Published online by Cambridge University Press: 07 December 2009
Introduction
Organisms can cope with a variable environment in which various actions are called for in a variety of ways, e.g.:
‘Red Queen’ evolution. Each individual performs the different types of actions with a pre-set frequency. There is some within-population/ species variation in these frequencies (i.e. it is a ‘quasi-species’ rather than a monomorphic species). Because many variants are present in the population, changes in the environment will cause relatively rapid change in the population by selection of available genotypes of the quasi-species. In such a case the rate of change is fairly independent of mutation rate.
Frequency-dependent selection. There are two or more subtypes in the population, each specialising on one or a subset of the actions. In contrast to the previous mode the within-population variation is not unimodal but multi-modal. Dependent on which actions are more in demand these subpopulations will increase/decrease. A clear-cut example is the distribution of the ‘rover’ and ‘sitter’ types in Drosophila, which do what the names suggest in foraging. The difference has been localized to two different alleles of a cGMP-dependent kinase gene which has plural effects, among which is a change in ion channels in the brain, ultimately leading to the two modes of exploiting food resources (Osborne et al., 1997; Sokolowski, 1997; Renger et al., 1999).
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