Abstract
Most studies of evolutionary responses to climate change have focused on phenological responses to warming, and provide only weak evidence for evolutionary adaptation. This could be because phenological changes are more weakly linked to fitness than more direct mechanisms of climate change impacts, such as selective mortality during extreme weather events which have immediate fitness consequences for the individuals involved. Studies examining these other mechanisms may be more likely to show evidence for evolutionary adaptation. To test this, we quantify regional population responses of a small resident passerine (winter wren Troglodytes troglodytes) to a measure of winter severity (number of frost days). Annual population growth rate was consistently negatively correlated with this measure, but the point at which different populations achieved stability (λ = 1) varied across regions and was closely correlated with the historic average number of frost days, providing strong evidence for local adaptation. Despite this, regional variation in abundance remained negatively related to the regional mean number of winter frost days, potentially as a result of a time-lag in the rate of evolutionary response to climate change. As expected from Bergmann's rule, individual wrens were heavier in colder regions, suggesting that local adaptation may be mediated through body size. However, there was no evidence for selective mortality of small individuals in cold years, with annual variation in mean body size uncorrelated with the number of winter frost days, so the extent to which local adaptation occurs through changes in body size, or another mechanism remains uncertain.
Highlights
Species’ distributions and range boundaries are widely regarded to be closely related to climate [1,2,3], indicative of limits in the evolutionary potential of species to adapt to climatic2016 The Authors
The number of winter frost days at which λ = 1 varied between regions (range = 6.9 to 11.5; figure 1), due to variation in the region-specific intercepts and the region × winter FD interaction. This FD value at stability was strongly correlated with FDH (r = 0.93, n = 10, p < 0.001; figure 1b), with a close to 1 : 1 relationship; wren populations were stable if they were exposed to a winter of equal severity to the long-term average
We identified the expected strong negative effect of winter severity upon wren populations, which was consistent across regions and matches previous studies [29,43]
Summary
Species’ distributions and range boundaries are widely regarded to be closely related to climate [1,2,3], indicative of limits in the evolutionary potential of species to adapt to climatic2016 The Authors. Projected impacts of climate change upon species’ extinction risk [10,11,12] have stimulated much interest in the potential for populations to exhibit evolutionary responses to climate change [13] To date, this has focused on phenological responses to changing environmental conditions, such as timing of breeding and migration, which are relatively measured traits with some fitness consequences [14]. This has focused on phenological responses to changing environmental conditions, such as timing of breeding and migration, which are relatively measured traits with some fitness consequences [14] In this case, the majority of studies suggest that much of the phenotypic responses observed results from individual plasticity [15,16,17,18], with relatively few studies providing evidence for local adaptation (but see [19,20]). Given that recent studies imply that the fitness consequences of mistimed breeding may be less severe than previously thought, at least in birds [21,22], there is a need to consider the evidence for populations exhibiting local adaptation to climate with respect to other mechanisms and traits
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