INBREEDING DEPRESSION DOESN'T MATTER: THE GENETIC BASIS OF MATING-SYSTEM EVOLUTION

Models of mating-system evolution commonly assume that inbreeding depression is independent of the genotype at loci determining the mating system. Because of the association that develops between genotypes at different loci in inbred populations, an individual that is heterozygous at a mating-type locus is more likely to be heterozygous at a fitness locus than is a randomly chosen individual. A modifier model for the evolution of self-fertilization in plants demonstrates that inbreeding depression is not an adequate descriptor of the relative fitness of inbred and outbred progeny. If inbreeding depression is primarily a result of segregation at overdominant loci, intermediate rates of self-fertilization may be favored, even if the inbreeding depression is less than 30%. Indeed, in some cases, mutants that cause some outcrossing can be introduced into completely selfing populations when the inbreeding depression is as little as 1%. If inbreeding depression is primarily a result of the expression of recessive lethals in inbred progeny, selfing can evolve in an initially random mating population, even when the inbreeding depression is over 70%.