Phylogeographic patterns and species delimitation in the endangered silverside "humboldtianum" clade (Pisces: Atherinopsidae) in central Mexico: understanding their evolutionary history.

The Atherinopsidae is the second largest group of freshwater fishes occupying central Mexico and is one of biological, cultural, and economic importance. The "humboldtianum" clade (Genus Chirostoma) is a "species flock" of nine described species that inhabit lacustrine ecosystems in central Mexico. The high morphological polymorphism within the group makes species identification difficult and thereby limits the development of research and management projects focusing on this group. In this study, we used phylogeographic and coalescent-based methods to understand the evolution of genetic variation among these species. The results revealed taxonomic inaccuracies and genetic admixture among species. Genetic variation was structured geographically, rather than taxonomically, and five closely related genetic groups were recovered. Two evolutionary pathways were found. First, a novel geographical arrangement of haplotypes was recovered that gave rise to the five recently (Pleistocene, < 1 Myr) derived genetic groups. The second pathway showed a recent intra-lacustrine genetic differentiation that could be associated with sympatric or ecological speciation. The current classification of the group is revised and includes a reduction in the number of valid species in the "humboldtianum" clade. Moreover, this study provides new insight into the biogeography and evolutionary history of this important group of fishes. [ABSTRACT FROM AUTHOR]

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