Geneticists have traditionally traced the flow of genes in order to study the movements and origins of peoples. They've studied particular variants of the Y chromosome in an attempt to determine which ethnic group in Asia is most closely related to the Amerindians.1 They've tried to determine the extent to which modern Europeans are descended from ancient Europeans of the Upper Paleolithic period who adopted farming, or from Neolithic immigrants from the Middle East.2 Researchers have used mitochondrial DNA (mtDNA) and Y-chromosome data to determine which groups contributed maternal and paternal ancestry to a mixed population—they have determined, for example, that most Mexican Y chromosomes are of Spanish origin, whereas most Mexican mtDNA is Amerindian.3 They have tried to analyze other ancient population movements in this way as well, most notably the original human expansion out of Africa.
There are two ways of looking at these types of informative gene variants. In the kind of analysis conducted to determine paternal and maternal ancestral lines, on the one hand, researchers are only interested in these gene variants as markers of past population movement and admixture rather than in the functions of the variants themselves. The assumption is that one Y chromosome functions just like any other and that all mtDNA variants have the same properties: That is to say, they're neutral. But if their properties varied—if the bearers of some Y-chromosome variants had noticeably higher fitness than those bearing other variants—this whole brand of analysis would be thrown into question, particularly when used to look far back into prehistory.
We, on the other hand, are interested in alleles because of their effects, precisely because they do make a difference. Generally, we're interested in how population movements and admixture have helped to spread new adaptive variants rather than in how using the variants can help us to track the movements.
Every new mutation, including any rare but important beneficial mutation, starts out as a single copy in one individual. It's local. If it's going to be important, if it's ever going to influence a significant fraction of the human species, it must first spread. Looking at the bigger picture, we can see that the flood of favorable mutations involved in the recent acceleration of human evolution will have major impacts only if they spread widely. Presumably, if our theories are correct, many of them are still in the midst of sweeping through large segments of our world population. This means that the average person today bears many of these new favorable mutations. You are, most likely, significantly different genetically from your ancestors of a few thousand years ago. And although natural selection hardly operates in a way guaranteed to maximize our convenience, it is still the case that many adaptive changes have welcome results. It's hard to argue against something that keeps you alive.
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