Characterizing the extent and drivers of intraspecific variation in alewives

Variation below the species level is vitally important to ecological and evolutionary processes, but is a topic that has received less attention and study than other forms of biological diversity. In this dissertation, I sought to enhance our understanding of the extent and drivers of intraspecific variation generally, and specifically in a single species of anadromous fish. Focusing on a culturally and ecologically important fish species native to New England, the alewife (Alosa pseudoharengus), I explored phenotypic and genetic variation between its two life history forms, as well as the potential drivers of these differences. I then investigated whether intraspecific competition and resource depletion affected variation within alewife populations, as well as how these factors shaped diet variation more generally.;I documented differences in body shape and diet source between anadromous (sea-run, migratory) alewife life history forms. I then compared these patterns of divergence to those observed in other landlocked and anadromous species pairs (e.g., salmonids) as a means of testing hypotheses about the drivers of the phenotypic divergence observed in alewives. This comparative analysis suggests that landlocked populations diverged from their anadromous ancestor as a result of their own ecological impacts, rather than as a result of differences in their migratory behavior or due to differences in their access to lacustrine habitats. This provides important inferential evidence for the idea that organisms can shape their own evolutionary trajectory; an idea that is not often considered when studying adaptation.;To explore the genetic variation underpinning the divergence among alewife life history forms, I utilized a genotype by sequencing approach to identified thousands of putative single nucleotide polymorphisms (SNPs) in alewives. I then analyzed this dataset to determine if there were SNPs that exhibited signals of being under selection in the landlocked alewife populations, with the ultimate goal determining if there is any evidence of genetic parallelism in these independently derived populations. These analyses indicated that as many as ten SNPs may be under selection in each of the landlocked populations, however, only two of these outlier SNPs were shared by two or more landlocked populations, suggesting that each population has evolved since becoming landlocked, but providing little support for the idea that the genetic mechanisms underlying landlocked alewife evolution are parallel. Instead, these results suggested two possibilities 1) that the divergence of alewife forms may result from convergence rather than parallelism, or 2) that if parallel adaptation is occurring it may not be due to a hard sweep at a few loci of large effect, but instead as a result of genetic changes less easily detected by genomes scans.;In the next two chapters I shifted my focus to the drivers of intraspecific variation within populations. Using anadromous alewives as a model species, I first tested the long-standing theory that suggests that intraspecific competition should increase the diversity of resources consumed by a population in anadromous alewives. For this, I utilized a survey of natural populations, as well as large-scale mesocosm experimental manipulations. My results suggest that intraspecific competition in alewife, which results from increasing density, actually decreases dietary diversity at the population level as a result of the strong effect of alewife on their resources. This provides an important contrast to existing work from other systems, which have used species with smaller per-capita effects to explore this phenomenon, and suggests that more work is needed to understand the drivers of variation within populations.;I then I revisited the idea that competition among conspecifics acts as a diversifying force. While this idea is tightly integrated into our thinking about adaptation and maintenance of variation, empirical tests of this concept have found inconclusive results. Additionally, recent theory suggests that resource depletion by consumers appears to modify the relationship between competition and diet breadth, and thus may help to explain some of the variation in empirical results. To test this recent theory, I compiled data from existing studies on consumer diet variation, resource depletion and population density, and performed a mixed-model analysis. My results indicate that the interaction strength of a consumer acts to limit the diversifying effect of competition, as a direct result of the extirpation of preferred resources at high densities. This suggests that for consumers with large direct effects on their resources, intraspecific competition will reduce (rather than promote) variation within populations. This work provides valuable information about the potential drivers of variation within populations, and suggests more work is needed to refine our understanding of the role of competition among conspecific consumers.