Evolutionary relationships and historical biogeography of pygmy sunfishes (percomorphacea elassoma)

Abstract

The mechanisms responsible for generation and maintenance of a freshwater biodiversity gradient in southeastern North America have received extensive scientific inquiry, but a synthetic understanding of their relative importance has not been attained. Recent advancements in DNA sequencing technologies and statistical methods are poised to generate and analyze the data necessary to identify such mechanisms, but this work should be preceded by a foundational knowledge of organismal biology and basic phylogeny. Pygmy sunfishes (Percomorphacea: Centrarchidae: Elassoma) are endemic to an aquatic biodiversity "hotspot" in the southeast, but are among the least-studied groups of North American freshwater fishes. I used molecular phylogenetics to infer evolutionary affinities within and relationships within Elassoma, and this information was used to address questions spanning a broad spatiotemporal scale. The goals of each study were to, 1) identify repeated phylogenetic and biogeographic patterns, 2) develop strategies to reconcile errors that previously impeded pattern recognition, and 3) assess putative processes that may explain the observed patterns. Chapter One includes a search of relevant literature and a discussion of taxonomic and conservation issues surrounding this group. Chapter Two is a test of hypothesized sister group relationships between Elassoma and other taxa within Percomorphacea, a problem that has plagued the systematic ichthyology community for decades. This study is the first to combine nuclear and mitochondrial protein-coding genes in a broadly sampled analysis of spiny ray-finned fishes. In addition, a new taxonomic proposal is presented that recognizes three major clades within Percomorphacea. Chapter Three reconstructs phylogenetic relationships among pygmy sunfish species using two nuclear genes and one mitochondrial gene. This study presents the first evidence for an Elassoma phylogeny that is independently corroborated by multiple loci and strongly supported in a combined analysis. In addition, this study identifies trends pertaining to the evolution of sexual dimorphism within the genus, and implicates sexual selection as a potential mechanism promoting ecological compatibility among closely related species. Chapter Four examines phylogeographic structure of the most widespread pygmy sunfish using the mitochondrial cytochrome b (CYTB) gene. This study reveals an unprecedented number of pre-Pleistocene lineages for a species of the Gulf-Atlantic Coastal Plain, and adjusts existing vicariance hypotheses to accord with Miocene geoclimatic events. Results of all three studies provide novel insights into the mechanisms responsible for the origin and maintenance of an aquatic biodiversity gradient in southeastern North America.