LAUSR

Significance Interspecific hybridization is now known to be common, raising the question of how species are maintained in the face of gene flow. Assortative mating among species can limit hybridization in the first place and can also help purge introgressed DNA if hybrids have reduced mating success. Here, we explore a further, indirect role of assortative mating as a barrier to gene flow. Under assortative mating, parents covary in their ancestry, and therefore “bundle” ancestry together in their offspring. This increases the population’s ancestry variance, allowing selection to purge introgressed DNA more efficiently. Using theoretical and empirical analyses, we show that the bundling effect can greatly increase the purging of introgressed DNA, contributing substantially to the maintenance of distinct species.