Tuesday, December 11, 2007

Accelerated Human Evolution

 
We frequently hear claims that humans have stopped evolving. Most of these claims have to do with medical advances that are now allowing people to survive who might have died in earlier times. The idea is that natural selection is no longer working so we have stopped evolving.

This is a flawed argument [Have Humans Stopped Evolving?]. Assuming that medical advances are having a significant impact on the world's population, it follows that the impact is to speed up evolution and not slow it down!

To understand this, you have to keep in mind that evolution is defined as ... [What Is Evolution?].
Evolution is a process that results in heritable changes in a population spread over many generations.
In the past, mutations that were detrimental were rapidly purged from the population (or kept at a very low level) because humans could not save those with genetic diseases such as diabetes. Today, those alleles are not being removed from the population by negative selection.

Thus, the frequency of alleles that used to be detrimental is increasing at a faster rate than in the past. Modern humans are evolving faster than before the advent of medical advances.

In addition to the major flaw in logic, there are many other things wrong with the claim that modern humans have stopped evolving. The claim carries with it a very loaded assumption that is never explicitly stated. The assumption is that humans have pretty much reached their optimal level of fitness for all other characteristics. For example, we are no longer selecting for higher intelligence, or a better immune system, or more efficient energy production, or stronger muscles, or any of a host of other things that might make us better adapted to all environments.

Why is this assumption necessary? Because nobody could possibly suggest that we have stopped evolving without assuming that we have reached optimal fitness for all those things in our present environment.

There's another problem with the claim. It assumes that adaptation is the only form of evolution. Nobody who understand random genetic drift would ever say that humans have stopped evolving because you can't stop drift.

Today I learned of another variation on this claim. Apparently there are people out there who say that biological evolution of humans has slowed (stopped?) because it has been supplanted by cultural evolution. I imagine that this is similar to the argument about medical advances. Presumably humans aren't adapting to different climates, for example, because we can make clothes and air-conditioned houses. Presumably we aren't dying of food shortages because, with the coming of agriculture, everyone has enough food.

The flaws in those arguments are the same; (a) there's more to evolution than natural selection and (b) lots of ongoing adaptations do not depend directly on the physical environment and cannot be replaced by culture.

A new paper about to be published in PNAS claims that human evolution has accelerated, not slowed, in the past 10,000 years (Hawks, et al. 2007). The first author is John Hawks of john hawks weblog. One of the press releases is from Scientific American [Culture Speeds Up Human Evolution]. The authors of the PNAS paper looked at variations within the human population and asked whether any of it showed evidence of selective sweeps. This would indicate that the alleles had moved rapidly toward fixation by natural selection. The results were surprising ...
We found very many human genes undergoing selection," says anthropologist Gregory Cochran of the University of Utah, a member of the team that analyzed the 3.9 million genes showing the most variation. "Most are very recent, so much so that the rate of human evolution over the past few thousand years is far greater than it has been over the past few million years."

"We believe that this can be explained by an increase in the strength of selection as people became agriculturalists—a major ecological change—and a vast increase in the number of favorable mutations as agriculture led to increased population size," he adds.
What they mean, of course, is that they have detected more presumed examples of natural selection in recent times than they deduce must have happened in the past. Whether this is an increase in the overall rate of evolution, as opposed to positive selection, is another thing entirely. I'll have to read the paper to see if they establish a baseline rate that includes all mechanisms of evolution.

The idea here is that with the switch from hunter-gatherer to farmer, humans created a new environment and this stimulated rapid natural selection in order to adapt to this new environement.
Roughly 10,000 years ago, humanity made the transition from living off the land to actively raising crops and domesticated animals. Because this concentrated populations, diseases such as malaria, smallpox and tuberculosis, among others, became more virulent. At the same time, the new agriculturally based diet offered its own challenges—including iron deficiency from lack of meat, cavities and, ultimately, shorter stature due to poor nutrition, says anthropologist John Hawks of the University of Wisconsin–Madison, another team member.

"Their bodies and teeth shrank. Their brains shrank, too," he adds. "But they started to get new alleles [alternative gene forms] that helped them digest the food more efficiently. New protective alleles allowed a fraction of people to survive the dread illnesses better."
The assumption here is that evolution among the hunter-gatherers had slowed down because they had become very well adapted to that environment. That's a big assumption but if that's what their data shows then I guess we'll just have to accept the fact that our ancestors were very fit.

I wonder how evolution proceeded from the time of Homo erectus to all of the groups that must have been present 10,000 years ago (i.e., Asians, native Americans, Africans, Europeans)? I suppose evolution was quite rapid until about 50,000 years ago then it slowed down as all the various groups became well-adapted to a hunter-gatherer form of existence?

Then evolution positive natural selection took off as all these groups switched to farming, which caused a change in the environment so that people were no longer well-adapted to the point where selection had slowed down? It's an easy prediction to test. All the new alleles should be present at high frequency in European, Asian and Middle Eastern groups but not in the existing hunter-gatherer groups who haven't been living in large agricultural communities.

It will be interesting to follow the discussion on John Hawks blog [Human Evolution Has Accelerated]. He promises to explain the work in considerable detail. One of the statements he makes today is quite interesting, he says,
It is a powerful paper because it shows why a rapid acceleration of our evolution is expected in theory, and it matches those expectations to real empirical data. It shows the absolute impossibility of a constant rate of selective change in humans, and that gives reality to our estimate of the amount of acceleration.
I'm anxious to find out why "theory" predicts a rapid acceleration of evolution natural selection. I'm also anxious to find out why it's impossible for there to be a relatively constant rate of adaptive change in the human lineage.

John includes the last paragraph of his paper on today's blog article. Here it is ...
It is sometimes claimed that the pace of human evolution should have slowed as cultural adaptation supplanted genetic adaptation. The high empirical number of recent adaptive variants would seem sufficient to refute this claim. It is important to note that the peak ages of new selected variants in our data do not reflect the highest intensity of selection, but merely our ability to detect selection. Due to the recent acceleration, many more new adaptive mutations should exist than have yet been ascertained, occurring at a faster and faster rate during historic times. Adaptive alleles with frequencies under 22% should then greatly outnumber those at higher frequencies. To the extent that new adaptive alleles continued to reflect demographic growth, the Neolithic and later periods would have experienced a rate of adaptive evolution more than 100 times higher than characterized most of human evolution. Cultural changes have reduced mortality rates, but variance in reproduction has continued to fuel genetic change. In our view, the rapid cultural evolution during the Late Pleistocene created vastly more opportunities for further genetic change, not fewer, as new avenues emerged for communication, social interactions, and creativity.
Call me skeptical ...


[Photo Credit: The photograph of the San tribesman, Klaas Kruiper, with his 4-year-old son is from (Save the San]

Hawks, J., Wang, J.T., Cochran, G., Harpending, H.C. and Moyzis, R.K. (2007) Recent acceleration of human adaptive evolution. PNAS in press

8 comments :

  1. I would think that humans will always be evolving, if only because bacteria and viruses will always be evolving...

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  2. Interestingly Greg Cochran was the guy who has suggested that all our chronic diseases are ultimately infectious. This fascinating theory is documented in Paul Ewald's "Plague Time"

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  3. We found very many human genes undergoing selection," says anthropologist Gregory Cochran of the University of Utah, a member of the team that analyzed the 3.9 million genes showing the most variation. "Most are very recent, so much so that the rate of human evolution over the past few thousand years is far greater than it has been over the past few million years.">

    How does he reconcile dramatically increased selection with a dramatically increased population size over the same period?

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  4. We found very many human genes undergoing selection," says anthropologist Gregory Cochran of the University of Utah, a member of the team that analyzed the 3.9 million genes showing the most variation.

    I would really like to know who wrote that,

    3.9 million genes... lol

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  5. I'm not a population geneticist by any means, but I think a larger effective population size means even slightly adaptive traits have a higher probability of being fixed. I like Hawk's blog, and I find the premise interesting and plausible, so I won't say I'm skeptical. Maybe we should all wait and see?

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  6. Dunbar says,

    I'm not a population geneticist by any means, but I think a larger effective population size means even slightly adaptive traits have a higher probability of being fixed.

    The probability of being fixed depends on the fixation coefficient (s). It's approximately 2s.

    If a beneficial allele has a fixation coefficient of 0.03—as in many of the examples in the paper—then it has a 6% chance of being fixed in the population.

    The rate of fixation depends on the size of the population. The larger the population the longer it takes.

    The biggest effect of population size is the number of mutations. With large populations it becomes increasingly likely that a particular beneficial mutations will occur in a finite amount of time.

    Hawks et al. say that the conversion to an agricultural society made the existing population less fit so it had to evolve by acquiring more adaptations. Fortunately for us, the population of our ancestors expanded enormously at just the right time.

    It's not clear to me why there would be such an expansion if people were less fit.

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  7. It's not clear to me why there would be such an expansion if people were less fit.

    It's known that average stature, life expectancy, and indications of morbidity visible in human remains, all deteriorated sharply after the agricultural revolution (that's what prompted Jared Diamond to quip that it was "the worst mistake in the history of the human race"). Yet, the availability of enough food to feed far more mouths enabled this less-fit population to expand rapidly. So, while remaining agnostic on the conclusions of that paper, I would have to say that the idea is by no means absurd and self-contradictory on its face.

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  8. That is only possible if you have a custom-made definition of fitness. By other definitions, if more descendants are made possible, fitness has increased, no matter how "crappy" the lifestyle may seem.

    Truth is, in evolution a shift in lifestyle hardly ever can be confidently described as an increase of fitness or "improvement" of adaptation; but simply a shift into a different mode of existence, with other balances and trade-offs (despite popular notions of progressism).

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