The oligogenic view of adaptation.

The traditional view is that evolution proceeds very slowly, over immense periods of time, driven by weak selection acting on innumerable genes of small effect. Recent studies of rapid evolution, in the laboratory and in the field, have given a radically different picture. Although beneficial mutations tend to be small in effect when they first appear, those that survive to spread and become fixed are usually among the minority with large effect. Hence, although hundreds of loci of small effect may contribute to variation in character state, adaptation is predominantly caused by alleles of large effect. This leads to the hope that the particular mutations responsible for adaptation to altered conditions of life can be identified and characterized. This has been achieved in some cases and may soon become routine. Furthermore, it raises the possibility that adaptive change can be predicted from a knowledge of genetics and ecology. Experimental evolution suggests that any given selection line that is adapting to changed conditions will follow one of a few themes (broadly speaking, loci), each of which may have many variations (mutations within the locus producing similar phenotypes). Hence, evolutionary change can be predicted only within limits, even in principle. Nevertheless, recent attempts to predict how very simple genomes change have been surprisingly successful, and we may be close to a new predictive understanding of the genetic basis of adaptation.