Hybridization and divergent selection have shaped the evolutionary history of grunts (genus: Haemulon)
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Speciation of marine organisms remains a contentious topic in evolutionary biology. Divergence is particularly difficult to explain in cases where multiple closely related species have the same geographical range and occupy similar habitats. Further, dispersal of pelagic larvae can facilitate genetic connectivity across broad distances. These represent sizable challenges for traditional ideas that consider geographic isolation the sole driver of divergence in marine systems. In recent years, concepts of ecological speciation, where disruptive selection can lead to divergence in the absence of reproductive isolation, have regained the attention of evolutionary biologists. This project focuses on understanding the process of divergence in species of the genus Haemulon, which is composed of multiple sympatric sister species in the Caribbean and Eastern Pacific. The first portion of the dissertation deals with the case of the sister species Haemulon flaviguttatum and H. maculicauda, where shared haplotypes and reduced divergence in the mitochondria indicate recent hybridization. However, hybridization led to scant introgression of the nuclear genome, which allows the maintenance of phenotypic differences between the two groups. Further, there is strong evidence for disruptive selection in coding regions of the nuclear genome. The second portion of this project deals with the Caribbean species H. carbonarium, H. flavolinetaum and H. macrostomum. The sequencing, annotation and analysis of the transcriptome revealed strong positive selection in genes related with reproductive isolation, metabolism, stress response and development of pharyngeal structures. Genes associated with the pharyngeal apparatus could be important for the divergence of the group, as these structures are strongly associated with diet. Hence, a hypothesis that divergence of the group is partly due to dietary partitions is proposed. Overall, this dissertation provides novel evidence that the radiation of grunts was directly influenced by divergent selection. By doing so, this project becomes part of a growing body of work that suggests ecological speciation is fundamental for explaining the outstanding diversity that characterizes coral reef systems.