Amphibian larval development is highly temperature sensitive; larvae develop faster at warmer temperatures and slower at cooler ones. Countergradient variation is a form of local adaptation where – for example, in the Wood frog (Rana sylvatica)- tadpoles living in colder ponds (due to more enclosed, shaded forest canopies) are adapted to develop at faster rates than tadpoles from populations living in warmer ponds (with more open, sunnier forest canopies). This allows tadpoles in cold ponds to developmentally catch up to tadpoles in warmer habitats. The original study (Skelly 2004) found that this countergradient variation occurs at very small 'microgeographic' spatial scales, i.e., Wood frog populations are adapting to local environmental conditions well within the distance that the frogs can disperse and interbreed within a metapopulation. Arietta and Skelly (2021) performed the identical experiment with the same Wood frog populations 17 years later (~ 6-9 frog generations) and found persistent microgeographic countergradient evolution as before with overall Wood frog embryonic developmental rates accelerated by 14-19%. During this period, the forest canopy over these ponds generally became more shaded, but pond temperatures still increased across the metapopulation. With warmer ponds, it would be expected that the Wood frog populations would evolve slower tadpole development rates, not faster. Critically, ponds experiencing more change (i.e., canopy cover and temperature change) also had declining populations and several disappeared altogether. Overall, their experiment illustrates how amphibians might be able to adapt to environmental change relatively quickly and at relatively small spatial scales, but, even so, there are limits on rescuing populations from rapid environmental change through evolutionary adaptation.
