Relative importance of isolation-by-environment and other determinants of gene flow in an alpine amphibian

A pattern of population structure called isolation-by-environment (IBE)
evolves when gene flow connecting populations in different habitats is
lower than expected. Although IBE is widespread, there is limited
information on its magnitude compared with other factors influencing gene
flow. We estimated the relative importance of IBE in the frog Rana
temporaria in the Swiss Alps, a geographic context in which IBE should be
relatively pronounced. The environmental factor potentially causing IBE
was the length of the growing season, which is highly correlated with
elevation. A sample of 992 individuals from 82 breeding sites were
genotyped at 1827 SNP markers; gene flow was estimated in four ways
related to FST, genetic distance, allele sharing, and distance on a
population-graph. Gravity modeling and random forest regression evaluated
the importance of six at-site covariates, ten between-site covariates, and
geographic distance. There was broad agreement among analysis methods and
measures of gene flow: isolation-by-distance (IBD) and habitat quality
between sites were of highest importance, the elevation and ruggedness of
the dispersal path were about half as important, and IBE was about 10-20%
as important as IBD. These results combine with other evidence to suggest
that population divergence across elevational gradients is underway in
amphibians.