Unravelling the evolutionary advantage of sex: a commentary on 'Mutation-selection balance and the evolutionary advantage of sex and recombination' by Brian Charlesworth.

Delving into the literature on the evolution of sex is like picking up a dusty, old, unpromising book and discovering that a fascinating mystery novel lies within it. What makes this tome unpromising is that explaining the evolution of sex seems so trivial at first. Isn't it obvious that sex generates variation ? The fact that nearly all eukaryotes engage in sex, at least occasionally , reinforces the view that the advantages of sex must be pervasive and strong. But cracks in this story appeared early on, when the first models were developed to investigate the fate of genes controlling the rate of sex and recombination (so-called 'modifiers '). The earliest work showed that genetic variants increasing recombination never spread when introduced into a population at equilibrium under selection While these papers focused on modifiers of recombi-nation, similar results apply to modifiers that alter the frequency of sex (e.g. Dolgin & Otto, 2003). In short, when the only evolutionary process acting is viability selection (no mutation, no departures from random mating, no drift, etc.), evolutionary theory predicts that populations should evolve lower and lower rates of sex and recombination. That's a pretty big crack. The underlying reason why evolution leads to reduced levels of sex and recombination in these models is that sex and recombination break apart the favourable gene combinations that have been built up by past selection. Consider the simplest case of a single selected locus, A, where selection favours hetero-zygotes. Modifiers that increase the frequency of sex increase the production of AA and aa offspring from heterozygous parents (who are more likely to have survived to reproduce). Sex therefore does produce more genetically variable offspring, but these offspring are less fit, and their deaths cause the demise of modifiers that promote sexual reproduction. Metaphorically, we have come to the point in the mystery where the protagonist has been murdered. And as good detectives, evolutionary biologists have been pursuing the culprit for decades in an attempt to find the real reason why sex and recombination have evolved and are so prevalent. Enter Brian Charlesworth and his classic (1990) paper, 'Mutation–selection balance and the evolutionary advantage of sex and recombination ' in Genetical Research. This paper explored whether deleterious mutations, added to a model of selection in an infinitely large diploid population, could favour sex and recombination. By producing more variable offspring, some of which carry more deleterious mutations …