Plastic Responses to Temperature Versus Local Adaptation at the Cold Extreme of the Climate Gradient
Climate is a strong selection agent at high elevations, but experimental examinations of how animals exclusive of highlands cope with its variation are scarce. We analysed temperature-induced variation of early ontogenetic traits in the alpine grasshopper Chorthippus cazurroi, and compared populations from the elevational extremes of the species distribution under laboratory conditions spanning natural temperature ranges. Neither elevation of origin, nor different growing temperatures, had a direct effect on nymph body size, but both factors contributed to size at hatching indirectly, via their effect on the duration of embryo development. Large emerging nymphs had a consistently greater survival, although small and fast-developing nymphs from highlands also performed well at low temperatures. Viability selection favoured fast-developing phenotypes in conditions in which plasticity delayed development, in a typical countergradient pattern. Growth in the successive stage did not compensate for slow development at hatching, thus responses at this early stage have potential long-lasting consequences. Although phenotypic selection during early development certifies the strength of selection imposed by cold temperatures in the laboratory, elevation clines of body size did not emerge in either nymphs or the wild parental generation. Differentiation in the wild may be levelled out by fecundity selection for large sizes, drift and gene flow resulting from the fragmentation and proximity of populations, or by micro-climatic differences that reduce the likelihood of directional selection. There is therefore potential for local adaptation to temperature, but a series of conditions typical of alpine environments and ectotherms may impair, confound or constrain full differentiation along the gradient.
KeywordsAlpine fauna Body size Countergradient selection Development Phenotypic selection Selection gradients
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