On the logic of Fisherian sexual selection
In Fishers model of sexual selection, a female preference for a male trait spreads together with the trait because their genetic bases become correlated. This can be interpreted as a greenbeard system: a preference gene, by inducing a female to mate with a trait-bearing male, favors itself because the male is disproportionately likely also to carry the preference gene. Here, we use this logic to argue that Fisherian sexual selection in diploids proceeds via two channels, corresponding to two reasons that trait-bearing males disproportionately carry preference genes: (i) trait-bearing males are disproportionately the product of matings between preference-bearing mothers and trait-bearing fathers, and thus trait and preference genes are correlated in trans; (ii) trait and preference genes come into gametic phase disequilibrium, and thus are correlated in cis. Gametic phase disequilibrium is generated by three distinct mechanisms: a recombination mechanism, a dominance mechanism, and a sexual admixture mechanism. The trans channel does not operate when sexual selection is restricted to the haploid phase, and therefore represents a fundamental difference between haploid and diploid models of sexual selection. We use simulation experiments to artificially eliminate the cis channel, and show that a preference gene can spread in its absence in the diploid model, but not in the haploid model. We further show that the cis and trans channels contribute equally to the spread of the preference when recombination between the preference and trait loci is free, but that the trans channel becomes substantially more important when linkage is tight.