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

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, life history) are most likely to be targets of natural selection? Do particular combinations of traits maximize fitness? In a semi-natural experiment, we analyzed 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 (semiarid North to humid South). We then released ca. 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) traits were measured in all snails before the release. Survival was recorded monthly during one year. However, we only detected significant selection on body size (MB), residual standard metabolic rate (SMRR) and growth rate. 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 growth rate 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.

进化生态学的核心议题之一，是探究不同功能性状如何影响适合度（fitness），以及该影响在种群间的变异模式。例如，表型性状是否会沿地理梯度对适合度产生趋同的影响？哪些生理、形态、生活史类性状最可能成为自然选择的作用靶标？特定的性状组合能否实现适合度的最大化？本研究依托半自然实验体系，对智利境内的入侵物种——庭园蜗牛（Cornu aspersum）的引入种群开展分析。该物种的这些引入种群尽管地理间距较大，但遗传分化水平极低。本研究旨在明确，蜗牛种群所受自然选择的强度、方向与形式，能否解释种群间的分化（或未出现分化的情况）。我们选取了沿1300公里纬度梯度分布、气候差异显著的三个种群（拉塞雷纳、孔斯蒂图西翁与瓦尔迪维亚，北部为半干旱气候，南部为湿润气候），并设置了同质园（common garden）/互易移植（reciprocal transplant）实验。在该纬度梯度的两个极端区域（拉塞雷纳与瓦尔迪维亚）的重复围栏生境中，我们分别投放了约450头每种群的个体——这些个体为野外捕获蜗牛繁殖两代后的后代。投放前，我们对所有个体的形态性状（体型、螺壳黑化程度）、生理性状（标准代谢率、消化效率）以及生活史性状（生长速率）进行了测定。在为期一年的实验周期内，我们每月记录蜗牛的存活状况。不过，研究仅检测到针对体型（MB）、残余标准代谢率（SMRR）与生长速率的显著选择作用。在拉塞雷纳饲养时，源自拉塞雷纳种群的蜗牛存活率显著高于孔斯蒂图西翁与瓦尔迪维亚种群的个体；但在瓦尔迪维亚，不同源种群的蜗牛存活率并无显著差异。值得关注的是，在拉塞雷纳区域，我们检测到体型与残余标准代谢率间的负向相关选择；而在瓦尔迪维亚区域，仅检测到针对生长速率与体型的定向选择，以及针对残余标准代谢率的稳定选择。上述结果表明，与能量分配相关的生理性状所受的选择作用普遍存在，且不受气候与地理间距的影响。