PFENNIG, K.S.: Differential Selection to Avoid Hybridization and its Role in Speciation by Reinforcement

The fitness effects of hybridization critically affect the outcome of speciation. When hybridization is costly, selection favors the evolution of female preferences that reduce heterospecific matings, and, consequently, enhances reproductive isolation between species (i.e., reinforcement). When the costs of heterospecific matings differ for females of two hybridizing species (e.g., when hybrid fitness is intermediate between the two pure-species types), however, selection on females to avoid hybridization will differ between species. Such differential selection to avoid hybridization could impede the reinforcement process. To address this issue, I examined the fitness effects of hybridization between two congeneric species of hybridizing toads (Spea multiplicata and S. bombifrons). I created four kinds of offspring: pure S. multiplicata, pure S. bombifrons, hybrids from crosses of S. multiplicata females and S. bombifrons males, and hybrids from crosses of S. bombifrons females and S. multiplicata males. I compared survival and age and size at metamorphosis among these groups. Hybrid offspring from crosses in which S. multiplicata was the female had significantly lower survival than pure S. multiplicata offspring. Hybrid offspring from crosses in which S. bombifrons was the female did no worse than the pure S. bombifrons offspring. Indeed, these hybrids out-performed pure S. bombifrons offspring by reaching metamorphosis faster. These data suggest that although S. multiplicata females are under selection to avoid hybridization with S. bombifrons, S. bombifrons females may be under selection to mate with S. multiplicata males in some circumstances. Yet, despite this differential selection to avoid hybridization, reinforcement is occurring between these species.