Selection limits

[1] The most obvious possible cause of reaching a limit (or plateau) when a population is under continued directional selection is that all of the additive-genetic variation (see additive genetic effects) related to that trait gets "used up" or fixed.

In reality, mutation, random genetic drift (especially in small populations), and gene flow from immigrants may stop some loci from becoming fixed for the "good" alleles.

As noted by Lerner and Dempster,[1] these factors are generally one of two types: 1) negative relations with Darwinian fitness; 2) non-additive gene action and/or genotype-environment interaction (although others are possible[3] [4] [2] [5]).

A negative relation with Darwinian fitness is a situation in which an allele that is "good" for the trait under directional selection is "bad" with respect to lifetime reproductive success.

[6] Non-additive gene action refers to such situations as heterozygote advantage, where heterozygous individuals have higher (or lower) values for a trait (such as body size) than do either of the two homozygotes.

For instance, somewhat different genes (a term that can refer to alleles or loci) tend to give the highest value of a trait depending on the season.

Results of a hypothetical replicated artificial selection experiment with three treatments. At generation 0, the base population of organisms had been randomly sampled to create six lines, two of which would be selectively bred for high values of the trait, two for low values of the trait, and two of which would not experience any intentional selection. The control lines diverged somewhat by random genetic drift and possibly unique mutations , but, overall, did not change in their average phenotype from the beginning of the experiment. Both of the high-selected lines reached apparent selection limits around generation 20. Both of the low-selected reached absolute limits near zero around generation 25.