EcoEvo Icelandic Arctic charr population divergence

Conceptual models of adaptive divergence and ecological speciation in
sympatry predict differential resource use, phenotype-environment
correlations, and reduced gene flow among diverging phenotypes. While
these predictions have been assessed in past studies connections among
them have rarely been assessed collectively. We examined relationships
among phenotypic, ecological, and genetic variation in Arctic charr
(Salvelinus alpinus) from six Icelandic localities that have undergone
varying degrees of divergence into sympatric benthic and pelagic morphs.
We characterized morphological variation with geometric morphometrics,
tested for differential resource use between morphs using stable isotopes,
and inferred the amount of gene flow from single nucleotide polymorphisms.
Analysis of stable isotopic signatures indicated that sympatric morphs
showed similar difference in resource use across populations, likely
arising from the common utilization of niche space within each population.
Carbon isotopic signature was also a significant predictor of individual
variation in body shape and size, suggesting that variation in benthic and
pelagic resource use is associated with phenotypic variation. The
estimated percentage of hybrids between sympatric morphs varied across
populations (from 0 to 15.6%) but, the majority of fish had genotypes
(ancestry coefficients) characteristic of pure morphs. Despite evidence of
reduced gene flow between sympatric morphs, we did not detect the expected
negative relationship between divergence in resource use and gene flow.
Three lakes showed the expected pattern, but morphs in the fourth showed
no detectable hybridization and had relatively low differences in resource
use between them. This coupled with the finding that resource use and
genetic differentiation had differential effects on body shape variation
across populations, suggests that reproductive isolation maintains
phenotypic divergence between benthic and pelagic morphs when the effects
of resource use are relatively low. Our ability to assess relationships
between phenotype, ecology, and genetics deepens our understanding of the
processes underlying adaptive divergence in sympatry.