Quantifying Reproductive Barriers in a Sympatric Pair of Darter Species

Quantifying and comparing the strengths of different reproductive barriers between diverging lineages is especially useful for determining the evolutionary mechanisms driving speciation. Etheostoma barrenense and Etheostoma zonale are closely related sympatric species of darters that are sexually dimorphic and exhibit clear differences in male nuptial coloration. Prior studies demonstrated that these species exhibit complete behavioral isolation, and that both intraspecific and interspecific variation in male coloration play a role in female choice, all consistent with speciation by sexual selection on male nuptial color. Remaining unclear, however, is whether behavioral isolation is the strongest reproductive barrier between these species or, alternatively, whether additional reproductive barriers are equally strong, which could implicate mechanisms other than sexual selection in speciation. Here, we compare the relative strengths of multiple reproductive barriers between the two focal species, measuring: (1) ecological isolation, (2) gametic incompatibility, (3) hybrid inviability, (4) conspecific sperm precedence, and comparing these measures to a previously estimated strength of behavioral isolation. We find that behavioral isolation is the strongest reproductive barrier measured to date and suggest it may be the only barrier that has evolved to completion. This result provides additional empirical evidence for speciation driven by sexual selection and provides insight into the maintenance of sympatric species in nature.