The maintenance of ecological and evolutionary variability in small populations

Small populations often fail to thrive, as the low levels of variation that are frequently present in small populations hinder the ability to acclimate and adapt to future environmental challenges. Despite this well-known phenomenon, many small populations persist but the factors that mediate the success of these populations are often poorly understood. In this dissertation, I conduct three studies that vary in scope and scale to better contextualize the ecological and evolutionary contexts that may lead to small population persistence. The first chapter synthesizes the theoretical and empirical evidence that allelic dominance can exert a strong influence on genetic diversity after a sharp population reduction. My findings demonstrate that dominance of one allele over another can increase additive genetic variation after a population reduction. However, maintaining the maximum amount of additive variation possible with allelic dominance may require more individuals than if two alleles have only additive effects. The second chapter focuses on elucidating the patterns of establishment and spread of three invasive anuran species that underwent sharp population declines upon their introduction to novel environments. The results from my study highlight different introduction and establishment histories that have all led to the success of each invasive species. The third and final chapter evaluates the maintenance of genomic variation in a small population with strong sexual selection, a process expected to erode variation. I tested the fit of empirical genomic and phenotypic variability data to theoretical models and found little support for the hypothesis of genic capture to maintain genetic variation. I thus discuss alternative mechanisms that may be responsible for maintaining diversity in this system. The maintenance of variation in small populations involves a complex interplay of ecological, environmental, and evolutionary forces but understanding how variation is sustained in small and imperiled populations is crucial for addressing challenges in conservation biology and the management of invasive species.