American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“Gene flow limits adaptation along steep environmental gradients”

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Judith C. Bachmann, Alexandra Jansen van Rensburg, Maria Cortazar-Chinarro, Anssi Laurila, and Josh Van Buskirk (March 2020)

Genetic adaptation in frogs is limited on steep climate gradients by dispersal connecting pops in very distinct habitats

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Male common frog (<i>Rana temporaria</i>) keeping an eye out for females at a breeding pond.<br />(Credit: Josh Van Buskirk)
Male common frog (Rana temporaria) keeping an eye out for females at a breeding pond.
(Credit: Josh Van Buskirk)

Abstract

When environmental variation is spatially continuous, dispersing individuals move among nearby sites with similar habitat conditions. But as an environmental gradient becomes steeper, gene flow may connect more divergent habitats, and this is predicted to reduce the slope of the adaptive cline that evolves. We compared quantitative genetic divergence of Rana temporaria frog populations along a 2000 m elevational gradient in eastern Switzerland (new experimental results) with divergence along a 1550 km latitudinal gradient in Fennoscandia (previously published results). Both studies found significant countergradient variation in larval development rate (i.e., animals from cold climates developed more rapidly). The cline was weaker with elevation than with latitude. Animals collected on both gradients were genotyped at ~2000 SNP markers, revealing that dispersal distance was 30% farther on the latitudinal gradient but 3.9 times greater with respect to environmental conditions on the elevational gradient. A meta-analysis on 19 experimental studies of anuran populations spanning temperature gradients revealed that countergradient variation in larval development, while significant overall, was weaker when measured on steeper gradients. These findings support the prediction that adaptive population divergence is less pronounced, and maladaptation more pervasive, on steep environmental gradients.

Pair of common frogs (<i>Rana temporaria</i>), already in amplexus, approaching a wetland.<br />(Credit: Josh Van Buskirk)
Pair of common frogs (Rana temporaria), already in amplexus, approaching a wetland.
(Credit: Josh Van Buskirk)