What is the Role of Drift in Evolution? I. Morphological Characters

Abstract

Our discussion so far has been non-controversial, but now, in attempting to assess the role in evolution of random genetic drift, we enter an area of the most profound disagreement. In the view of some population geneticists, drift is of quite minor importance save in special circumstances (for example, when an allele frequency is close to zero). In the view of others, the genetic composition of natural populations is substantially affected by drift, whatever the frequencies of the alleles involved. Now it will be clear from earlier chapters that one’s opinion on the possible importance of drift will be intimately bound up with one’s opinion on the possible existence of neutral alleles. Suppose it is commonly the case that many alleles at a locus are neutral inter se, that is, they have equal effects on fitness. The relative frequencies of such alleles will be purely a matter of chance. Different species would often differ in their allelic composition at a homologous locus, but in many cases this difference would arise through the chance fixation of one allele in one species and a different allele in the other. In a similar manner, completely isolated populations of the same species would gradually diverge genetically. Different sub-populations of the same population might or might not diverge, the degree of divergence being governed, in such cases of neutral alleles, by the amount of migration between sub-populations. Polymorphism, in these cases, arises through the chance spread of alleles that happen to have escaped chance extinction and yet have not reached fixation (Kimura and Ohta 1971).

“ Oh, quite enough to get, sir, as the soldier said ven they ordered him three hundred and fifty lashes ”, replied Sam.

“ You must not tell us what the soldier, or any other man, said, sir ”, interposed the judge; “ It ’ s not evidence”. Charles Dickens, Pickwick Papers