From Ben Haller
Your article on early Europeans discusses a mutation that leads to increased fertility and asks “why doesn’t everybody have this?” (22 January, p 9). You conclude that the mutation might have been re-introduced into the modern human population only 50,000 years ago through “cross-breeding with other species”.
Increased fertility and having more offspring can lower the actual survival rate of your genes. The offspring may all die due to overstretched resources, or may be stunted in such a way as to make them undesirable mates in the next generation. Humans have evolved, like all organisms, to walk a line between maximising the number of offspring and maximising the fitness of those offspring. To find that a gene that disturbs that delicate equilibrium has not been selected for is completely unsurprising, just as it is unsurprising that we do not generally bear litters of babies, having evolved instead to (usually) maximise the fitness of one infant at a time.
So in all likelihood, the mutation discussed is usually slightly harmful and only occasionally beneficial, and thus has been preserved at a low rate in the population. Perhaps for some reason it is more often beneficial in the European climate, and so occurs more often there. This is no more mysterious than Europeans having lighter skin than Africans, and certainly I see no justification for theories about cross-species breeding.
Menlo Park, California, US
