Blind Cave Tetra (Astyanax mexicanus), a form of Mexican Tetra, also known as Blind Tetra or Blind Cavefish. Natural selection theory explains that fish living in a completely dark environment may lose their capacity for vision over time. Genetic changes (mutations) leading to a loss of eye function would not be a survival disadvantage and thus would be passed on to offspring. Studies have shown that there are at least two distinct genetic lines of the Blind Tetra indicating that the trait may have evolved separately in different populations - an example of convergent evolution.