Abstract
In the short term selection is by far the most important systematic force changing gene frequencies in populations. In this chapter we look at some of the ways in which it acts under natural conditions. Evolutionary change maybe studied in continuously varying characters, but the relation between phenotypic response and gene frequency is less easy to follow than with characters controlled by major genes. We therefore need to study situations where different alleles coexist in a population, a condition known as genetic polymorphism. E.B. Ford (1975) has defined genetic polymorphism as ‘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’.
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