Parallel evolution occurs when, under adaptive divergence conditions, similar phenotypic traits evolve independently from similar ancestral states among closely related organisms. These events of parallel evolution occur during ecological speciation when reproductive isolation evolves due to adaptive divergence in association to exploitation of alternative resources. In this sense, speciation can be considered as a process where different stages occur in a continuum. This is from a continuous phenotypic variation without reproductive isolation, to discontinuous adaptive differences with complete reproductive isolation, where sexual selection plays an important role as a force generating the reproductive isolation which reinforce the genetic and phenotypic differentiation of populations. The genus Astyanax is characterized by a remarkable adaptability to diverse environmental conditions. In some Astyanax populations from Mesoamerica a parallel evolution process of ecologically divergent morphs (i.e. elongate and deep-body) have been suggested, possibly related to an ecological speciation process. In this PhD thesis, the parallel evolution and the ecological speciation process in Astyanax from México and Central America were evaluated, using morphological, ecological, genetic, and mate choice analyses. First, I determined the shared morphological patterns of parallel evolution and the unique patterns of divergence between sympatric morphs pairs of Astyanax from three geographic systems: Lake Catemaco, Lakes Ocotalito-Metzabok and San Juan River basin, using body and head shape, and trophic morphology analysis. Second, I characterized the patterns of late ontogenetic allometric trajectories of trophic morphology between sympatric Astyanax morphs from Lake Catemaco and San Juan River. Then, I determined the ecological divergence of Astyanax morphs from San Juan River system by analyzing their trophic level and its correspondence with the genetic differentiation among morphs using mitochondrial and nuclear markers. Finally, I determined the role of mate choice of females as promotor of reproductive isolation between sympatric Astyanax morphs from Lake Catemaco.

In the geographic systems analyzed, I found shared patterns of morphological divergence between sympatric morphs associated to parallel evolution mainly related to trophic morphology divergence. The unique patterns of divergence between morphs were related to historical contingency, finding a more similar body shape between sympatric morphs that between allopatric ones (Chapter II). In the ontogenetic allometric trajectories of trophic morphology in Astyanax morphs from Lake Catemaco and San Juan River, I found parallel trajectories between morphs from Lake Catemaco and divergent trajectories between morphs from San Juan River. These results show the importance of changes in allometric patterns of trophic morphology in the diversification of Astyanax morphs in Mesoamerica (Chapter III). In Chapter IV, divergent morphs of Astyanax from San Juan River system showed trophic ecology differentiation using stable isotopes, with the elongate-body morph showing a higher trophic level. Also, the deep-body morph showed a wider trophic niche in Lakes Nicaragua and Managua. Despite the high morphological and ecological differentiation between morphs, I found a weak genetic differentiation between morphs, according to an incipient ecological speciation scenario. These results, together with previous investigations and accordingly to the colonization history of Astyanax from South America, suggest a geographic cline in the ecological speciation process, where southern morphs (San Juan River system) show an advanced state of ecological speciation in comparison with morphs from northern systems (Lake Catemaco and Lakes Ocotalito-Metzabok). Finally, for morphs from Lake Catemaco, I did not find assortative mating toward its conspecific morph based on association time (Chapter V). This lack of assortative mating is in agreement to an initial state in the process of ecological speciation, where reproductive isolation is absent. This PhD investigation supports the growing evidence of the genus Astyanax as a model in evolutionary biology, mainly in parallel evolution and in ecological specialization and speciation.

Tesis doctoral presentada en la Universidad Nacional Autónoma de México adscrita al Posgrado en Ciencias Biológicas del Instituto de Biología, 157 pp.