Abstract
Populations undergoing rapid climate-driven range expansion experience distinct selection regimes dominated both by increased dispersal at the leading edges and steep environmental gradients. Characterisation of traits associated with such expansions provides insight into the selection pressures and evolutionary constraints that shape demographic and evolutionary responses. Here we investigate patterns in three components of wing morphology (size, shape, colour) often linked to dispersal ability and thermoregulation, along latitudinal gradients of range expansion in the Speckled Wood butterfly (Pararge aegeria) in Britain (two regions of expansion in England and Scotland). We measured 774 males from 54 sites spanning 799 km with a 10-year mean average temperature gradient of 4 °C. A geometric morphometric method was used to investigate variation in size and shape of forewings and hindwings; colour, pattern, and contrast of the wings were examined using a measure of lightness (inverse degree of melanism). Overall, wing size increased with latitude by ∼2% per 100 km, consistent with Bergmann’s rule. Forewings became more rounded and hindwings more elongated with history of colonisation, possibly reflecting selection for increased dispersal ability. Contrary to thermal melanism expectations, wing colour was lighter where larvae developed at cooler temperatures and unrelated to long-term temperature. Changes in wing spot pattern were also detected. High heterogeneity in variance among sites for all of the traits studied may reflect evolutionary time-lags and genetic drift due to colonisation of new habitats. Our study suggests that temperature-sensitive plastic responses for size and colour interact with selection for dispersal traits (wing size and shape). Whilst the plastic and evolutionary responses may in some cases act antagonistically, the rapid expansion of P. aegeria implies an overall reinforcing effect between these two mechanisms.
Highlights
A population may respond to climate change either by altering its phenotype to maintain local fitness, or by shifting distribution and/or phenology to track its climatic envelope (Parmesan &Yohe, 2003; Macgregor et al, 2019)
We considered that we had low confidence of the colonisation year for grids in which the first record of P. aegeria coincided with or preceded the onset of good recording (i.e. P. aegeria first recorded in or before the third well-recorded year), since it was unclear whether such records represented true colonisation or the discovery of preexisting populations
Each of the environmental factors show a consistent effect on forewing and hindwing size, latitude seems to have a stronger effect on hindwings compared to forewings (Table 1; Fig.3)
Summary
A population may respond to climate change either by altering its phenotype to maintain local fitness, or by shifting distribution and/or phenology to track its climatic envelope (Parmesan &Yohe, 2003; Macgregor et al, 2019). A population may respond to climate change either by altering its phenotype to maintain local fitness, or by shifting distribution and/or phenology to track its climatic envelope Any resulting phenotypic changes are created by evolutionary responses to both changing local environments and the process of range expansion itself. Phenotype-environment optima under equilibrium conditions, may be overridden or obscured by the expansion process as mal-adapted genotypes can surf on the range front due to genetic drift (Burton & Travis, 2008). Phenotypic responses to climate change have been reported for correlates of dispersal (Thomas et al, 2001; Hill, Griffiths & Thomas, 2011), body size (Daufresne, Lengfellner & Sommer, 2009), and colour lightness (linked to thermal tolerance; Zeuss et al, 2014)
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