Data from: The role of selection and historical factors in driving population differentiation along an elevational gradient in an island bird

Adaptation to local environmental conditions and the range dynamics of
populations can influence evolutionary divergence along environmental
gradients. Thus, it is important to investigate patterns of both
phenotypic and genetic variation among populations to reveal the
respective roles of these two types of factors in driving population
differentiation. Here, we test for evidence of phenotypic and genetic
structure across populations of a passerine bird (Zosterops borbonicus)
distributed along a steep elevational gradient on the island of Réunion.
Using eleven microsatellite loci screened in 401 individuals from 18
localities distributed along the gradient, we found that genetic
differentiation occurred at two spatial levels: (i) between two main
population groups corresponding to highland and lowland areas,
respectively, and (ii) within each of these two groups. In contrast,
several morphological traits varied gradually along the gradient.
Comparison of neutral genetic differentiation (FST) and phenotypic
differentiation (PST) showed that PST largely exceeds FST at several
morphological traits, which is consistent with a role for local adaptation
in driving morphological divergence along the gradient. Overall, our
results revealed an area of secondary contact mid-way up the gradient
between two major, cryptic, population groups likely diverged in
allopatry. Remarkably, local adaptation has shaped phenotypic
differentiation irrespective of population history, resulting in different
patterns of variation along the elevational gradient. Our findings
underscore the importance of understanding both historical and selective
factors when trying to explain variation along environmental gradients.