Thorax size (highly correlated to whole body size) is a widely accepted measure of dispersal ability in Lepidoptera as it indicates flight muscle investment ( Srygley & Chai, 1990 Hill, Thomas & Lewis, 1999 Berwaerts, Van Dyck & Aerts, 2002). Dispersal in insects, of which flight performance is a key component, may be affected by several factors including morphological, physiological, metabolic, and behavioural traits ( Betts & Wootton, 1988 Berwaerts, Van Dyck & Aerts, 2002 Niitepõld et al., 2009 Flockhart et al., 2017 Renault, 2020). Density-dependent selection is predicted to favour good dispersers colonising new habitat patches, where they encounter much lower intraspecific competition, resulting in increased fecundity and intrinsic growth rates at the range edge. This cline in dispersal ability is a product of spatial selection, resulting from the combined effects of spatial sorting (assortative mating of dispersive genotypes on the range edge) and density-dependent selection ( Phillips, Brown & Shine, 2010). Indeed, a common phenotypic signature of range expansion, found in many species, is increased dispersal ability towards the leading edge of a shifting range ( Hughes, Hill & Dytham, 2003 Simmons & Thomas, 2004 Phillips, Anderson & Schapire, 2006). 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).Ĭontemporary evolution of dispersal traits is often closely linked to the process of range shifts towards cooler climates ( Parmesan, 2006 Hickling et al., 2006). Moreover, temperature-dependent reaction norms, and genetic correlations among traits, may help or hinder adaptation to the new environment ( Pujol et al., 2018). 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). Any resulting phenotypic changes are therefore created by evolutionary responses to both changing local environments and the process of range expansion itself. During a range expansion populations on the range front are subject to multiple selection pressures ( Phillips, Brown & Shine, 2010). aegeria implies an overall reinforcing effect between these two mechanisms.Ī 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). Whilst the plastic and evolutionary responses may in some cases act antagonistically, the rapid expansion of P. Our study suggests that temperature-sensitive plastic responses for size and colour interact with selection for dispersal traits (wing size and shape). 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. Changes in wing spot pattern were also detected. Contrary to thermal melanism expectations, wing colour was lighter where larvae developed at cooler temperatures and unrelated to long-term temperature. Forewings became more rounded and hindwings more elongated with history of colonisation, possibly reflecting selection for increased dispersal ability. Overall, wing size increased with latitude by ∼2% per 100 km, consistent with Bergmann’s rule. 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). We measured 774 males from 54 sites spanning 799 km with a 10-year mean average temperature gradient of 4 ☌. 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). Characterisation of traits associated with such expansions provides insight into the selection pressures and evolutionary constraints that shape demographic and evolutionary responses. Populations undergoing rapid climate-driven range expansion experience distinct selection regimes dominated both by increased dispersal at the leading edges and steep environmental gradients.
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