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When the previous generation of life scientists was coming up through the academy, there was a widespread assumption, not always articulated by professors, that human evolution had all but stopped. It had certainly shaped our prehuman ancestors — Australopithecus, Paranthropus, and the rest of the ape-men and man-apes in our bushy lineage — but once Homo sapiens developed agriculture and language, it was thought, we stopped changing. It was as though, having achieved its aim by the seventh day, evolution rested. “That was the stereotype that I learned,” says population geneticist and anthropologist Henry Harpending. “We showed up 45,000 years ago and haven’t changed since then.”
The idea makes a rough-and-ready kind of sense. Natural selection derives its power to transform from the survival of some and the demise of others, and from differential reproductive success. But we nurse our sick back to health, and mating is no longer a privilege that males beat each other senseless to secure. As a result, even the less fit get to pass on their genes. Promiscuity and sperm competition have given way to spiritual love; the fittest and the unfit are treated as equals, and equally flourish. With the advent of culture and our fine sensibilities, the assumption was, natural selection went by the board.
Moreover, evolution had never been observed in humans, except in a few odd cases, so the conclusion was drawn that it wasn’t happening. One can’t fault the logic. The most famous case of adaptive change in humans, that of sickle cell trait as an evolutionary response to malaria, seemed to prove the point that human evolution must be rare: Even in as dire and malaria-stricken an environment as West Africa, the only response evolution has been able to come up with is an imperfect defense that can cause serious health problems along with its solitary benefit. Selection pressures as strong as those brought about by endemic malaria are uncommon, and civilization was thought to wash out those less powerful.
But since the turn of the millennium, genomics has undergone a revolution. With the completion of such landmark studies as the Human Genome Project and the publication of HapMap, scientists finally have access to the particles of evolution. They can inspect vast stretches of DNA from people of all ethnicities, and the colossal amount of information suddenly available has spurred a revision of the old static picture that will render it unrecognizable. Harpending and a host of researchers have discovered in our DNA evidence that culture, far from halting evolution, appears to accelerate it.
John Hawks started out as a “fossil guy” studying under Milford Wolpoff, a paleoanthropologist who is the leading proponent of the faintly heretical multiregional theory of human evolution. Coming to genetics from such a background has perhaps given Hawks the stomach to wield unfashionable hypotheses. In December of last year, he, Harpending, and others published a paper whose central finding, that evolution in humans is observable and accelerating, would have been nonsensical to many geneticists 20 years ago. Up to 10 percent of the human genome appears to be evolving at the maximum rate, more quickly than ever before in human history.
“Seven percent is a minimum,” Hawks says. “It’s an amazing number,” and one that is difficult to square with the prevailing view of natural selection’s power. Because most mutations have a neutral effect on their carriers, making them neither fitter nor less fit, neither more fertile nor sterile, only slightly different, those changes are invisible to natural selection. They spread or don’t spread through a population by chance, in a process called genetic drift, which is often thought of as the agent of more change than natural selection. But the changes that Hawks detected, if he is correct, are too consistent from person to person, from nationality to nationality, to have been caused by genetic drift alone.
By looking at the data from HapMap, a massive survey of the genetic differences between selected populations from around the world, Hawks identified gene variants, or alleles, that were present in many people’s DNA, but not in everyone’s. These alleles seemed to be moving, over time, through populations in a way that matched mathematical predictions of what natural selection should look like on the genomic level. And though Hawks doesn’t know why possession of the new alleles should be advantageous, he doesn’t need to know. The signature that natural selection inscribes on the genome is legible even when the import of the message is unclear.
Fixation of the allele that allows European adults to digest lactose is nearly complete, after arising some 8,000 years ago and undergoing strong positive selection. Following the curve predicted by R.A. Fisher, the allele spread rapidly long after it first appeared. Data for the spread of different lactose-digestion alleles in Africa is incomplete, but what is available suggests the gene is far from fixed there.
HapMap can reveal where natural selection has occurred thanks to the tendency of DNA “neighborhoods” to be inherited in blocks that do not change much, if at all, from parent to offspring. When an organism reproduces, adjacent DNA sticks together and is passed on as a unit to the offspring. Such sections of linked DNA are called haplotypes; HapMap is a directory of them. A given haplotype is nearly identical among family members, but populations that have had recent contact with each other, such as the French and the Spanish, or the Cherokee and the Inuit, also tend to share it.
One of the characteristics of this linkage is that it is strong over short distances on a chromosome and weak over long distances. This is because mutations are rare but equally likely at every location, so they happen less often in a small region of DNA than in a large region. Over many generations, mutations nibble away at the edges of haplotypes and poke holes in their interiors, and the routine reshuffling of nucleotides, called recombination, can move linked sections of DNA far from one another, thereby breaking the linkage. Thus, the length of a haplotype roughly indicates its age, as does the amount of variation within it. Since mutations and recombination occur at a predictable rate, by comparing haplotypes from two populations, one can determine their degree of relatedness and thus estimate how long ago they diverged. So, for example, the San of southern Africa and the Han Chinese, would tend not to share haplotypes because their populations diverged long ago, and those they did share would be short or contain a great deal of variation.
Because haplotypes are similar from population to population, differences are easy to spot; a variation on a familiar haplotype background is like a smear of red paint on a white wall. If one looks at a haplotype in 100 individuals, and 90 of them are identical but 10 show the exact same variation, the odds are vanishingly small that random processes generated the same mutation 10 different times. Such a site is a candidate for one undergoing natural selection, because only a mutation that confers some kind of advantage will be propagated reliably through the population.
If the trait under selection produces a significant enough adaptive advantage, the allele responsible for the trait will rise in frequency so quickly that it will drag a long haplotype along with it before recombination and mutation can break it down to a short haplotype. So a rare allele on a long haplotype is an indication of strong and recent selection.
Hawks’s analysis of the HapMap data yielded many such candidate sites, but some of his colleagues were unimpressed. “They didn’t like the idea,” he says. An anonymous reviewer of his paper claimed not to think that natural selection could possibly be important in recent evolution, “so much so that they said positive selection happens rarely, if ever.”
An oft-cited example of evolution in historic times is the spread of the mutation that allows humans to digest milk in adulthood. It seems to have arisen around 8,000 years ago and has since spread to all parts of the world, though there are still plenty of us without it: One in 50 Swedes and nine out of 10 Asian Americans lack the mutation. The lactose intolerant are, at least in this respect, as the first Homo sapiens were.
“There are five versions of the lactase drinking gene, so five different populations have mutations that let them drink milk,” says Hawks. Because of this, many mutations conferring the same benefit are unlikely to have become common by genetic drift, but Hawks knows of practicing geneticists who find the idea that natural selection was the agent of their propagation to be preposterous. He’s incredulous at what he sees as such scientists’ fundamental misprision of the field’s core principle. “This is Darwin’s field,” he says. “Darwin talks about evolution, and Darwinism is about natural selection. But these people don’t believe in natural selection — except way back when, when chimps and humans were the same.”
By Hawks’s own description, his research “depends on a view of evolution that’s dominated by natural selection. When I look at the evolutionary processes leading to humans I’m thinking, what’s the adaptive change that’s happening? What are the constraints on our adaptation?” One of those, he says, “is demography.”
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