of their skull, which is propounded to increase water flow to the opposite side of the face, functionally enhancing sensory input and spatial mapping in the dark waters of caves. Scientists suggest that gene cystathionine beta synthase-a mutation restricts blood flow to cavefish eyes during a critical stage of growth so the eyes are covered by skin. Currently, about 30 cave populations are known, dispersed over three geographically distinct areas in a karst region of San Luis Potosí and far southern Tamaulipas, northeastern Mexico. Among the various cave population are at least three with only full cave forms (blind and without pigment), at least eleven with cave, "normal" and intermediate forms, and at least one with both cave and "normal" forms but no intermediates. Studies suggest at least two distinct genetic lineages occur among the blind populations, and the current distribution of populations arose by at least five independent invasions. Furthermore, cave populations have a very recent origin (< 20,000 years) in which blindness or reduced vision evolved convergently after surface ancestors populated several caves independently and at different times. This recent origin suggests that the phenotypic changes in cavefish populations, namely eye degeneration, arose as a result of the high fixation of genetic variants present in surface fish populations in a short period of time. The eyed and eyeless forms of A. mexicanus, being members of the same species, are closely related and can interbreed making this species an excellent model organism for examining convergent and parallel evolution, regressive evolution in cave animals, and the genetic basis of regressive traits. This, combined with the ease of maintaining the species in captivity, has made it the most studied cavefish and likely also the most studied cave organism overall. The blind and colorless cave form of A. mexicanus is sometimes recognized as a separate species, A. jordani, but this leaves the remaining A. mexicanus as a paraphyletic species and A. jordani as polyphyletic. The Cueva Chica Cave in the southern part of the Sierra del Abra system is the type locality for A. jordani. Other blind populations were initially also recognized as separate species, including antrobius described in 1946 from the Pachón Cave and hubbsi described in 1947 from the Los Sabinos Cave (both subsequently merged into jordani/mexicanus). The most divergent cave population is the one in Los Sabinos. Another cave-adapted population of Astyanax, varying from blind and depigmented to individuals