Energy expenditure and body size are targets of natural selection across a wide geographic range, in a terrestrial invertebrate

Abstract

© 2015 The Authors. One of the central questions in evolutionary ecology is how different functional capacities impact fitness, and how it varies across populations. For instance, do phenotypic attributes influence fitness similarly across geographic gradients? Which traits (physiological, morphological and life history) are most likely to be targets of natural selection? Do particular combinations of traits maximize fitness? In a semi-natural experiment, we analysed introduced populations of an invasive species, the garden snail (Cornu aspersum) in Chile, which show low levels of genetic differentiation in spite of the distance. Specifically, we addressed whether the magnitude, sign and form of selection in snail populations could explain the differentiation (or its absence) among populations. A common garden/reciprocal transplant experiment was performed in three populations (La Serena, Constitución and Valdivia) that span a 1300-km latitudinal gradient and differ markedly in climate (semi-arid north to humid south). We released c. 450 individuals per population (two generations after field-captured snails) in replicated enclosures at the range extremes (La Serena and Valdivia). Morphological (size and shell darkness), physiological (standard metabolic rate and digestive efficiency) and life-history [growth rate (GR)] traits were measured in all snails before the release. Survival was recorded monthly during 1 year. Survival was significantly higher in snails from La Serena than in snails from Constitución and Valdivia, when raised at La Serena. However, at Valdivia, survival was not different among source populations. Interestingly, we found negative correlational selection in M B and SMR R at La Serena, whereas at Valdivia we only found directional selection on GR and M B , and stabilizing selection on SMR R . These results suggest that selection on physiological traits related with energy allocation is pervasive, irrespective of climate and distance.