From individuals to species: how natural selection and phenotypic plasticity shape ecomorphological evolution in freshwater mussels

Abstract

Adaptation is the hallmark of evolutionary biology, explaining how species achieve ecological success through natural selection. However, adaptation is challenging to identify leading to frequent ‘just-so stories’ to explain the adaptive features of organisms. At the core of adaptive studies is the motivation to find the fit between morphological and functional diversity. Here I used the freshwater mussels of North America as a study system to investigate the fit between morphological and ecological traits both within and across species. I used comparative and experimental inferences to identify the evolutionary mechanisms driving ecomorphological patterns. My first chapter identified ecomorphological patterns within and across species between shell thickness, shell anterior thickening, and flow rate. Across species, I found widespread convergence in these traits showing that natural selection produces the following adaptations to riverine flow rates: thick and anteriorly thickened shells in high flow rates (likely for stability in the substratum) and thin and uniformly thickened shells in low flow rates (likely for burrowing efficiency). Additionally, within species, I found a creditably positive relationship between shell thickness and flow rate, effectively mirroring interspecific relationships albeit at different scales. Intraspecific processes may therefore be partially responsible for the evolvability and ecological diversification of the clade. Although I identified this intraspecific ecomorphological pattern, I could not identify the mechanism producing this pattern. To address this, in my second chapter I conducted a common garden experiment on a morphologically variable species, Pyganodon grandis. The morphology of this species varies predictably between lake and stream environments and I investigated if this relationship was due to phenotypic plasticity or genetic differentiation. By rearing siblings from a single female’s broodstock, I minimized genetic variation, and released ~6,000 marked individuals into nine sites (four streams, five lakes). Two years after release, I recaptured a total of 70 individuals from both stream and lake sites showing significant shell shape differences between habitats and no shell shape differences between recaptured siblings and wild P. grandis reared at the same site, showing definitively that phenotypic plasticity rather than genetic differentiation is driving ecomorphological patterns. In my third and final chapter I ran a fluvial experiment investigating the function of mussel posterior ‘ribbed’ sculpture. I measured water velocity magnitude, direction, and streambed erosion surrounding mussel models with sculpture and with their sculpture manually removed. In opposition to previous studies, I found more streambed erosion associated with sculptured models. However, mussel orientation to streamflow was the more significant driver to variations in water velocity magnitude, direction, and streambed erosion. This body of work illustrates the complementary nature of phylogenetic comparative methods and experiments to finding the evolutionary mechanisms of phenotypic variation. Lastly, the role of phenotypic plasticity in macroevolutionary outcomes has seldom been investigated but the widespread convergence of ecomorphological traits in chapter 1 and common garden experiment in chapter 2 suggest plasticity may be a key mechanism to macroevolutionary diversification.