December 13, 2007

I, Mutant: Adaptation Goes On

Blogging on Peer-Reviewed ResearchAs we all know, natural selection is a driving force of evolution. Modern humans, popular conceptions often hold, have surrounded themselves with technology that allows them to effectively step out of the natural world into an environment largely of their own design. Therefore, we don't expect to see evolution in humans resulting in changes on a par with organisms that have no such control over their environment. When we look at our own relatives of a generation or three preceding us, we don't see much difference. Humans just don't appear to be evolving anymore.

That's not the case, though. It's just a matter of how closely we care to look. If we zoom in to the genetic level, there's a lot of change occurring. Some of these changes aren't visible on the phenotypic level under the circumstances that we face in our day-to-day lives; disease-resistance only becomes obvious when we're exposed to a particular disease, and if we have the genes we're not likely to even know we've been exposed. Others are so commonplace that we don't look for them at all. Here in America, for instance, only a few of us who don't carry the mutation ourselves ever consider how lucky the rest of you are to have inherited a gene to make lactase. Without that mutation, we unfortunates need to pop a pill before we can think about digesting ice cream. In short, its very easy to take it for granted that we are what we are and aren't evolving anymore.

By analyzing a recently-available dataset of 3.9 million human genotypes, the authors of Recent acceleration of human adaptive evolution demonstrate that the popular notion simply doesn't hold up to scrutiny. In fact, advances in technology that have led to increasing population size over the past millennia have sped up human evolution significantly. While some of these changes are readily apparent when looking back over a mere 10,000 years of history, it's now possible to have insight into how much we've really evolved over 50,000 years in ways that we can't see by simply examining ourselves morphologically. When all is said and done, examination of a number of human populations reveals thousands of selection events — instances where a mutation arose and was positively selected, increasing in prevalence in humanity as a whole, beginning some 80,000 years ago and spreading over the globe as those populations diverged. It appears from the study that allele diversification reached a peak sometime between 8,000 and 5,250 years ago, but calming down doesn't mean it's stopped. We're still evolving, and that's good news.

We're all mutants, after all. You're not as much like grandpa as you might have thought.

The paper will eventually be published in the Proceedings of the National Academy of Sciences, but for now preprint copies are available here. If you prefer to wait for PNAS, on the other hand, check the following citation/link every so often:

Hawks J, Wang E, Cochran G, Harpending H, Moyzis R (2007) Recent acceleration of human adaptive evolution. PNAS published whenever they get around to it. 10.1073/pnas.0707650104
Thanks to Dr. John Hawks for sending a preprint. His blog is an interesting read as well, especially if you've looked this paper over and are now scratching your head. A particularly useful FAQ (properly a RAQ?) is available there, too.

While we're on the subject, here are excerpts from some commentary on this study from two of the investigators that help point out its importance.
The vastly over-represented set of neuronal related genes under accelerated selection suggests the complex culture we have created is exerting selective pressure back onto ourselves. It is in contradiction to the commonly held belief that somehow humans stopped evolving due to a variety of technological and medical advances. Our findings suggest humans today are indeed different than humans 40,000 years ago and are still evolving.

Eric Wang

...Many assume that evolution somehow makes us better at doing what we do, that is that evolution is good for us. But evolution favors genes that out-reproduce other genes, while as humans we have completely different purposes and preferences.

Consider the textbook favorite sickle cell anemia. Carriers, with one copy of the mutant gene, have a slight advantage in environments with a lot of Falciparum malaria. Individuals with two copies of the mutant either die in early childhood else suffer impaired and uncomfortable lives. Today in northern countries there is no Falciparum malaria yet there are tens of thousands of sufferers of sickle cell anemia and tens of thousand of families who have lost precious offspring to it. It is an ugly scar of a past environment. Many of us think that there are many such systems in our populations like cystic fibrosis, Tay-Sachs disease, favism, etc. These are genomic garbage today responsible for much misery, and the first ask of genetic manipulation in humans ought to be to patch these. While many aspects of human genetic manipulation are scary, simply fixing well understood broken genes poses no apparent risk to anyone.

Henry Harpending

Both quotes taken from commentary on Google News.

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