Evolution of Mating Systems: Outcrossing versus Selfing

Mating systems vary enormously among groups of organisms. This has led to diverse definitions and approaches for investigating their evolution and maintenance. Animal mating systems are characterized by different patterns of parental investment in offspring and variation in the extent to which sexual selection shapes male and female traits (see chapter VII.4). A primary focus of most studies is determining the causes and consequences of variation in mate number for females and males. In contrast, in hermaphrodite organisms, particularly plants, the emphasis is largely on determining the incidence of crossand self-fertilization and its fitness consequences. Most studies of matingsystem evolution have been conducted on plants and largely concern the ecological and genetic mechanisms responsible for evolutionary transitions from outcrossing to predominant self-fertilization, and the extent to which mixed mating can be maintained as a stable strategy. Models of mating-system evolution involve the balance between the transmission advantage of alleles affecting the selfing rate and the reduced fitness of self-fertilized offspring because of inbreeding depression. Selfing provides reproductive assurance whenever outcrossing is limited by the availability of pollinators or mates and thus low density often plays an important role in matingsystem transitions. Reconstruction of the evolutionary history of mating systems using phylogenies indicates that transitions to predominant selfing have occurred on numerous occasions, although selfing lineages are often short-lived because of the negative effects of selfing on the genome.