Abstract Major advances have been made to study speciation through the lens of genomics, yet much remains to be understood about the processes of reproductive isolation and how different reproductive barriers evolve during species divergence. We conducted a comprehensive study to characterise reproductive isolation between two sympatric populations with both a recent evolutionary history and extensive phenotypic and genetic differences: the planktivorous and the small-benthic morphs of Arctic charr (Salvelinus alpinus, Linneaeus 1758) from Thingvallavatn, Iceland. We assessed both the absolute and relative strengths of reproductive barriers through a fishing survey, a mating experiment and a common-garden experiment involving the offspring of each morph and F1 hybrids. Our findings provide compelling evidence for asymmetric gene flow between the two morphs. Reproductive isolation in the planktivorous charr from the small benthic charr appeared to be nearly complete, owing to temporal differences in spawning habitats over time. In contrast, this barrier appeared to be less effective in the small-benthic charr, where assortative mating and postmating isolation like fertilization failures and/or embryo mortality made up for partial reproductive isolation. While our results support the idea that the most likely barriers to evolve are prezygotic, they also emphasize that intrinsic postzygotic barriers may also arise at early stage of ecological speciation.
Horta-Lacueva et al. (Wed,) studied this question.