Rethinking local adaptation: Mind the environment!

Abstract

Spatial heterogeneity in environmental conditions has led to adaptive genetic differentiation and the development of home-site fitness advantage (local adaptation) among populations of many widespread plant species such as forest trees. Although its overall patterns have been well characterised, earlier studies on adaptation have predominantly used long-term averages of environmental variables to describe local climates. Subsequently, only little is currently known about more complex patterns of variation in potential selective forces and how they affect adaptive processes. Furthermore, we also lack a good understanding of why adaptive traits often vary within populations despite clear evidence of local selection. Because the capacity of a population to respond to changes in its home environment depends on the amount of genetic variation that it contains, an understanding of these patterns is fundamental to predicting how extant populations will cope with climate change. In this paper, I call attention to these two issues and discuss adaptation in heterogeneous environments using studies mainly on Finnish populations of Scots pine (Pinus sylvestris L.) as an example. In this geographic area, population means in growth cessation are closely related to the latitude at the population’s home site, indicating adaptation to local environments. Yet the trait varies considerably also within populations, with the highest levels of phenotypic variation found in the central part of the latitudinal gradient. Increased variation further north may have a significant genetic component. In stable local environments this would indicate that populations with less variation are more optimally adapted to their home site environments. On the other hand, climate data show that growing season temperature conditions within Finland become temporally more variable towards the north which might contribute to different levels of trait variation. Collectively, these findings demonstrate the weaknesses of focusing only on long-term averages of environmental variables or trait means when examining adaptation in natural populations. Moreover, better integrated analyses of both genetic and environmental variation might help in disentangling the mechanisms that maintain adaptive genetic diversity and adaptive capacity in natural populations of perennial species under changing environmental conditions.