The Blind Mexican Cave Fish

This is the blind Mexican cavefish (Astyanax mexicanus) along with its sighted cousin the Mexican tetra. 
Credit: Professor Richard Borowsky

 

The blind Mexican cavefish (Astyanax mexicanus) have not only lost their sight but have adapted to perpetual darkness by also losing their pigment (albinism) and having altered sleep patterns. New research published in BioMed Central's open access journal BMC Evolutionary Biology shows that the cavefish are an example of convergent evolution, with several populations repeatedly, and independently, losing their sight and pigmentation.

The blind cavefish and the surface dwelling Mexican tetra, despite appearances, are the same species and can interbreed. The cavefish are simply a variant of the Mexican tetra, albeit one adapted to living in complete darkness. A team of researchers from Portugal, America, and Mexico studied the DNA from 11 populations of cavefish (from three geographic regions) and 10 populations of their surface dwelling cousins to help understand the evolutionary origin of the physical differences between them.
While results from the genotyping showed that the surface populations were genetically very similar, the story for the cave populations was very different. 

 

The cave forms had a much lower genetic diversity, probably as a result of limited space and food. Not surprisingly the cave populations with the most influx from the surface had the highest diversity. In fact there seemed to be a great deal of migration in both directions.

It has been thought that historically at least two groups of fish lived in the rivers of Sierra de El Abra, Mexico. One group originally colonized the caves, but became extinct on the surface. A different population then restocked the rivers and also invaded the caves. more here