If you want a succinct description of how scientists are finding human genes that have experienced natural selection over the past few thousand years, you could do worse than read Ann Gibbons’s three-page “news focus” in this week’s Science, “Tracing evolution’s recent fingerprints” (a bit of a mixed metaphor, that). The article is a useful summary for both biologists and laypeople, especially if you’re curious about whether we’re still evolving. (The answer, of course, is “yes.”)
Starting with recent suggestions that the EPAS1 gene could have been selected for oxygen transport in high-altitude populations, Gibbons sumarizes new statistical approaches (many of them taken here at the U of C) to detecting selection in H. sapiens. As you may know, these analysis have suggested that a surprisingly large fraction of our genome has been under fairly recent selection, with that selection based on adaptation to things like oxygen, diet, disease, and the like.
One of the problems of these studies, as Gibbons notes, is that statistics is not sufficient to show selection: “Finally, few teams have been able to prove that a particular allele actually affects the function of a trait under selection.” I think it’s unwise to say that your case for selection is conclusive without showing that the genetic variants you’re studying make a physiological difference to their carriers. And, of course, the ultimate “proof” of selection is to connect those physiological differences to reproductive output: i.e., that there really was selection.
Gibbons talks a bit about physiological studies (there aren’t many of these), but showing that genetic variants really do affect reproductive fitness is even harder. For one thing, that selection might have occurred in our ancestors, and not be going on so much today. Or, the selection could be very weak, and, though sufficient to cause significant evolution over centuries, might be undetectable in just one or two generations of an experiment. Long-term cohort studies, like those of Steve Stearns and his colleagues, might be useful here, but are still limited if selection is weak. I worry whether the new era of bioinformatics will gull us into accepting conclusions that are based solely on statical analysis of gene-frequency data.
also a recent piece in the NY times on that issue:
http://www.nytimes.com/2010/07/20/science/20adapt.html
…statistics is not sufficient to show selection.
The proper approach is that statistics may be sufficient to disprove genetic drift. That by itself substantiates selection but does not identify its cause.
The new era of bioinformatics has shown that selection was widespread in humans in the recent past, but we don’t know yet what much of that selection was *for*.
Thank you, it is pay-walled but I will try to get to it.
Meanwhile, as laypeople, I’m confused.
I was under the, probably mistaken, impression that sweeps as such _are_ caused by selection albeit: 1) it works on the genome, not the “phenome” 2) many (most?) alleles are free riders 3) they may not go to fixation.
Isn’t it a conflation between observing the selection process at work on genes to demand “there really was selection” with 1) a gene set-physiological set certified mapping 2) enough tending to fixation to show up as observable differences in reproductive output?
In the last case I’m thinking of how a large population AFAIU may more easily undergo the “weak” selection described in the article. [Which btw comes across as a funny term for me, since it would fish up smaller fitness differences, and so as a mechanism act stronger and broader in those circumstances!]
Or in other words, shouldn’t both types of investigations stand by themselves as conclusive, but with different scope?
Next to last line – statical>statistical?
I was scrolling from the bottom up and got a bit of a surprise. I know lesser apes are smart but I didn’t think they were that smart.