Abstract:
Despite the enormous advances in genetics, links between phenotypes and geno‐
types have been made for only a few nonmodel organisms. However, such links can
be essential to understand mechanisms of ecological speciation. The Costa Rican
endemic Mangrove Warbler subspecies provides an excellent subject to study differ‐
entiation with gene flow, as it is distributed along a strong precipitation gradient on
the Pacific coast with no strong geographic barriers to isolate populations. Mangrove
Warbler populations could be subject to divergent selection driven by precipitation,
which influences soil salinity levels, which in turn influences forest structure and
food resources. We used single nucleotide polymorphisms (SNPs) and morphological
traits to examine the balance between neutral genetic and phenotypic divergence to
determine whether selection has acted on traits and genes with functions related to
specific environmental variables. We present evidence showing: (a) associations be‐
tween environmental variables and SNPs, identifying candidate genes related to bill
morphology (BMP) and osmoregulation, (b) absence of population genetic structure
in neutrally evolving markers, (c) divergence in bill size across the precipitation gradi‐
ent, and (d) strong phenotypic differentiation (PST) which largely exceeds neutral ge‐
netic differentiation (FST) in bill size. Our results indicate an important role for salinity,
forest structure, and resource availability in maintaining phenotypic divergence of
Mangrove Warblers through natural selection. Our findings add to the growing body
of literature identifying the processes involved in phenotypic differentiation along
environmental gradients in the face of gene flow.
