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

Abstract

Summary 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 MB and SMRR at La Serena, whereas at Valdivia we only found directional selection on GR and MB, and stabilizing selection on SMRR. These results suggest that selection on physiological traits related with energy allocation is pervasive, irrespective of climate and distance.

Snails in the sun: Strategies of terrestrial gastropods to cope with hot and dry conditions

“…Individuals with average‐to‐reduced standard metabolic rates were supported by selection, providing support for an energetic definition of fitness. These results were corroborated by transplantation experiments with Cornu aspersum in Chilean field sites (Bartheld et al, ). In this species, artificial selection for lower metabolism early in ontogeny was associated with larger egg and adult sizes and a faster size‐specific growth rate at the expense of shell production (Czarnoleski et al, ).…”

Repeatability of metabolic rate is lower for animals living under field versus laboratory conditions

“…There is some evidence that metabolic reaction norms are under selection (Bartheld et al, 2015;Terblanche et al, 2009), so the lower repeatability estimates obtained in the wild do not necessarily indicate that metabolic rates will not evolve. However, the differences in repeatabilities that we report do have implications for the level of inference that can be made from laboratory estimates to the temporal consistency of metabolism in the wild.…”

Selection on the morphology–physiology‐performance nexus: Lessons from freshwater stickleback morphs

“…Negative directional selection gradients have been reported underlying reduced SMR via survival probability in an invasive population of garden snails (Cornu aspersum) in central Chile (Artacho & Nespolo, 2009). Yet when surveyed less than a decade later, stabilizing selection on SMR was inferred in the same population and at the same geographic location, in which mortality was low for snails originating from multiple climatic zones; negative selection gradients were inferred in a contrasting climactic zone (Bartheld et al, 2015). Similar patterns of interannual variation in selection gradients on basal metabolic rate have also been observed in wild blue tits, Cyanistes caeruleus (Nilsson & Nilsson, 2016).…”