Countergradient variation and compensatory growth in Moor frog (Rana arvalis) along a replicated latitudinal gradient

Abstract: For evolution to occur over time, it is necessary for animals and plants to show phenotypic variation. If the individuals within populations of a species do not show observable differences among themselves, there will be a lack of driving force for natural selection to act on and decide which characteristic gets inherited from one generation to the next. It is hence important to study phenotypic variation, especially against environmental gradients such as latitude and altitude, which gives us an insight into the pattern of change according to essential factors such as temperature and length of seasons. The latter would impose time constraints on growing populations, leading to periods of unfavourable conditions limiting their growth and development. In many cases, such organisms would tend to compensate for the period of slow growth and catch up to the others that did not have to endure the same situation and grow to the same size as them. This study aims to find differences in three key larval life-history traits of the moor frog (Rana arvalis), namely metamorphic mass, larval period, and growth rate, and find what kind of pattern is observed in case of these phenotypic variations. It also aims to find differences in the strength of compensatory response between populations from lower and higher latitudes. A common garden experiment was conducted with populations originating from both sides of the Baltic Sea, ranging from southern Sweden and Latvia to central Finland and northern Sweden. It was expected that the northern populations would grow faster and show a countergradient variation pattern since they are faced with more strict time constraints at higher latitudes, but in most cases, we observed a co-gradient pattern, wherein the environmental effect amplifies the individual’s genetic predisposition instead of opposing it. We also observed a stronger compensatory response in the northern populations as compared to their southern counterparts. Effects of climate change and subsequent rise in temperatures making the environment unpredictable over time could be used to speculate about the reason behind the results obtained. Epigenetics could also be used as an approach to study long lasting changes in an organism’s gene expression to make it adapt better to changing conditions and hence show different patterns of variation from studies in the past. Studying such changes, expected or not, is important to keep up with the needs of the species that require conservation, and will help conservation biologists to formulate strategies that would be effective even in the face of constant change in the world.