This is actually a problem, because of the way that evolution is supposed to operate. Beneficial mutations are supposed to spread through the population. If this happens, then all but the most fit allele will be eliminated. And according to the theory of evolution this must have happened quite often, because many genes are different between different species. The hemoglobin gene is often different, for example, implying that some mutation spread through an entire species and eliminated all other alleles. How then can we possibly have 2 alleles differing by 5 base pairs, both common in the population? It must be that mutations to one of the alleles spread through the populatinn but did not eliminate the other allele. And this had to happen at least 5 times for each such pair of alleles, a total of about 30,000 times in the course of human evolution! I think this stretches credulity, even if one accepts such mechanisms as balancing selection.
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