Speciation is at the core of evolutionary biology and is the driving force behind biodiversity. Thus, a fundamental goal is to understand the factors, both historical and contemporaneous, that generate, structure and maintain species biodiversity and the undertaking requires a combination of approaches that can span the temporal scale of the speciation. Darters, a sexually dichromatic and species-rich teleost fish clade endemic to North America, and particularly the Orangethroat darter clade Ceasia, represent an excellent model for investigating the evolutionary processes that contribute to phenotypic and species diversity. I took an integrative approach utilizing molecular markers in combination with empirical and comparative approaches to elucidate the following four mechanisms: dispersal via paleodrainage connections, contemporaneous hybridization, postmating isolating barriers, and premating isolation via brilliant visual signals. (i) Given the geographic range of Ceasia was likely influenced by shifting riverine dispersal routes during Pleistocene glacial cycles, I incorporated divergence time estimates of mitochondrial and nuclear phylogenies and modifications of river connectivity over time to investigate the contribution of historical and contemporaneous dispersal. Stability and isolation within and subsequent recolonization from three glacial refugia east and west of the Mississippi River and Pleistocene dispersal between highland regions through the Teays-Mahomet paleodrainage have shaped the present-day genetic variation and geographic distribution of Ceasia. (ii) I employed a suite of polymorphic microsatellite markers and mitochondrial profiles to characterize the extent, direction and temporal scale of hybridization events between Etheostoma bison, a species in the Ceasia clade, and E. caeruleum, a distantly related congener. This population genetic approach revealed a previously undocumented hybrid zone and provided evidence that hybridization is contemporaneous, resulting in a bimodal distribution of genotypes. The presence of first and second-generation hybrids indicated the ability of F1 hybrids to backcross into the parental species, mediating introgression of the mitochondrial genome across species boundaries. In addition, the mitochondrial profiles revealed bi-directional introgression, although the primary signature suggests a higher frequency of hybridization between E. caeruleum females and E. bison males. (iii) I conducted noncompetitive and competitive experimental crosses between Ceasia and E. caeruleum to investigate whether the biased gene flow in a mosaic hybrid zone was associated with postmating isolating barriers. Overall, gametic incompatibility and hybrid inviability were negligible in hybrid formation, whereas conspecific sperm precedence was strong in E. caeruleum clutches, contrary to the primary direction of gene flow (E. caeruleum to Ceasia). This pattern was equivocal across areas with and without introgressive hybridization, indicating postmating barriers do not structure the mosaic hybrid zone. Furthermore, a reduction of hybridity (proportion of admixture) across juvenile and adult life-history stages suggested a fourth postmating barrier, specifically ecological selection against hybrids, could also be acting to maintain the bimodal hybrid zone. (iv) In darters, sexual dichromatism manifests as brilliant carotenoid-, pigment-, and melanin-based colors in male visual signals, providing insight into the tradeoff of between natural selection (predator- or habitat-mediated selection) and sexual selection (mate choice and species recognition). I took a comparative approach to investigate the adaptive evolution of sexual dichromatism and its association with reproductive isolation. First, the evolution of dichromatism in darters is associated with different evolutionary optima in benthic and hyperbenthic (mid-water) habitats. In the hyper-benthic habitat, a narrow distribution around a low dichromatism optimum suggested that dichromatism evolution was constrained potentially by predation-mediated selection against conspicuousness, whereas a wider distribution of dichromatism surrounding a higher optimum in the benthic habitat could be evidence of the greater influence of habitat-mediated selection in diverse microhabitats and the ecological release from predation pressure. The evolution of dichromatism was not associated with elevated diversification rates in darters, instead it had an effect further along the speciation continuum, specifically on the probability of hybridization occurrence. Among sympatric darters of varying ages, dichromatic lineages had a lower probability of hybridization compared to monochromatic lineages, advocating the function of dichromatism in species maintenance rather than formation. The combination of the approaches above (i.e. phylogeographic, population genetic, empirical tests, and phylogenetic comparative analyses) clarified the contribution of various evolutionary mechanisms that ultimately shaped the genetic architecture and species diversity of darters.
|Advisor:||Near, Thomas J.|
|Commitee:||Alonzo, Suzanne H., Donoghue, Michael J., Holsinger, Kent|
|Department:||Ecology and Evolutionary Biology|
|School Location:||United States -- Connecticut|
|Source:||DAI-B 74/05(E), Dissertation Abstracts International|
|Keywords:||Historical biogeography, Introgressive hybridization, Mosaic hybrid zone, Orangethroat darters, Postmating isolation, Reproductive isolating barriers, Sexual dichromatism|
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