Data from: Divergent natural selection promotes immigrant inviability at early and late stages of evolutionary divergence

Natural selection's role in speciation has been of fundamental
importance since Darwin first outlined his theory. Recently, work has
focused on understanding how selection drives trait divergence, and
subsequently reproductive isolation. ‘Immigrant inviability’, a barrier
that arises from selection against immigrants in their non-native
environment, appears to be of particular importance. Although immigrant
inviability is likely ubiquitous, we know relatively little about how
selection acts on traits to drive immigrant inviability, and how important
immigrant inviability is at early-versus-late stages of divergence. We
present a study evaluating the role of predation in the evolution of
immigrant inviability in recently-diverged population pairs and a
well-established species pair of Brachyrhaphis fishes. We evaluate
performance in a high-predation environment by assessing survival in the
presence of a predator, and swimming endurance in a low-predation
environment. We find strong signatures of local adaptation and immigrant
inviability of roughly the same magnitude both early and late in
divergence. We find remarkably conserved selection for burst-speed
swimming (important in predator evasion), and selection for increased size
in low-predation environments. Our results highlight the consistency with
which selection acts during speciation, and suggest that similar factors
might promote initial population differentiation and maintain
differentiation at late stages of divergence.