Together, generation time, gene flow and a lack of genetic variation constitute a triumvirate of outstanding reasons why climate-change scientists have felt safe in ignoring evolution even as they make sweeping claims about the impending consequences of changing climate on biodiversity [8]. Into this headwind, Kavanagh et al. [4] have provided an example of contemporary divergence in a suite of traits that have responded to contrasting thermal environments. Instead of changing climate, these researchers studied the response to thermal variation that emerged when a small number of grayling (Thymallus thymallus) colonized the upper reaches of a watershed within which they began breeding in both cold (north facing) and warm (south facing) drainages. Specifically, the researchers studied within a common garden setting embryos and larvae collected from representative warm and cold drainages (two of each). Within this common setting, individuals collected from cold drainages exhibited higher growth rates and higher efficiencies of conversion between yolk mass and hatchling mass. At the same time, cold-drainage animals also developed muscle mass at a higher rate. Collectively, these patterns are consistent with countergradient variation in which selection can promote traits that compensate for an environmental gradient such that the degree of phenotypic variation across the gradient is less than would be expected otherwise [9].
Without additional contextual information these findings might have been deemed interesting primarily as a confirmation of previous results seen in other taxa. The context that yielded divergence, however, makes all the difference in this case. Even as they bred in these contrasting environments, the grayling mingled in a reservoir into which the streams emptied. The timing of the colonization is known (just 22 generations ago), the thermal environments of the drainages are well characterized, and the existence of ongoing gene flow among them has been documented. In spite of a reasonably long generation time (around five years), a small founding population, and ongoing genetic exchange, a suite of traits have diverged in remarkable fashion. In fact, Kavanagh et al. have shown that contemporary divergence is possible in spite of the existence of multiple hurdles that are commonly believed to provide good reasons not to consider as significant evolutionary responses to changing climate.
What lessons should climate change researchers take away from the findings of Kavanagh et al.? There are two worth highlighting. First, textbook-style descriptions of the possible constraints on adaptation should not be viewed as a good reason not to interrogate systems about the potential for evolutionary response [10]. Arguably, grayling in this study had three strikes against them and yet they showed substantial divergence in critical traits over contemporary timescales. The second lesson is more sobering. None of the embryos in Kavanagh et al.'s study were able to tolerate a rearing temperature of just 12°C, even though grayling from other parts of Europe are able to survive at such temperatures. As Scandinavia warms, it is not clear whether the grayling in lake Lesjaskogsvatnet will be able to endure, or whether their relatives from warmer climes will have to supplant them for the species to persist locally.
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