Digest: On the temporal evolution of reproductive isolation*

Speciation research has long focused on understanding how reproductive barriers—sometimes referred to as the “currency” of speciation—contribute to genetic isolation between divergent lineages (Coyne and Orr ). These reproductive barriers include those that occur before mating takes place (e.g., sexual, habitat, and temporal isolation) and after (e.g., gametic incompatibility, hybrid sterility, and sexual/ecological selection against hybrids). Given that the speciation process is typically too long to observe in its entirety, researchers often approach the problem of understanding how barriers evolve by studying barriers between taxa at different stages of divergence. This technique provides snapshots along the speciation process, and is useful for inferring the relative importance of barriers across speciation. However, to date most studies have either tested for a limited number of barriers (e.g., sexual isolation, hybrid viability; Coyne and Orr ; Mendelson ) between many pairs of species at different levels of divergence, or have tested for many barriers between one or a few pairs of lineages at the earliest stages of divergence (Schemske ; Nosil ).