Acceleration's discontents01 Jun 2008
The June Scientific American (no link available) has an article on page 32 about the “therapeutic value of blogging.” That’s some relief, after the stories a couple of months ago about blogging being potentially deadly.
And it’s no small irony, considering that the article I found on the previous two pages had great potential to give me therapeutic opportunities here.
In the article, titled, “Need for speed?” David Biello wrote up some of the human genetics results of the past 6 months, placing them as a point-counterpoint presentation of our acceleration result.
First, he cites Gregory Cochran, who does as good a job explaining our result in one sentence as I’ve seen:
"We found very many human genes undergoing selection" ... "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."
In that form, it is hard to see how anyone could disagree. Clearly, agriculture was a major ecological shift for humans, and it imposed new selection pressures associated with diet, disease, social organization and other ecological factors. At the same time, the population grew and more people meant more mutations. That’s the story; the rest is detail filled in by anthropology, genomics, and math.
Biello then cites another recent study that partially confirms our results. That study, by Lluis Quintana-Murci and colleagues, found a much smaller number of selected genes (55), but what is important is that every one of these genes has an FST greater than 0.65. In other words, in every one of these cases, an allele that is vanishingly rare in most of the world has reached a frequency over 80 percent in one population. As allele frequencies go, these are extreme differences – much, much larger than the average genetic difference between populations, characterized by an FST around 0.1. We also found a few such alleles in our survey of selected genes, but the vast majority of genes have not generated such extreme differences in frequency – mainly because they haven’t been around long enough. In other words, the Quintana-Murci study confirms the distribution of positively selected alleles, across the range where it overlaps with other studies, including ours.
Then Biello turns to the doubters. Noah Rosenberg coauthored a study earlier this year that reported polymorphism data from a sample of populations around the world.
"We are a young species," remarks geneticist Noah Rosenberg of the University of Michigan at Ann Arbor, who participated in a comprehensive study of genetic variation that appeared in Nature in February. "Different human populations have not been separated for long enough periods of time to develop their own new alleles."
Now, I never hold quotes in the press against people, because they represent a very small portion of what they may have said to a writer, and there are many opportunities for miscommunication. Still, I have to write about this, because it’s about my work! So I’ll try to describe the misconceptions illustrated by the article.
I am pretty sure that Rosenberg must know that his statement in the article is false. For one thing, “developing” a new allele is simply mutation, and mutation occurs continuously. All human populations have rare alleles that have originated recently and remain distributed only across small areas. Rosenberg’s surveys of gene variation have identified many such alleles.
But more important to the current question, positive selection carries an allele to high frequency very rapidly – much more quickly than the 50,000-year or longer span of time we are talking about. An allele with a five percent fitness edge can go from zero to fixation in several hundred generations – in humans, they can make very large frequency changes in a thousand years.
If we took the quote at face value, Rosenberg would be saying that human evolution is impossible – and that new selected alleles like lactase persistence and sickle cell simply cannot exist. We may be a young species (although I would argue the point), but that doesn’t mean that we have stopped evolving!
Two prominent geneticists quoted in the article suggest that a bottleneck may explain the pattern of human genetic variation. Here also, I have to be cautious interpreting their quotes – because even though they may seem relevant, they are referring to their own research papers, which don’t actually address the question of linkage disequilibrium and positive selection.
Marcus Feldman suggests that a series of bottlenecks are a likely explanation for the pattern of human genetic variation, in particular, the decreasing gradient of genetic diversity with increasing distance from Africa. This is the “serial founder effect” scenario that I have written about before. I criticized Feldman’s and other papers on this subject this spring, referring to “the Stanford school of genetic orthodoxy.” My basic point is that all of the results are assumed to support the idea of bottlenecks: no one has yet tested the hypothesis. Even simulations that show the credibility of the concept do not test the hypothesis, because they do not examine credible alternatives, either demographic or selective.
More important, bottlenecks during the dispersal from Africa 50,000 years ago cannot possibly explain linkage blocks concentrated in coding genes with a mean age of 5500 years!
Why is there such difficulty understanding natural selection? I find it quite incredible that many of the scientists who would rail against ignoring Darwin in public schools at the same time actively root out Darwin’s theory from their graduate students. Still, there it is. One prominent geneticist (I won’t give the name) recently asked me, “You don’t really think that lactase was selected, do you?” Many really believe that natural selection has stopped and that recent human evolution reflects nothing more than the cumulative effects of bottlenecks.
What is amazing to me is that these same geneticists embrace hypotheses of population history that cannot possibly have happened. The other geneticists quoted in the article, Carlos Bustamante and his graduate student Kirk Lohmueller, wrote a paper earlier this spring arguing that deleterious mutations have reached high frequency in Europeans (moreso than Africans) because of a bottleneck during European history. The press reported this work as “Whites genetically weaker than blacks, study finds.” The hypothesis in the paper is that protein-coding sites otherwise conserved in most mammals may differ among humans because of relaxed selection in a bottleneck.
Here’s why they’re wrong: their bottleneck is impossible. They propose that the European population was a small, isolated population of 5,700 effective individuals from 214,000 years ago up to the Last Glacial Maximum. I suppose I should take some encouragement that they believe Neandertals were European ancestors (because otherwise, where exactly would this small, isolated population of Europeans have lived). But it’s still quite impossible – it implies no gene flow between Africans and Europeans across that entire span. You see, that is the only way that genetic drift can lead to this kind of result – large differences in frequencies between continents for hundreds of deleterious alleles. It takes a bottleneck of exceptional length, along with complete isolation.
In what has become a troubling trend, these details were hidden away in the online supplementary information of the paper. It is no surprise that most people read only the paper’s conclusions, without critically evaluating the methods. But when the assumptions are hidden so that it takes an effort to look at them, you can understand that the paper does not receive the kind of scrutiny that it deserves. These are not obscure laboratory techniques; they are the basic evidence on which the conclusions were based.
Now, Bustamante knows that positive selection has been very important in recent human evolution, because he wrote an important paper on the subject in 2005. I wrote about the paper at the time – it was one of the works that really got us thinking about acceleration in the first place. So why in the world did their more recent paper adopt such a ridiculous model of population history?
In any event, I don’t think that either of these studies from earlier this year are relevant to our acceleration results. They address different aspects of genetic variation. However, acceleration may help to explain the high frequencies of some gene variants conserved in other mammals – the results explained by Lohmueller and colleagues as relaxed selection under a bottleneck.
The acceleration of recent positive selection would predict that many otherwise conserved gene variants may be segregating in humans, because they are the targets of positive selection. These conserved sites are among those most likely to show a strong sign of recent selection, because adaptive changes on them are necessarily rare (we know they’re rare, because they haven’t happened very often among other species). Most such sites are still conserved in humans – it’s just not possible to change their function in adaptive ways. But the massive ecological changes of recent human history have created the opportunity for adaptive responses that are not present in other mammalian lineages. We shouldn’t be surprised to see that some such changes are currently underway.
Now, that’s a different interpretation of the same data, and it’s a testable hypothesis. Are these conserved sites in regions that show other signs of positive selection? If they are, then acceleration explains the data. I’m looking into it now.