Data from: Extraordinarily rapid speciation in a marine fish

Divergent selection may initiate ecological speciation extremely rapidly.
How often and at what pace ecological speciation proceeds to yield strong
reproductive isolation is more uncertain. Here, we document a case of
extraordinarily rapid speciation associated with ecological selection in
the postglacial Baltic Sea. European flounders (Platichthys flesus) in the
Baltic exhibit two contrasting reproductive behaviors: pelagic and
demersal spawning. Demersal spawning enables flounders to thrive in the
low salinity of the Northern Baltic, where eggs cannot achieve neutral
buoyancy. We show that demersal and pelagic flounders are a species pair
arising from a recent event of speciation. Despite having a parapatric
distribution with extensive overlap, the two species are reciprocally
monophyletic and show strongly bimodal genotypic clustering and no
evidence of contemporary migration, suggesting strong reproductive
isolation. Divergence across the genome is weak but shows strong
signatures of selection, a pattern suggestive of a recent ecological
speciation event. We propose that spawning behavior in Baltic flounders is
the trait under ecologically based selection causing reproductive
isolation, directly implicating a process of ecological speciation. We
evaluated different possible evolutionary scenarios under the approximate
Bayesian computation framework and estimate that the speciation process
started in allopatry ∼2,400 generations ago, following the colonization of
the Baltic by the demersal lineage. This is faster than most known cases
of ecological speciation and represents the most rapid event of speciation
ever reported for any marine vertebrate.