The generation and maintenance of diversity in a rapid adaptive radiation
Parnell, Nicholas Francis
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The Lake Malawi cichlid fishes are a pre-eminent example of adaptive evolutionary radiation. The diversity of species (nearly 1000 extant) is mirrored by an array of variation in dozens of phenotypes (e.g. trophic morphology, tooth shape, color patterns, behavior, development). The unique characteristics of this system have produced unparalleled diversity with very little genetic differentiation between species. This dissertation is composed of four studies addressing different aspects of the variation in the LM cichlids and the mechanisms generating and maintaining this level of diversity at multiple biological levels. Community-level diversity is investigated using null model analysis of species co-occurrence data. We detect signals of non-random community assembly at only the broadest and finest spatial scales. Based on the unique ecological and evolutionary characteristics of this assemblage we suggest that different mechanisms are responsible for these patterns. A core‟ group of species is posited to act as a foundation on which these diverse communities are created as a result of fine-scale species interactions. We identify both positive and negative depth-based correlations between species and suggest these interactions play an important role in species diversity in these fish. The Lake Malawi cichlids exhibit an array of trophic morphologies which may play a role in the fine-scale species interactions described in chapter one. In the second chapter we build a genetic model to predict the evolution of jaw morphology and a complex functional jaw trait. We use a complex biomechanical system, the 4-bar jaw linkage, to simulate trait evolution during interspecific hybridizations. We find rampant transgression (trait values beyond parental distributions) in jaw function in a large proportion of potential crosses. This result is characterized by a lack of novel morphological components but rather is the result of recombinations of existing component traits thus producing functional novelty. In the third chapter we create a laboratory cross of one of the parental combinations suggested from the genetic model. The results of this study serve as a proof of principle to the simulations as we observe a large proportion of transgressive 4-bar function in the F2. As predicted this diversity is produced in the absence of transgressive morphology. We contrast these results between this complex system and data generated from several simple jaw lever traits and report differences in the patterns. Using quantitative trait locus (QTL) mapping approaches we examine the genetic basis for complex and simple jaw traits and discuss correlative patterns within and between systems. Finally we examine the genetic architecture of sex-determination and color morphs in this hybrid cross. We find both ZW and XY sex systems segregating as well as linkage to sex-specific color patterns. Several loci and epistatic interactions are associated with sex-determination and color morphs in this cross. The orange-blotch (OB) color is found associated with ZW as predicted from previous work but a previously undescribed (in these species) male nuptial color (blue) is found associated with both ZW and XY genetic systems as well as other loci segregating for sex-determination. These results are discussed in the context of models of sex chromosome evolution as a result of sexual conflict and the potential importance of sexual selection in the diversification of Lake Malawi cichlids. Overall we observe various mechanisms generating and maintaining diversity at different levels of biological organization. We use community co-occurrence analyses, genetic simulation, and QTL analysis of an F2 hybrid population to examine these mechanisms in this rapidly radiating assemblage. These results bolster our understanding of the origins of diversity and the interplay between variation and aspects of evolution in all biological systems.