Ecological opportunity promotes diversifying selection and facilitates rapid phenotypic divergence in Icelandic Arctic charr

Ecological opportunity can promote rapid adaptive diversification across multiple populations if niche availability is high, phenotypic trade-offs facilitate diversifying selection, and spatial dynamics as well as phenotypic divergence reduce gene flow. We investigated populations of Icelandic Arctic charr (Salvelinus alpinus) to evaluate whether phenotypic diversification is the result of common niche availability, diversifying selection, and/or a reduction in gene flow across populations. Arctic charr invaded freshwater Icelandic lakes within the last 10,000 years and different populations have either diversified into coexisting benthic and pelagic morphs or evolved as monomorphic populations. Using single nucleotide polymorphisms we evaluated genetic population structure to determine if it was consistent with the repeated evolution of morphs in sympatry. We inferred resource use with stable isotopic signatures and characterized morphological variation with geometric morphometrics. Adult charr were sampled from five Icelandic lakes and a river containing anadromous charr. Sympatric morphs were genetically more similar to each other than to phenotypically similar morphs in other lake populations. Populations appear to be evolving in response to similar ecological gradients and aspects of benthic-pelagic niche availability based on similar resource use patterns. We detected varied degrees of morphological divergence between sympatric morphs and phenotype-environment correlations suggest that habitat specific phenotypic trade-offs are facilitating diversifying selection within populations. Lastly, habitat divergence leads to reductions in gene flow between morphs, which may further advance phenotypic diversification. These lines of evidence suggest that high ecological opportunity and diversifying selection facilitate rapid morphological divergence and adaptive diversification.