Hybridization—interbreeding between species—is generally thought to be both deleterious (e.g., because it often results in sterile or inviable offspring) and a force that reduces biodiversity (e.g., via species collapse). However, hybridization can also enhance fitness and generate novel traits (by generating new genetic combinations), suggesting that hybridization can actually enhance diversity. Our work suggests that hybrid fitness depends on a complex interplay of genetic interactions, the environment, and even individual condition or developmental history. One of our goals is to understand how these various factors contribute to hybridization in nature.

We focus this work on S. bombifrons. The females of this species can actually benefit by hybridizing with S. multiplicata males. However, they only benefit when they breed in highly ephemeral ponds where hybrid tadpoles have a fitness advantage of faster development. Ponds vary in space and time in their ephemerality, so females have evolved the ability to modify whether or not they mate with heterospecifics depending on the prevailing fitness consequences of hybridization. In short, females prefer males of their own species when hybridization is costly, but switch their preferences and prefer S. multiplicata males when hybridization is beneficial. Mate choice thereby contributes to hybridization when it is beneficial and isolation between species when it is not.

Consequently, gene flow between species (introgression) can vary in both time and space. Using S. bombifrons, we are investigating how variation in the fitness consequences of hybridization impacts behavioral evolution and the distribution of species.