Out of the cradle: expansion from the natal ecosystem erodes morphological specificity in endemic Kamchatkan charrs (genus Salvelinus)

The contribution of postglacial adaptive radiations to maintaining regional biodiversity of freshwater fish remains debatable. To uncover whether such radiations can lead to the emergence of new populations in neighbouring ecosystems, we have reconstructed the evolutionary history of the endemic Salvelinus malma assemblage from Lake Kronotskoe in Kamchatka. We traced the dispersal of endemic evolutionary lineages from the lake into neighbouring basins and quantified the morphological divergence between the lacustrine populations and their derivatives. To this end, we first examined the phylogeny, genetic differentiation, current hybridization and historical gene flow of eleven charr groups using genome-wide SNPs, followed by a charr morphometric comparison. We found that the Lake Kronotskoe lineages of the predatory and littoral benthivorous charrs invaded small peripheral waterbodies via temporary riverbed interconnections during the Holocene and continue to migrate downstream from the landlocked ecosystem, hybridizing with the anadromous S. malma. These dispersal processes have resulted in three new isolated populations and one hybrid anadromous population. The evolution of morphotypes in derived populations occurred independently of their phylogenetic history, resulting in the generalized morphology that is characteristic of pelagic or riverine dwellers. These findings show that the postglacial expansion of adaptive morphs from the cradle ecosystem bolsters regional diversity, and that migrants quickly lose ancestral adaptive traits, forming new morphotypes in response to changing environmental pressures.