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After reading the article “Darwin Lives! Modern humans are still evolving” are you convinced that humans continuing to evolve?From what you have learned about adaptation, natural selection, and genetic drift, choose one side of this debate and discuss one clue that humans are either continuing to adapt or have ceased any evolutionary advancement.Remember, as explained in the lecture, a key to evolution is the passing on of genes. Evolution through natural selection does not work as “survival of the fittest” but rather, “survival of the genes from the most reproductively fit” (unfortunately, that doesn’t have the same ring to it!)
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Friday, Oct. 23, 2009
Darwin Lives! Modern Humans Are
Still Evolving
By Eben Harrell
Modern Homo sapiens is still evolving. Despite the long-held view that natural selection has ceased to affect
humans because almost everybody now lives long enough to have children, a new study of a contemporary
Massachusetts population offers evidence of evolution still in action.
A team of scientists led by Yale University evolutionary biologist Stephen Stearns suggests that if the
natural selection of fitter traits is no longer driven by survival, perhaps it owes to differences in women’s
fertility. “Variations in reproductive success still exist among humans, and therefore some traits related to
fertility continue to be shaped by natural selection,” Stearns says. That is, women who have more children
are more likely to pass on certain traits to their progeny. (See the top 10 scientific discoveries of 2008.)
Stearns’ team examined the vital statistics of 2,238 postmenopausal women participating in the
Framingham Heart Study, which has tracked the medical histories of some 14,000 residents of
Framingham, Mass., since 1948. Investigators searched for correlations between women’s physical
characteristics — including height, weight, blood pressure and cholesterol levels — and the number of
offspring they produced. According to their findings, it was stout, slightly plump (but not obese) women
who tended to have more children — “Women with very low body fat don’t ovulate,” Stearns explains — as
did women with lower blood pressure and cholesterol levels. Using a sophisticated statistical analysis that
controlled for any social or cultural factors that could impact childbearing, researchers determined that
these characteristics were passed on genetically from mothers to daughters and granddaughters.
If these trends were to continue with no cultural changes in the town for the next 10 generations, by 2409
the average Framingham woman would be 2 cm (0.8 in) shorter, 1 kg (2.2 lb.) heavier, have a healthier
heart, have her first child five months earlier and enter menopause 10 months later than a woman today,
the study found. “That rate of evolution is slow but pretty similar to what we see in other plants and
animals. Humans don’t seem to be any exception,” Stearns says. (See TIME’s photo-essay “Happy 200th
Darwin Day.”)
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Douglas Ewbank, a demographer at the University of Pennsylvania who undertook the statistical analysis
for the study, which was published Oct. 21 in the Proceedings of the National Academy of Sciences (PNAS),
says that because cultural factors tend to have a much more prominent impact than natural selection in the
shaping of future generations, people tend to write off the effect of evolution. “Those changes we predict for
2409 could be wiped out by something as simple as a new school-lunch program. But whatever happens, it’s
likely that in 2409, Framingham women will be 2 cm shorter and 1 kg heavier than they would have been
without natural selection. Evolution is a very slow process. We don’t see it if we look at our grandparents,
but it’s there.”
Other recent genetic research has backed up that notion. One study, published in PNAS in 2007 and led by
John Hawks, an anthropologist at the University of Wisconsin at Madison, found that some 1,800 human
gene variations had become widespread in recent generations because of their modern-day evolutionary
benefits. Among those genetic changes, discovered by examining more than 3 million DNA variants in 269
individuals: mutations that allow people to digest milk or resist malaria and others that govern brain
development. (Watch TIME’s video “Darwin and Lincoln: Birthdays and Evolution.”)
But not all evolutionary changes make inherent sense. Since the Industrial Revolution, modern humans
have grown taller and stronger, so it’s easy to assume that evolution is making humans fitter. But according
to anthropologist Peter McAllister, author of Manthropology: the Science of Inadequate Modern Man, the
contemporary male has evolved, at least physically, into “the sorriest cohort of masculine Homo sapiens to
ever walk the planet.” Thanks to genetic differences, an average Neanderthal woman, McAllister notes,
could have whupped Arnold Schwarzenegger at his muscular peak in an arm-wrestling match. And
prehistoric Australian Aborigines, who typically built up great strength in their joints and muscles through
childhood and adolescence, could have easily beat Usain Bolt in a 100-m dash.
Steve Jones, an evolutionary biologist at University College London who has previously held that human
evolution was nearing its end, says the Framingham study is indeed an important example of how natural
selection still operates through inherited differences in reproductive ability. But Jones argues that variation
in female fertility — as measured in the Framingham study — is a much less important factor in human
evolution than differences in male fertility. Sperm hold a much higher chance of carrying an error or
mutation than an egg, especially among older men. “While it used to be that men had many children in
older age to many different women, now men tend to have only a few children at a younger age with one
wife. The drop in the number of older fathers has had a major effect on the rate of mutation and has at least
reduced the amount of new diversity — the raw material of evolution. Darwin’s machine has not stopped,
but it surely has slowed greatly,” Jones says. (See TIME’s special report on the environment.)
Despite evidence that human evolution still functions, biologists concede that it’s anyone’s guess where it
will take us from here. Artificial selection in the form of genetic medicine could push natural selection into
obsolescence, but a lethal pandemic or other cataclysm could suddenly make natural selection central to the
future of the species. Whatever happens, Jones says, it is worth remembering that Darwin’s beautiful theory
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has suffered a long history of abuse. The bastard science of eugenics, he says, will haunt humanity as long as
people are tempted to confuse evolution with improvement. “Uniquely in the living world, what makes
humans what we are is in our minds, in our society, and not in our evolution,” he says.
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