Gene flow limits adaptation along steep environmental gradients

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.