cost
(e.g. energy needed to build weapons
or adornments), low “per-event” cost
(features built just once), high risk
(e.g. injury or death), and potentially
high gains (e.g. greater numbers of
offspring)
Parenting effort
✤
Parenting effort = effort
expended on growing/raising
offspring
‣
Feed, protect, teach
✤ Characterized by low “upfront” costs (no competitive
weapons or adornments),
high “per-event” costs (each
reproduction requires full
investment), low risk, and
moderate potential gains
Different reproductive payoff curves
✤
Mating effort requires large
“up-front” expenditures to
get started
✤
Once reproductively
competitive, small
investments in mating effort
can lead to great advantages
✤
Parenting effort requires less
effort to be viable, but each
reproduction requires the
full amount of effort
Mating Effort Payoff Curve:
*
The Jake Ryan Effect
Number of girls
who would accept an
invitation to the dance
* Characters from the fantastic John Hughes’ film, Sixteen Candles.
15
10
5
0
Parenting Effort Payoff Curve:
The Samantha Baker [Non-]Effect
Chance of getting
pregnant at the dance*
Still Sixteen Candles…
15
10
5
0
* NOT an accurate
representation of
the probability of
getting pregnant
Differences in obligatory parental
investment
✤
Within species, males and females generally differ in the amount of energy
they are obliged (required) to invest to successfully reproduce
The obligatory amount varies by species, sex, and
environment
✤
vs.
vs.
Differences in obligatory parental
investment
✤
Obligatory costs include post-natal investment that is necessary for offspring to
survive; in birds, e.g., after eggs are laid either males or females could incubate the
eggs and feed the offspring, but if these things don’t happen, individuals do not
reproduce
✤
In mammals, there is always a significant sex difference in obligate investment due
to the costs of internal gestation, parturition, and lactation.
✤
In other organisms, the degree of sex difference in obligate investment can vary
greatly, from an insignificant difference to a huge difference.
✤
Egg-laying animals, for example, often show more equal levels of obligate
investment, because either sex is capable of incubating an egg, and in some species,
cooperation between two individuals is required to maintain constant incubation
✤
It is the AMOUNT of difference in obligate investment between males and females
in a particular population that is important
Parental Investment Theory
(Robert Trivers, 1972)
✤
Behavioral patterns (strategies) can
be reliably predicted based upon
differences in obligatory parental
investment
✤
The sex with lower obligatory
investment, usually males, should
be more motivated to obtain
mating opportunities
✤
The sex with higher obligatory
investment, usually females,
should be more choosy about
whom he or she mates with
You guys, he’s trying!
Proof of concept
“Pregnant” male seahorse
✤
In “sex-reversed” species, like
seahorses, we see the opposite
patterns
✤
Male seahorses have higher obligate
parental investment than do females,
because the carry the fertilized eggs in
a pouch until they hatch (often
referred to as male “pregnancy”)
✤
Among seahorses, females tend to be
more sexually motivated and
aggressive, while males tend to be
more choosy about their mates
What are the constraints on
reproductive success?
✤
For female mammals:
✤
Reproductive success is not limited by access to mates, but by
resources that affect one’s own rate of reproduction
✤
There is no shortage of available genetic material (i.e. sperm),
therefore, no need to compete for access to that (one male can
produce enough sperm to enable reproduction with dozens of
females)
✤
Rate of reproduction can be affected by one’s own somatic
resources (e.g. fat stores), availability of nutritional resources,
availability of “weaning foods,” availability of alloparents
What are the constraints on
reproductive success?
✤
For male mammals:
✤
Sperm is plentiful and cheap to produce; therefore, reproduction is
not limited by access to resources (i.e. nutritional resources
necessary to produce sperm)
✤
Female reproductive effort (gestation and lactation) is very costly
and thus in limited supply. Male reproductive success is limited
by his ability to access female reproductive effort.
✤
If a male can obtain exclusive access to multiple females, he can
have much much greater reproductive success than other males
who will be shut out of reproductive opportunities
✤
Monogamy levels the playing field between males, ensuring access
to costly reproductive resources for all males
✤
Despite the different constraints on reproductive success, males and females have, on
average, the same reproductive success (each offspring has one mother and one father)
✤
But, males have more variation in reproductive success than females do (some males
have high RS while others have low RS; most females have similar RS)
👩🏻 1 👩🏻 2 👩🏻 3 👩🏻 4 👩🏻 5 👩🏻 6 👩🏻 7 Total
👨🏻 1
8
0
0
0
0
0
0
8
👨🏻 2
0
7
0
0
0
0
0
7
👨🏻 3
0
0
6
7
5
0
0
18
👨🏻 4
0
0
0
0
0
6
0
6
👨🏻 5
0
0
0
0
0
0
9
9
👨🏻 6
0
0
0
0
0
0
0
0
👨🏻 7
0
0
0
0
1
0
0
1
Total
8
7
6
7
6
6
9
Average offspring per woman = 7; range = 6-9
Average offspring per man = 7; range = 0-18
Greater Variation
Why share a male mate?
✤
In the example above, some males had multiple wives/reproductive
partners, leaving others with none
✤
Why would females choose to mate with someone who has already
mated with someone else?
✤
Two broad classes of scenarios:
‣ Males provide no investment beyond genetic quality (thus no reason to keep one
around for oneself; get the sperm and go)
‣ Males do provide investment, and they vary (differ) in their own access to, or ability
or willingness to share, resources with their mate and offspring
When monogamy is enforced (even with
some cheating or serial monogamy):
👩🏻 1 👩🏻 2 👩🏻 3 👩🏻 4 👩🏻 5 👩🏻 6 👩🏻 7 Total
👨🏻 1
8
0
0
0
0
0
0
8
👨🏻 2
0
7
0
0
0
0
0
7
👨🏻 3
0
0
6
0
0
0
0
6
👨🏻 4
0
0
0
9
0
0
0
9
👨🏻 5
0
0
0
0
5
0
0
5
👨🏻 6
0
0
0
0
0
7
0
7
👨🏻 7
0
0
0
0
2
0
5
7
Total
8
7
6
9
7
7
9
Average offspring per woman = 7; range = 6-9
Average offspring per man = 7; range = 6-9
Same Variation
Differences in obligate investment produce
these differences in reproductive variance
✤
Reproductive variance is the
range of variation in reproductive
success between members of the
same sex
✤
The higher investing sex will
have less variance in
reproductive success (i.e. all
individuals will have about
same success)
✤
The lower investing sex will
have greater variance (i.e. there is
a greater difference in success
between members of that sex)
Data from Hill & Hurtado, 1996
✤
Ethiopian man with 11 wives and 77 children
✤
Ten other men are left without wives or children
✤
Each wife has 5-8 children
Women gain less in RS by competing
for access to mates
✤
A polyandrous family in
Tibet
✤
Female total
reproduction is still
limited by how many
babies SHE can have in a
year
✤
This system seems odd,
as males are sharing a
mate instead of
competing for additional
mates
TOPIC I: Basics of Behavioral Ecology & Evolution
1. What exactly is the phenotypic gambit? Is it the same as phenotype??
The phenotype is simply what the organism looks like (broadly). Technically, it can
include features that are not visible, such as hormonal profiles or blood types, but it is the
traits that are ​expressed​ in the organism.
The ​phenotypic gambit​ is simply a short cut, a tool that researchers use. Basically,
researchers take as the first assumption that any common phenotype observed in a
population is there because that particular phenotype conferred some selective advantage
compared to other phenotypes. So, for example, if we see that primates typically give birth
to only one offspring at a time, as opposed to, say, cats that give birth to about 4-6 offspring
at a time, we want to know why primates have that particular phenotype. For behavioral
ecologists (which includes evolutionary anthropologists), our first guess is “there is a
selective advantage for this species to giving birth to only one offspring at a time.” From
there we can think about how having more offspring at a time might have been costly for
primate ancestors and come up with hypotheses as to why we see the specific phenotype
we do in primates. The phenotypic gambit can be compared to other first assumptions,
such as randomness (“it doesn’t matter how many offspring members of a species have, it’s
just random”) or religion (“primates have one offspring at a time because God planned it
that way”) or group selection (“primates have only one offspring at a time to control their
population”) or a variety of other possible hypotheses.
2. Stabilizing Selection – is that the same thing as no selection?​ ​What is Directional
Selection?
The short answer is, ​NO​, if a trait is under stabilizing selection it does not mean that
selection has stopped.
“Selection” means simply that some traits, or some expressions (versions) of traits,
affect an individual’s ability to survive and reproduce better than do other traits, or other
expressions. There is therefore selection for the favorable traits. Selection can either be
directional​ (i.e. the average expression of the trait in the population is ​changing​) or
stabilizing​ (i.e. the average expression of the trait in the population is being ​maintained​). If
there is no selection at all (meaning that one version of the trait is as good as any other in
terms of survival and reproduction), the trait will simply wander. It is actually pretty hard
to think of a trait that is not under either directional or stabilizing selection since most
traits will affect one’s ​fitness​ (in the evolutionary sense, meaning, potential reproductive
success) in some way or another. For example, I was sitting here trying to come up with a
good example of a trait that is just ​drifting​ (i.e., a trait not under stabilizing selection) and
first thought about hair color, but in fact, even if there is not a strong survival cost to hair
color, there may be a substantial social and reproductive cost to an individual who is born
with a non-typical hair color. Think, for example, of albinism (no pigmentation in hair or
skin). People often find people with albinism to be “strange looking” and in many times
and cultures there have been prejudices against such people. That would certainly confer a
fitness cost on the person with that trait.
Any trait for which there is a fitness cost if the expression of the trait is too far from
the norm can be considered to be under ​stabilizing selection​. The process of natural
selection is ​maintaining that trait in it’s typical form​. Remember that there will still always
be variation around the average expression of the trait, that is, not everyone will exhibit the
exact same expression of the trait, but if there is stabilizing selection, that average ​should
not change​ substantially from one generation to the next.
Most traits, most of the time, are under stabilizing selection. ​Directional selection
occurs if there is a ​selection event​, that is, a change in the environment that changes the
costs and benefits of different expressions of the trait. Changes in the environment could
include ecological changes (e.g. changes in average temperature, changes in available food
sources, etc.) or social changes (e.g. a population crash or boom, changes in sex ratio, etc.).
When directional selection is occurring, the average phenotype is changing. For example,
when pollution killed the lichen on the oak trees in London, the peppered moths
underwent ​directional selection​ that favored darker colored moths. Eventually, the
darker-colored moths became common. When environmental standards started cleaning
up the soot and lichen starting coming back, ​directional selection ​occurred again, pushing
for lighter coloration again in some moth populations.
3. What maintains genetic variation? And how is variation random (and selection
systematic)?
Selection is systematic because only those traits that, on average, result in greater
reproductive success are “selected,” that is, are likely to be transmitted to future
generations. Those traits that produce greater on average costs than benefits (ultimately
measured in the currency of reproductive success) are selected ​against​. There is nothing,
therefore, random about which traits are likely to get passed on.
But the traits themselves ​arise​ from random variation. The variation comes from
information in our genes. The human genome has somewhere between 20,000 and 25,000
genes, that is, specific loci on our DNA. Each one of those loci has some number of possible
alleles: for some there are only two possible alleles (e.g. dominant and recessive genes), but
for many there are more than two. The combinatorial possibilities are, essentially, infinite.
Written language provides a good example. There are only 26 letters in the English
language, but they can combine into hundreds of thousands words (actually, relatively few
in English — only about 600,000 — compared to say, Arabic, for example), and those can
combine into an infinite number of completely novel and unique sentences. Likely the
exact sentence above has never been uttered or written by anyone before. Now imagine a
language with 25,000 different letters and you’ll get a sense of the vast possibility of unique
variation held in the human genome. Some of the mathy folk among you might argue with
me that while the number is ​Very Large​ it is not actually infinite (there are, after all,
technically only a finite number of combinations that can be produced based on the
variants available for 25,000 loci in the genome, though that number is incredibly huge). In
the case of genetic combinations, though, it ​is​ actually infinite. This is because, in addition
to the combinatorial explosion that we have from the sheer number of genes, mutation
occurs continuously (albeit at low rates) adding in new variation all the time.
In addition, most genes influence multiple traits, all genes interact with the
environment (natural and social) to produce a specific expression of a trait (this is why
identical twins, while very similar to one another, are ​NOT​ the same person despite having
identical genotypes… they are, in fact, clones of one another, yet all of you who know
identical twins, know that they are subtly, and sometimes pretty obviously, different from
one another), and most genes do not function in an “on/off” way, but rather work like dials,
making something more or less likely, more or less sensitive, more or less pronounced, etc.
Selection​ narrows the available ​alleles​ and thus the available ​expressions​ of traits.
Some traits will come to ​fixation,​ meaning that they do not vary (meaningfully, within the
species). For example, humans, and all mammals, birds, and reptiles, have an underlying
body plan that includes five digits on each limb. For us, that is five fingers on each hand
and five toes on each foot. For horses, those digits are still there, but they’ve been
reconfigured by natural selection so that most of them are very small and make up part of
the “hock” (ankle)… the hoof is, essentially, the toenail of the middle digit (amazing!). This
suggests that the “five digit” trait is super ancient, having arisen long before there were
such categories as mammals and birds, let alone humans! Other traits have come to
fixation much more recently. All normally developing humans are ​bipedal​, that is, we walk
upright on two legs. Although there are obviously exceptions, these can most correctly be
considered as deviations from the normal human body plan (this does ​NOT​ mean that
people with no legs, or non-functioning legs, or lost legs or anything else are not human, or
less human, or “damaged,” it just means that they diverge from the species-typical
phenotype. This concept will ​also​ come back many times….).
Other traits will never come to fixation in that same way because it is beneficial for
there to be variation in the trait. ​All ​personality traits are like that. There is no
species-typical personality for humans, rather there are huge amounts of variation in our
tendencies to be open to new experiences, to be comfortable with others, to be
introspective, etc. If we think about that variation from an adaptive perspective, we realize
that having differences would be favored by natural selection because often, if everyone
else is the same, the one with a unique set of traits has an advantage (thinking outside the
box, as it were). If everyone is a truthful person, a rare liar who appears due to random
recombination or mutation will have a huge strategic advantage. If we are all a little
different, then we will each be likely to find our ecological niche. There are lots of nuances
to this argument, but they are beyond the scope of this class!
Remember that variation should be measured at many different levels. Humans, as
a species, are cooperative compared to other species (we are the only species that I’m
aware of for which you take 300 unrelated, unknown individuals, enclose them in a small
steel cylinder, and which don’t immediately devolve into threats, fights, and deaths). But
some individual humans are more cooperative than others. We will continue to talk about
this kind of variation throughout the course.
4. Behavior is genetic… but how?
As I outlined in the lectures, behavior is genetic because the mechanisms that
produce behavior (e.g. central and peripheral nervous system) are produced through
instructions encoded in your genes. Your genes do not dictate your specific behavior at a
given time, but they produce the capacity for behavior. There is no behavior outside of
what is made possible through the structures that produce it. For example, staying under
water for two hours (as blue whales do) is a behavior, but not one that humans can
produce. Writing a sonnet that no one else has ever written is also a behavior. It is not
directly encoded in your genes (there is no sonnet-writing gene and there is certainly no
gene for writing the words “O how much more doth beauty beauteous seem / By that sweet
ornament which truth doth give!), but it ​is​ your genes that allow you to comprehend and
produce language, that have shaped a mind sensitive to patterns and rhythm, allow you to
experience love and awe, seek recognition for your wit or creativity or sensitivity, etc.
5. Can you explain the difference between the “proximate explanation” and the
“ultimate explanation?”
The “proximate explanation” of a phenomenon is the immediate cause of that
phenomenon (proximate = proximity). The “ultimate explanation” explains why that
phenomenon was favored by natural selection (or, if it wasn’t specifically favored by
natural selection, why it exists in the organism). An example. Why did you fall in love with
your spouse? Proximate explanation(s): he made me laugh, I found him extremely sexy,
we both love the opera, we shared the same values, I just felt happy when I was around
him. Ultimate explanation(s): humans are sensitive to cues to being able to communicate
and cooperate with a potential partner (e.g. shared interests, sense of humor) because
finding a parenting partner with whom one could more easily communicate/cooperate
would have led to greater reproductive success among human ancestors than not being
sensitive to those cues. Men and women find certain scents sexually arousing because
those scents act as a cue to genetic compatibility; those who reproduced with individuals
with different genetic makeup from their own had higher reproductive success than those
who did not. The emotion of “falling in love” allowed our ancestors to pursue their own
mate preferences in the face of parent-offspring conflict over mates, thus increasing one’s
own reproductive fitness even at the cost of parents’ inclusive fitness. Etc.
6. What is the naturalistic fallacy?
The naturalistic fallacy is the ​false belief​ that whether or not something is “natural”
is related to whether or not that things is “morally good.” In short, there is, simply, no
relationship between “natural” and “morally good,” but we tend to confuse the two a lot.
This causes a lot of controversy when scholars apply a evolutionary framework to
questions of human behavior. An evolutionary perspective seeks to ​explain​ behavior from
the perspective of what has been favored by natural selection (resulted, on average, in
greater survival and reproduction in an ancestral population). Thus, there is definitely the
sense that behaviors that have been explained from this perspective are “natural.” So, for
example, I could say that there is good evidence that men tend to experience a greater
desire for sexual variety (lots of sex partners) than women do, and that this is explainable
from a natural selection perspective because men, like other mammalian males, can greatly
increase their reproductive success by increasing their number of sex partners, whereas
women, like other female mammals, cannot do so. The mistake, the ​fallacy,​ is interpreting
this to mean that it is ​morally acceptable​ for men to cheat on their wives or girlfriends, to
use women for sex, etc. There is, simply, no connection between what is natural and what
is moral, because it is ​human societies​ that determine what is moral. Natural selection is
silent on that point. We can very easily see that the two are unrelated because we can
easily find examples of all four possible combinations:
Natural
Artificial
Morally good
●Vitamins and minerals found in fruits
and vegetables are more accessible by
our bodies than manufactured ones.
●The tendency to love and cherish our
children; the tendency to value fairness
●Antibiotics. Save millions of lives from
what should be minor cuts or injuries
every year.
●The rule of law. There are always
conflicts of interests; law (generally)
protects us from those.
Morally bad
●Smallpox. Cholera. Malaria.
Flesh-eating virus. Polio. Etc.
●The tendency to treat people in our
in-group best, typically at a cost to
people in our out-group (discrimination)
●Heroin. Noise pollution. Air pollution.
Etc.
●Laws that codify the subjugation of
certain subsets of people: pro-slavery
laws, laws that treat women as property
to be traded between men, etc.
TOPIC II: Sexual Selection
1. Continuous vs. categorical traits
The term “continuous” or “categorical” refers to the distribution of variation on the
trait. Most of the traits that we are interested in in this class are ​continuously varying
traits.​ This means that variation on the trait is on a continuum. For example, intelligence
is a continuously varying trait. If we could line every human being up in order of
intelligence, there would be someone who is the smartest person in the world, and
someone who is the least smart person in the world, and every possible value of
intelligence in between would be represented in a ​normal distribution​. A “normal
distribution” refers to the famous “bell curve.” What this shows is that ​most people​ ​are of
average intelligence (that is, of course, what makes it the average). As we move away from
the average in either direction, we see fewer and fewer people showing that value of the
trait. There are very few geniuses relative to average-intelligence people, and very few
low-functioning intelligence people relative to average-intelligence people. Hence the
typical bell-shaped distribution. We would see a similar pattern if we lined every
individual in the world up in terms of skin tone, in terms of eye color, in terms of
extroversion, in terms of visual acuity, in terms of how much they enjoy smooth jazz, in
terms of BMI, etc.
Height is a continuously varying trait:
But not ​all​ traits are continuous. Most of the ones that are not refer to those traits
that have reached fixation (as described above) or those that refer to ​social categories
(which are not biological!). For example, if we lined every individual in the world up in
terms of whether or not they can perceive color, we would end up with four
non-continuous ​GROUPS​: (1) those who perceive the full range of color available to
humans, (2) those who are red-green color blind, (3) those who are blue-yellow colorblind,
and (4) those who are completely color blind. While there will be some variation within
those groups in terms of how sharp the color perception is or other features, it would be
inaccurate to say that it’s a ​continuously varying​ trait. Rather, there are four natural
categories​. Also, they will not make anything that looks like a bell curve if we tried to put
them all in a histogram. Instead, we would see a very large cluster of those who can see
color, a much smaller cluster of those who are red-green colorblind, another small cluster
of those who are blue-yellow colorblind, and a very small cluster of those who are totally
colorblind.
Social categories, by definition, are categorical. You either are or are not a CSUF
student. You are a citizen of a specific country (or multiple countries), but there are no
“in-between” values. Here’s what that looks like if we try to “line everyone up:”
As with all definitions, these things break down when we drill down far enough. I
just said there are no “in-between” values of citizenship, but of course that’s not entirely
true. How might we consider someone with a temporary visa, or a naturalized citizen, or
an expatriate, etc. But this is true of ​ALL CATEGORIES​. In the end, there are always some
sorts of “in-between” values. Species, for example, is a categorical variable. You are either
a human or a cat or a walrus or a bumblebee, etc. That seems quite obvious when we’re
talking about humans and cats and walruses and bumblebees, but among closely-related
species there is no ​real​ categorical divide. The definitions of the categories are
data-driven​ (that is, looking at the of distributions of traits) and there are sometimes
arguments (biologists may argue about whether two populations of insects represent the
same or different species). Again, the details of this argument are beyond the scope of this
class​.
2. Sexual vs. Natural Selection – are they the same thing, or different?
It is most accurate to characterize sexual selection as a ​special case​ of natural
selection. Natural selection, as outlined in Darwin’s three postulates, is quite broad, and
covers all variation that “affects an individual’s ability to survive and reproduce.” Sexual
selection is a subset that focuses on the selective effects of the ​sexual b
​ ehaviors and
preferences of conspecifics (individuals of the same species). The underlying mechanism is
the same: individuals vary on their expression of a trait, some versions or expressions of
that trait make them better able to reproduce (​specifically​ ​because they are either more
attractive to the opposite sex or because they make them more successful at competing for
access to mates​), and those traits get passed on to offspring. More successful traits,
successful by definition because they result in greater reproduction, are passed on to ​more
offspring (or more successful offspring) and become more common in the population. Less
successful traits eventually become ​selected out​, i.e. the cease to show up regularly in the
population. What make ​sexual selection​ so much more interesting and more powerful than
the standard natural selection is that there can be a “runaway” process, leading to really
extravagant features that would not otherwise be favored by natural selection.
Take, for example, the evolutionary biologist’s favorite example of the peacock’s tail.
The bright plumage and long feathers of the male peacock provide some information to
females about that male’s ​genetic quality​ and ​current condition​ (i.e. health). It is
energetically costly to grow and maintain such a tail – energy used to grow and maintain
the tail is energy that is not being used to find food, fight illness, or avoid predators, among
other important things. But because the quality of the tail correlates with the male’s
genetic quality, females ​prefer to mate with​ males with long, bright tails. In the beginning, it
may have been a subtle thing, a few colored feathers. But males who had a version of the
trait that “cheated,” i.e. produced colored feathers even though they were of lower genetic
quality, would also get mating opportunities. There is then selective pressure on females to
be ​more​ discriminating, looking for tails that are brighter and longer. This results in
greater selective pressure on males to produce ever longer and brighter tails, even at a
greater energetic cost to themselves. Over many generations, the trait becomes
exaggerated because those males with the longest and brightest tails in each generation get
massive reproductive payoffs, even at the cost, for example, of an earlier death. Eventually
the trait hits a wall, where it cannot be exaggerated any longer without becoming too costly
to bear, for example, leading to death before reproduction is possible. Just about any trait
can get “hit upon” as an indicator of quality that then gets exaggerated due to this sort of
preference, for example, deep croaks in frogs, shiny fins in fish, loud singing in Howler
Monkeys, and artistic creativity in humans. This is an example of ​intersexual selection
(inter = between; intersexual selection is a selective force occurring between the sexes
when individuals of one sex are choosy about whom they have sex with).
Intrasexual selection​ works the same way except that rather than being driven by
female’s sexual preferences, it’s driven by the ability to out-compete other males for ​access
to females (intra = within; intrasexual selection is a selective force occurring within a single
sex and individuals within that sex compete with one another for access to reproductive
partners). A small advantage — for example, hard boney bumps on top of one’s head — can
get exaggerated over the generations until it becomes something very costly and
exaggerated like the giant antlers of red deer bulls.
Scientists have recreated this sort of “runaway” selection in the lab many times, and
we can easily see how strong sexual advantage can create rapid and dramatic change. If
you remove the sexual advantage of the trait (for example, if you could make all peahens
indifferent to peacock tails), the trait would quickly (over several generations) revert to a
less costly version, for example, a plain, short brown tail. The same goes for traits
produced via ​intrasexual selection​, if you could remove the competitive advantage of the
trait (for example, convince all red deer to solve their problems through talking), it would
quickly (over several generations) revert to a less costly variant. The ​unexaggerated​ trait is
the one that is said to be favored by “normal” natural selection.
● Intrasexual selection​ refers to direct competition between individuals over
access to mates. That means that the competitors are, generally, individuals
of the same species and the same sex. It would not do my cat any good to
beat up our dog to get her mates. Nor, from a ​differential​ ​reproduction​ point
of view, would it do any good for my female cat to assault my male cat in
order to get ​his​ mates. We will, of course, come back to how homosexual
orientation fits into all this in the coming weeks, don’t worry!!
● Intersexual selection​ refers to the effect that the ​preferences o
​ f one sex have
on the phenotypes of the opposite sex. Female peacocks (called peahens)
favor brightly colored feathers because an individual’s current condition
(nutrition, parasite load, disease status, etc.) and genes affect his ability to
produce colorful feathers. Over evolutionary time, this preference produces
a competitive arena for male peacocks, leading to ever more brightly colored
and extravagant feathers in an effort to be most appealing to females.
● THESE PROCESSES ARE NOT MUTUALLY EXCLUSIVE, AND IT’S OFTEN
DIFFICULT TO DECIDE WHETHER A TRAIT HAS BEEN FAVORED BY
INTERSEXUAL SELECTION OR INTRASEXUAL SELECTION. ​But,
TYPICALLY, intrasexual selection produces weapons (things used to
fight others over access to mates, e.g. antlers, teeth, huge bodies, gossip,
etc.) and intersexual selection produces adornments (things used to
ATTRACT mates, e.g. bright feathers, courtship dances, melodious calls,
mad guitar skills, etc.). Such adornments are expected to provide some
INFORMATION to mates, otherwise, there’s not much point in
preferring them​.
3. What exactly is kin selection (inclusive fitness)? Can you give an example?
Let’s first define plain old ​reproductive success​, or ​fitness​. It’s exactly what it sounds
like… an individual’s success at reproducing. As noted above, this may be measured in
terms of how many offspring an individual produces, but really that’s only part of the
equation. Imagine there are two birds of the same species: one lays 12 eggs and the other
lays 4 eggs. If we just count eggs, we will conclude that the one that laid 12 eggs had
greater reproductive success. But that bird has to divide her resources (time, food, heat,
etc.) among 12 eggs, while the other only has to divide her resources amongst four. We
may find that the one who laid 12 eggs exhausts herself and her resources and ends up
with only two living offspring, while the one with four eggs is able to take care of all of
them and they all survive. Then, it turns out, the one who laid four eggs actually had higher
reproductive success. BUT… it is still more complicated because it is possible, for example,
that the one who started out with 12, but ended up with two, ended up with the ​BEST​ two
(because she produced so many and they had such a variety of traits that those who
survived had better than ​average​ traits for the species). Perhaps her two living offspring
go on to mate earlier than any of the other bird’s offspring, and have, on average, one more
egg per year than the other bird’s offspring. In the end, the one who laid the 12 eggs, in
that case, ​would ​have the higher reproductive success because, ultimately, she left more
copies of her genes in subsequent generations than the other. ​*PHEW!*
Short answer: ​Reproductive success is measured by ​how many copies of an
individual’s genes​ are passed on to subsequent generations. (We will usually just talk
about numbers of offspring because it’s easier to conceptualize).
Inclusive fitness, or kin selection, refers to the fact that you are not the only vehicle
for ​your genes​ to get into future generations. You share an average of 50% of your genes
with any full siblings (with the same mom AND same dad) you have. If your sibling
reproduces, they will pass on 50% of ​their​ genes, which will, on average, include 25% of
your​ genes. Voila! You have some reproductive success without reproducing! This is
referred to as “inclusive fitness,” that is, fitness you gain from a relative reproducing. You
share, on average 25% of your genes of your genes with any half siblings (someone who
has EITHER the same mother OR the same father as you), and 12.5% of your genes with
any first cousins. If your half sibling reproduces, on average, he or she will transmit about
12.5% of ​your genes​ into the next generation; if your first cousin reproduces, about 6.25%
of ​your genes​ will get transmitted to the next generation. As you can see, the genetic
benefit you receive from a relative reproducing drops off pretty quickly… exponentially, in
fact. You get a ​much​ bigger fitness benefit if your full sibling successfully reproduces than
if your first cousin successfully reproduces. Hamilton’s Rule (Kin Selection) simply states
that an individual should be more willing to incur costs to help someone more closely
related to them, because they can provide inclusive fitness benefits. The equation rb>c
quantifies that insight. ​r​ is the coefficient of relatedness, that is, the percentage of genes
shared with that individual. The highest it can be, with the exception of identical twins, is
.5 (50%). ​b​ refers to the fitness benefit that the individual being helped would gain, and ​c​ is
the cost to the helper (all of these “amounts” are thought of in terms of units of
reproductive success, either gained or lost). So, let’s say an individual is starving. I could
help that individual, but it will come at a cost to me. To put arbitrary numbers there, let’s
say that by helping, I will provide 10 fitness units of benefit to the starving individual at a
cost of 3 fitness units to me. If this person is completely unrelated to me, Hamilton’s Rule
would be calculated as
0*10 ≯ 3
0 ≯3
If it was my first cousin, the equation would be:
.125*10 ≯3
1.25 ≯3
If it was my half sibling, the equation would be
.25*10 ≯3
2.5 ≯3
If it was my full sibling, the equation would be
.5*10>3
5>3
In this example, it would only be ​genetically​ beneficial to me to help the starving person if
he were my full sibling. For most animals, this is a pretty good predictor of whether or not
they might incur a cost to help someone (along with a few other contexts), that is, for the
contexts in which cooperation is likely to occur. Don’t worry, though, humans cooperate in
lots of ways that go far beyond kin selection! We’ll learn about those in the coming weeks.
4. Can you explain the difference between fixed reproductive strategies and
facultative reproductive strategies? Are facultative strategies under human control?
Fixed​ strategies simply mean that they don’t vary. The only real “fixed”
reproductive strategies that I can think of that humans have is the male orgasm… given the
appropriate physical stimulation, orgasm is fairly reliable (unless there is something not
working quite right). Many other species, however, have fixed reproductive strategies…
e.g. stereotyped mating behavior, positions, preferences. ​Facultative​ strategies are those
that are ​sensitive to the environment ​and can be ​adjusted accordingly.​ The environment can
be external (amount of food one has, or wealth, or territory, or social status, or competition,
etc. ) or internal (one’s own bodily resources, hormonal levels, reproductive status, etc.).
The variation in strategies (the adjustment) can be ​either​ conscious (and under our
control) or non-conscious (and not under our control). For example, one may make a
conscious decision to have sex (or ​NOT​ have sex) with someone who is not his/her spouse.
This may be related to a conscious evaluation of the potential costs and benefits of such an
action. On the other hand, one may experience an ​unconscious​ feeling of sexual desire for a
person other than his/her spouse. This may be provoked by subtle cues to one’s partner’s
qualities (genes, resources), or suggestion that the relationship is not stable, or cues to an
alternative partner’s qualities (genes, resources, etc.). Both of those examples would be
examples of facultative reproductive strategies: the desire is not within our control, but the
behavior is. Both are related to the environment.
5. Obligate parental investment and parental investment theory
Obligate parental investment​ refers to the ​bare minimum​ that an individual ​must
invest (is ​obligated ​to invest) in offspring in order to produce a viable offspring. This
investment can be time, energy, resources, calories, etc. In ​all​ sexually reproducing species
there is some difference between males and females in obligate parental investment. That
is because females, by definition, produce the larger gamete (which requires more
energetic resources), so ​even if neither parent invests anything after conception​ (such as
many types of fish), the female starts off with a higher obligate investment. For a female
fish to successfully produce a viable offspring, she must produce a large number of
energetically costly eggs; a male fish must only produce a large number of energetically
cheap sperm. Birds have a similar situation since their eggs also develop outside of the
mom’s body, but the differences is that bird eggs and hatchlings typically require a great
deal of care before they can fend for themselves. Mammals face a more dramatic difference
in obligate parental investment because of internal gestation and lactation. The ​bare
minimum​ a female mammal can invest is producing an egg, gestating the egg, giving birth,
and producing milk for the infant; the ​bare minimum​ a male mammal can invest is
producing sperm.
Trivers’ ​Parental Investment Theory​ states that this difference in obligate parental
investment can explain almost all sex differences in strategies and behaviors because the
costs​ and ​benefits​ of different behaviors depend upon one’s own [biological] obligation in
investing in offspring. [Please note here, that when I talk about “obligate investment” I am
referring to the biological necessity, not a moral, legal, or social obligation… that is another
topic.] The gist of parental investment theory is that the sex with higher obligate
investment – typically females – will be less sexually motivated and more choosy about sex
partners, while the sex with lower obligate investment – typically males – will be more
sexually motivated (for example being willing to take greater risks to obtain sexual
opportunities) and less choosy about their sex partners.
Why?
Well, the sex with greater investment is going to be in higher demand. They are the
limiting resource. A male mammal can reproduce as many times as the number of females
he can convince to have sex with him. A female mammal can only reproduce as often as her
species’ gestation and lactation times will allow (in humans that’s about once every 3 years
prior to modern interventions such as bottle-feeding, etc.). So males are going to compete
over access to females. Females need not take great risks to find a mate, because the mates
will come looking or them. (Notice that bars often have “Ladies’ Nite” events, but almost
never have “Dudes’ Nite” events. Why? If you bring the ladies in, men will show up, and
pay extra to do so. You can bring all the men in you want, and women won’t pay extra to be
there… they can find men anywhere they want.)
In terms of choosiness, males of most species need not be overly choosy in their
reproductive partner because they don’t need to invest much in the offspring. If the female
is not the very best quality, a male can seek another mating opportunity mere minutes later
and try again. A female mammal, on the other hand, is likely to be extremely choosy about
whom she mates with because she is going to bear the cost of that reproduction whether it
was a good decision or bad.
We can see these patterns clearly in human male and female behavior, BUT it is
naïve to think that in humans, obligate parental investment ends with the biological
production of the offspring. The greatest human adaptation is our capacity for culture
(essentially, learning from others) – it is this ability that has allowed the human species to
inhabit just about every ecological niche on the planet, to transform the environment to
benefit ourselves, and, yes, even to destroy the environment and drive other species to
early extinction. What it requires is a lengthy “learning period,” otherwise known as
childhood. This is a part of the necessary investment in human children in order to make
them reproductively competitive adults. Technically, this investment can be provided by
moms or dads or anyone else (hence the reason that many kids grow up just fine with just a
dad or just a mom), but it is likely that the best strategy for individuals of our species
across evolutionary time has been for men and women to invest in ​biparental care​, that is,
both parents investing in offspring. Because of this, human males have lots of adaptations
for investing in offspring… and women ​compete​ over highly-investing males. Thus, while
men are not a “limiting resource” in terms of their sperm, they often are a limiting resource
in terms of their time, resources, social networks, knowledge, caring qualities, etc.
The implications of Parental Investment Theory are almost endless (when followed
to their logical conclusions), but the center around these ideas:
● The less investing sex will be more sexually motivated, will take greater risks
and pay greater costs in order to obtain mating opportunities. The more
investing sex will be less willing to take risks or pay costs in order to obtain
mating opportunities (because they don’t need to), and will be generally less
motivated by obtaining sexual opportunities.
● The more investing sex will be more choosy about whom they have sex with,
will therefore often delay sex or require some form of investment prior to
having sex (for example, many insects require a “nuptial gift” of food before
consenting to sex). The less investing sex will be less choosy about whom
they have sex with, will prefer to have sex sooner, and will require little or no
incentive to have sex.
6. What exactly is reproductive variance?
Know your terms!!
Reproductive success​ = the number of viable offspring produced by an individual
Reproductive variance​ = the amount of ​variation in reproductive success
Typically, the less-investing sex has greater reproductive variance than the
more-investing sex. When an individual has less investment, he (or she) can increase his
(or her) reproductive success by trying to obtain more mating opportunities. Because
there is ​competition​ over mating opportunities, those individuals who are most competitive
or attractive will have much higher reproductive success than those individuals who are
least competitive or attractive. Think about this in real terms. If some guy on campus is
super sexy and all the girls want to date him, if he decides to date multiple women at once
that is going to leave fewer women for the rest of the guys. If another guy is particularly
unattractive, this may reduce his chances of getting any date to zero. In the ancestral past
(when there was no contraception and likely little religious or moral strictures regarding
sexual behavior), “dating” was likely a pretty good proxy for “reproduction.” So, the guy
who gets lots of dates can also ostensibly have lots of babies. The guy who gets no dates
has zero babies. The more-investing sex, however, does not experience so much variation
in reproductive success. Because each female has to invest heavily in each reproductive
event, that limits her ability to have really great reproductive success. Because males
compete for access to mates so that they can increase their own reproductive success, this
limits the number of females who have very low reproductive success (in non-human
animals or in the human ancestral past; no so much in our modern society in which many
women choose not to reproduce).
A brief example: Imagine a “village” of 5 men and 5 women. The chart shows how
many babies each man has had with each woman. Remember that this can include
polygyny, remarriage, or infidelity (cheating).
Woman 1
Woman 2
Woman 3
Woman 4
Woman 5
Totals
Man 1
2
3
2
4
0
11
Man 2
0
0
0
0
3
3
Man 3
0
0
0
0
0
0
Man 4
1
0
0
0
0
1
Man 5
0
0
0
0
0
0
Totals
3
3
2
4
3
Average number of offspring per woman = 3, ​range = 2-4
Average number of offspring per man = 3, ​range = 0-11
Men and women have the same average reproductive success (in this case,
simply measured as number of offspring), but the range of variation among men is
much greater than the range of variation among women. ​ When there is high variance
in outcomes (as among men in this example), individuals should invest a lot in competition.
If you can’t compete, you go home a loser. If you win the competition, you take the lion’s
share of the reproductive success. When there is low variance in outcomes (as among
women in this example), it is not worth it to invest much in competition. If you “win,” you
get 4; if you “lose,” you get 2. It’s not a big enough difference to take many risks over. This
has implications in male and female behavior that go far beyond simple reproduction.
(This is also a good example of the difference between mean, median, and mode.
Among women in this example, the mean number of children is 3, the median number of
children is 3, and the modal number of children is 3. Among men in this example, the mean
number of children is 3, but the median is 1, and mode is 0. Here’s a …