Thermal environment, survival and local adaptation in the common frog, Rana temporaria

Natural selection of organisms results in differences between populations in response to environmental conditions - that is, local adaptation. Understanding heterogeneity at this level requires identification of the different selection pressures that cause this differentiation. Tadpoles of the common frog, Rana temporaria, from northern Sweden show superior growth in warmer incubation conditions than tadpoles from the south, and vice versa in cooler conditions. A plausible explanation for this is that the thermal optimum in the northern population has been shifted by selection to correspond better with the relatively warmer developmental conditions experienced in the north compared with the south (northern ponds are warmer because they are shallower and tend to heat more quickly following the melting of snow). To test this hypothesis, we performed an experiment in which frogs from both regions were allowed to develop in water temperatures representative of those in their natural breeding ponds and in those at the other climatic extreme. Tadpoles in the 'warm' (northern) part of the range had markedly reduced survival at low temperatures compared with those from the cooler region in the south, with a non-significant difference at relatively high temperatures. Thus, northern frogs appear to have evolved towards becoming high-temperature 'specialists', with local adaptation being upheld by hard selection on tadpole survival.