The characin Astyanax fasciatus is found both in rivers and streams and in underground caves, but differs in its appearance depending on where it is found.
Now a new study, due to be published in the November issue of the journal Molecular Phylogenetics and Evolution, has finally provided evidence as to how the different forms have evolved.
Strecker, Faundez and Wilkens of the University of Hamburg, Germany, used a molecular technique to look at the mitochondrial DNA (mtDNA) cytochrome b gene of 26 surface and nine cave populations of A. fasciatus and created an evolutionary family tree, called a phylogeny, with the results.
The new phylogeny shows that there are seven distinct groups called clades within the fish studied, which represent their geographical patterns of distribution.
Interestingly, some odd things seemed to have happened to A. fasciatus during its history, and those specimens collected from caves weren't always found to be most closely related to specimens collected from above ground nearby.
For example, the cave-dwelling forms from Piedras, Sabinos, Tinaja, and Curva, have greatly reduced eyes and lack pigment, and form a seperate cluster. However, they're not a sister group to the surface populations from the same region.
Nor do the fasciatus from Belize appear to be most closely related to their relatives in the Yucatan.
The authors propose that cave-dwelling forms have been invaded with populations of surface-dwelling fasciatus, probably influenced by Pleistocene climate change:
"The high level of genetic divergence among the different clades shows that differing haplotype lineages must have reinvaded the surface waters from the south and/or back-colonized them from residual habitats and also penetrated into the caves.
"Nested clade analyses show that recurrent gene flow as well as historic processes like past fragmentation and range expansion have influenced current populations of A. fasciatus in Central and North America.
"Different haplotype clades of the phylogeny are not compatible with the present taxonomy of Astyanax and, therefore, we propose the application of a single systematic unit, called A. fasciatus."
For more details see: Strecker U, Faundez VH, Wilkens H. (2004) - Phylogeography of surface and cave Astyanax (Teleostei) from Central and North America based on cytochrome b sequence data. Mol Phylogenet Evol. 2004 Nov;33(2):469-81.