EVOLUTION OF A BENEFICIAL ALLELE UNDER PARTIAL SELFING AND INBREEDING DEPRESSION.

Abstract

Classical population genetics has often focused on the dynamics of one or a few genes in isolation from the rest of the genome. Recently, however, there has been a recognition that the selective environment of a gene includes the entire genome (Charlesworth 1994; Peck 1994; Barton 1995). This study investigates the influence of a genomic trait, inbreeding depression, on the evolution of a beneficial allele at a single locus under partial selfing. Much of the effect of inbreeding depression is accounted for by its reduction of the selfing rate experienced in the population. In addition, however, I find evidence for a secondary consequence of a genomic background of inbreeding depression under partial selfing, namely, associations between the beneficial allele and the background. These associations, while small, may have a measurable effect on the evolution of advantageous alleles in a background of inbreeding depression. Inbreeding depression is the decrement in fitness associated with inbreeding compared to outbreeding. In selfcompatible plants, this is generally measured as 8 = 1 (wslwO), where wi is the average fitness of selfed progeny and w0 is the average fitness of outcrossed progeny. Inbreeding depression has been studied extensively as an important factor in the evolution of self-fertilization in plants, and is thought to be present in most sexual species (Charlesworth and Charlesworth 1987). Experimental evidence now generally supports the hypothesis that a substantial proportion of the inbreeding depression observed in plants is due to recurrent mutation to deleterious recessive or nearly recessive alleles at many loci (Crow 1993; Ritland 1996). An important property of this form of inbreeding depression is that it is dynamic and interacts with the breeding system, so that the input of new mutations is balanced by a purging of recessives through exposure to selection due to the increase in homozygosity that accompanies self-fertilization (Lande and Schemske 1985). Evolution in partially selfing populations has features that distinguish it from evolution in random-mating populations, both from a single-locus and a multilocus perspective. At a single locus, levels of dominance and inbreeding can interact to affect the fate of individual alleles. In a phenomenon known as Haldane's Sieve, recessive beneficial mu-