Abstract
What are the genes and traits that respond to selection and cause prezygotic reproductive isolation between species? This question has been hard to answer because genomes are large, the targets of selection may be scattered across the genome (Sabeti et al., ) and different genes may respond to the same selective pressure in different populations (Scheinfeldt et al., ). In this issue of Molecular Ecology , Weber et al. ( ) use a clever comparative approach and leading‐edge transcriptomic methods to identify the species and genes under positive selection for divergence between brittle stars (the echinoderm class Ophiuroidea) in the Ophioderma longicauda species complex. They found convincing evidence of positive or diversifying selection acting on two genes encoding ion channels that form part of the signal transduction cascade within the sperm in response to pheromones. Evidence for selection was concentrated in genes from one species (called C5, with internal fertilization and female parental care of brooded juveniles and not in the other species (called C3, with more conventional broadcast spawning and planktonic development of embryos and larvae). That analysis greatly extends the range of taxa, life history traits and molecules that are associated with positive selection in speciation. It also illustrates some of the current limitations on the application of RNA seq methods in the search for the targets of selection in nonmodel organisms like brittle stars. From both points of view, the new work by Weber et al. ( ) has important implications for our understanding of speciation in the ocean.