Adaptation to elevation but limited local adaptation in an amphibian

We performed a reciprocal transplant experiment to estimate “parallel”
adaptation to elevation and “unique” adaptation to local sites at the same
elevation, using the frog Rana temporaria in the Swiss Alps. It is
important to distinguish these two processes because they have different
implications for population structure and ecological specialization.
Larvae were reared from hatching to metamorphosis within enclosures
installed in their pond of origin, in three foreign ponds at the same
elevation, and in four ponds at different elevation (1500-2000 m higher or
lower). There were two source populations from each elevation, and adults
were held in a common environment for one year before they were crossed to
produce offspring for the experiment. Fitness was a measure that
integrated larval survival, development rate, and body size. Parallel
adaptation to elevation was indicated by an advantage at the home
elevation (11.5% fitness difference at low elevation and 47% at high
elevation). This effect was stronger than that observed in most other
studies, according to a survey of previous transplant experiments across
elevation (N = 8 animal species and 71 plants). Unique local adaptation
within elevational zones was only 0.3-0.7 times as strong as parallel
adaptation, probably because gene flow is comparatively high among nearby
wetlands at the same elevation. The home-elevation advantage may reduce
gene flow across the elevational gradient and enable the evolution of
habitat races specialized on elevation.