Thesis #17 – When such trade-offs arise from antagonistic pleiotropic effects of genetic variants, they sometimes maintain genetic variation for functional characters, and thus selectable genetic variation for patterns of aging.
There are two kinds of trade-off which are important to distinguish when thinking about the evolution of aging: genetic trade-offs and non-genetic trade-offs.
Cases with genetic trade-offs are called antagonistic pleiotropy, a term I invented in the early 1980s. In genetics, the term pleiotropy refers to genetic variants which have multiple effects. With antagonistic pleiotropy, genes have variants with opposed effects on different components of fitness. Imagine, for example, an allele that gives rise to faster egg-laying in young adult insects, with the antagonistic pleiotropic effect of a reduced rate of adult survival thereafter. If the increase in early fecundity is large enough relative to the reduction in adult survival, natural selection will favor the genetic variants that sacrifice later survival for earlier reproduction.
This is where the Forces of Natural Selection have a critical role to play. The key factor is adult age. If the beneficial effect on reproduction is early in adulthood, but the adverse effect on survival is late in adulthood, the biased weightings of the Forces of Natural Selection in favor of early versus late ages strongly favor the evolution of increased early reproduction and decreased later survival. In this respect, natural selection plays the role of an unfair merchant, who tips the scale in favor of themselves when you aren’t looking. [That is why a Western symbol of Justice is a blind-folded woman holding up a pair of scales.] Natural selection is inherently biased in favor of early reproduction, all other things being equal.
There are three main possibilities for the evolution of genes that have antagonistic pleiotropy affecting aging. The first possibility is that survival costs of increased early reproduction are too great and too early relative to the benefits, and such genes will not spread by natural selection. The second possibility if that a gene gives rise to a significant net increase in fitness in every genotype in which it occurs. In such cases, natural selection will sweep genes like this to fixation if they arise with some frequency in a population.
The third possibility is that genes with antagonistic pleiotropy give some genotypes with increased fitness, but other genotypes with decreased fitness. In this case, such genetic variants first increase in frequency thanks to natural selection. But they do not sweep through populations all the way to fixation. They remain polymorphic, and can be detected in evolutionary and genetic experiments, revealing the antagonistic pleiotropic effect.