The Powers of Natural Selection

A very important and common feature of animal populations is polymorphism. This has been defined by Ford (1975) as follows. ‘Genetic polymorphism is the occurrence together in the same locality of two or more discontinuous forms of a species in such proportions that the rarest of them cannot be maintained merely by recurrent mutation’. Since the forms are discontinuous they must be controlled by major genes (or sometimes supergenes, that is, as mentioned in my first section, groups of very strongly linked major genes). If one allele is spreading in a population at the expense of another (or others), we speak of transient polymorphism, since the process can end in the total elimination of the less advantageous allele (or alleles), leaving only one type (Sheppard, 1975). However it often happens that more than one type persists in populations for very long periods: this is called balanced polymorphism.

One way this can occur is when there is a balance of advantage and disadvantage between the two or more types. An excellent example of this is the case of the moth Cleora repandata, studied by Kettlewell in the Black Wood of Rannoch, ‘one of the few surviving indigenous pine-forests in this country’ (Ford, 1975). Though this wood is ‘remote from pollution’, 10% of the moths were found to be melanics. The pale forms are well camouflaged on the lichen-covered pine-tree trunks, where the melanics are conspicuous. But the pale forms are much more conspicuous when flying through the dark pinewood. ‘Thus the pale and dark phases each have advantages and disadvantages’ (Ford, 1975). Hence both types are selected – rather more of the pale type, since the balance of advantage is in their favour.

There were thus two optimum phenotypes, but this was not disruptive selection, and the population was not split in two, because both optima were in one ecological niche: the same individuals rested on tree-trunks and flew through the wood. However, there was a problem: there were two optima and three genotypes. Moths intermediate between pale and black would be conspicuous in both situations. Natural selection solved the problem by making blackness dominant and paleness recessive – that is, by making the heterozygote black.

When a new mutant appears, its heterozygote is normally intermediate between the two homozygotes, so how is dominance achieved? Strictly speaking, dominance is a property of phenotypic characters, for when an allele controls several characters it may be dominant for some and recessive for others. However it is convenient to speak of dominant and recessive alleles. The way dominance is selected was suggested by Fisher, and proved by observation and experiment by Fisher himself, Ford, Kettlewell and Sheppard (Ford, 1973, 1975, Sheppard, 1975). Out of the sub-population of heterozygotes, selection favours individuals in which modifier polygenes have modified the action of the two alleles, enhancing the effect of one and suppressing the effect of the other. This selection process continues until the heterozygote is phenotypically exactly like one of the homozygotes. Because the rarer allele (in this case the allele for blackness) is present in considerable numbers in heterozygote, and very rare in homozygote form, as can be shown from the Hardy-Weinberg equation, the rarer allele will be selected to be dominant – here the black one.

Kettlewell showed that this explains the dominance of the industrial melanics. Between about 8000 and 5000 BC, Britain was covered with pine-woods, and clearly many moth species would have evolved dominant melanics. After 5000, owing to climate change, deciduous forests replaced the pine-woods except in a few places like Rannoch. Since then, every time melanics appeared by recurrent mutation, they were promptly eliminated – until the industrial revolution, when they became more advantageous than the pale forms in polluted districts. The modifiers selected millennia ago were still present, so the melanics were dominant (Ford, 1975).