Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

If a population is exposed to directional selection (see IV.6.2), for example for increased body size, the average size of organism in the population initially changes rapidly; however, after a certain time, the population will cease to react to selection. At least three phenomena can be responsible for the loss of evolutionary plasticity. The simplest (and mostly incorrect) explanation is exhaustion of genetically determined variability in the population. It follows that this is an incorrect explanation, amongst other things, from the fact that the population almost always retains the ability to respond to selection in the opposite direction (Lynch & Walsh 1998).

Another possible explanation is genetic homeostasis (see IV.9.2). Genetic homeostasis is a complicated phenomenon in which a key role is apparently played by the pleiotropic effects of a large number of genes, i.e. their effect on a number of traits, and also epistatic interactions amongst the individual genes. If, for example, through artificial selection during the experiment, we increase the frequency in the population of alleles that have a negative effect on the viability of the organism in the homozygote state or are even lethal in the homozygote state, the population will cease to respond to directional selection at the moment when the intensity of the directed artificial selection equals the intensity of natural selection, acting in the opposite direction.

The third cause of loss of responsiveness in the population could be the existence of supergenes. The population can still have a large amount of additive genetically determined variability; however, this variability is contained primarily inside supergenes and, unless recombination occurs between the loci inside the supergene, it is not available for evolution (Sheppard 1958). Cases are known where the population resists intensive selection for dozens of generations without changing at all through the effect of this selection and suddenly, as if without any reason, it begins to respond to the selection pressure again. The main reason for the renewal of evolutionary responsiveness could be the formation of a rare recombinant in which the alleles in closely neighbouring loci within certain alleles of the supergene are separated.