The polymorphic prelude to Bateson–Dobzhansky–Muller incompatibilities

Speciation research has largely assumed that the genetic causes of reproductive isolation are the work of fixed, divergent alleles that interact to cause genetic problems in hybrids: Bateson–Dobzhansky–Muller incompatibilities. However, many recent studies demonstrate substantial heritable polymorphism within species for hybrid incompatibility, herein called variable reproductive isolation (VRI). In this review, I outline the causes and importance of this general phenomenon. I also identify the new challenges of quantifying the relative contributions to reproductive isolation of fixed alleles versus polymorphisms, and the change in these contributions over the course of speciation. Explicit integration of VRI into speciation theory will help to quantify the relative roles of genetic drift and selection in speciation, but this synthesis requires substantial new contributions from both theory and empirical studies.