Morphological variance in cave and surface populations of Asellus aquaticus challenges the fluctuating selection and greater male variability hypotheses

Introduction Fluctuating selection, the changes in the strength and/or direction of natural selection stemming from fine-scale spatio-temporal environmental variation, is a fundamental, yet rarely tested explanation for the maintenance of phenotypic variation within populations. Asellus aquaticus is an aquatic isopod inhabiting both surface (highly variable, complex environment) and cave (stable, simple environment) habitats, providing an excellent model to test the fluctuating selection hypothesis. The inclusion of multiple populations allows to explore variance patterns among-populations. We predicted that within-population variance (V wp ) is lower in caves due to negligible environmental variation. Regarding among-population variance (V ip ) the study was exploratory as in caves, (i) lower variance could be expected due to their high environmental similarity, but (ii) higher variance could be expected due to the mutual isolation of caves. Finaly, we tested whether males exhibit higher morphological variance than females following predictions of the greater male variability hypothesis. Methods We analysed the variance of 12 functional morphological traits of eight cave and nine surface A. aquaticus populations. Two statistical approaches were applied. Trait variances were determined for each sex using Bayesian double-hierarchical models. Then overall V wp and V ip was estimated for each sex and habitat using Bayesian generalised linear models. Results Overall V wp did not differ between the habitats but in some traits, variance was larger in cave than in surface populations (12.5% of cases). Overall V ip was larger in caves. There were no sex-related V wp patterns, but we found higher V ip in males than in females in the cave habitat. Discussion Our results reject both the fluctuating selection and greater male variability hypotheses. We suggest that equal V wp between habitats is a result of several processes overriding the marked habitat difference in environmental heterogeneity. Higher V ip in caves suggests the importance of isolation allowing local adaptation to cave specificities. Higher V ip in males in caves suggests the role of sexual selection in among-cave population divergence. These results underscore unmeasured differences among the caves and challenge the general view that caves are uniform replicates of simple and stable environments. For solid evolutionary inference, we recommend considering patterns of variance besides means.