Update II: John Hawks leaves a comment.
Update: Kambiz has much more comment.
Were neandertal and modern human cranial differences produced by natural selection or genetic drift?:
... Here we use a variety of statistical tests founded on explicit predictions from quantitative- and population-genetic theory to show that genetic drift can explain cranial differences between Neandertals and modern humans. These tests are based on thirty-seven standard cranial measurements from a sample of 2524 modern humans from 30 populations and 20 Neandertal fossils. As a further test, we compare our results for modern human cranial measurements with those for a genetic dataset consisting of 377 microsatellites typed for a sample of 1056 modern humans from 52 populations. We conclude that rather than requiring special adaptive accounts, Neandertal and modern human crania may simply represent two outcomes from a vast space of random evolutionary possibilities.
I am generally skeptical of drift as a catchall explanation (it often serves as a deus ex machina, just as sexual selection has become), but from what little I have gleaned from paleoanthropology it seems that some workers contend that the morphological differences between Neandertals and "modern" humans are overemphasized. I'm thinking here body form, e.g., the short & stocky build typical of Arctic peoples. Obviously the consistent patterns of changes in size and proportion of large mammalian species (larger & stockier the further north) across many taxa point to common selective pressures due to environment; but what about cranium? I simply don't know. I do know that human bone structure and teeth have become less robust over the last 10,000 years, perhaps due to agriculture. This might simply be relaxing the selection for more robust physiques. I hope John takes a minute from his grant application and comments....
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Nicholas Wade emphasized the importance of drift in small isolated populations. Can we say that the neanderthal population at one point consisted of very small isolated groups ? If that's happened, then their features being caused by drift is a possibility.
On the other hand , our modern features are gracile, resulting from the "domestication" of human beings starting from 50k years ago . This was most probably selection in action , which was favouring more social, less agressive people. A side effect of rooting extreme aggressiveness out might have been rooting out robustness . This may have been one pathway.
SO our difference are probably a combination of both processes , always acting in concert and their strenghts varying .
Can we say that the neanderthal population at one point consisted of very small isolated groups ? If that's happened, then their features being caused by drift is a possibility.
the bottleneck might be correct. the main issue is that you need a lot of inter-demic barrier for gene flow not to equilibrate alleles which differ in frequency due to drift across two populations. in other words, if neandertal and proto-modern populations stayed very, very, separate then sure, it could just be drift. on the other hand, if some admixture occurred over time it could quickly swamp out of the differences. this is why islands often exhibit such effects from drift, there is very little intermarriage with people who don't live on the island (e.g., sardinia).
This was most probably selection in action , which was favouring more social, less agressive people.
think smarter, not harder ;-) i doubt people are "less aggressive," at least on average.
(Not for publication)
Deus (god) ex machina, not deux.
It all comes from the comparison of long-term rates (always slow) and short-term rates (often fast). So if you compare cranial differentiation in living people (high) to some ancient hominid, you will always estimate selection as being low or absent -- especially if you assume, as this study did, that present cranial differentiation among humans is neutral.
In other words, the conclusion is an artifact of the method.
When a study like this tests for stabilizing selection, then you can start to believe it. Other related work by Jim Cheverud is better, but still faces the rate problem and still has no account of stabilizing selection. Anyway, this is an active area of my research, so I probably won't write about it for a while.
--John