showing intermediate features, is known from the Granadas Cave, part of the Balsas River drainage in Guerrero, southern Mexico, but it is a part of A. aeneus (itself sometimes included in A. mexicanus). Evolution research The surface and cave forms of the Mexican tetra have proven powerful subjects for scientists studying evolution. When the surface-dwelling ancestors of current cave populations entered the subterranean environment, the change in ecological conditions rendered their phenotype—which included many biological functions dependent on the presence of light—subject to natural selection and genetic drift. One of the most striking changes to evolve was the loss of eyes. This is referred to as a "regressive trait" because the surface fish that originally colonized caves possessed eyes. In addition to regressive traits, cave forms evolved "constructive traits". In contrast to regressive traits, the purpose or benefit of constructive traits is generally accepted. Active research focuses on the mechanisms driving the evolution of regressive traits, such as the loss of eyes, in A. mexicanus. Recent studies have produced evidence that the mechanism may be direct selection, or indirect selection through antagonistic pleiotropy, rather than genetic drift and neutral mutation, the traditionally favored hypothesis for regressive evolution. The blind form of the Mexican tetra is different from the surface-dwelling form in a number of ways, including having unpigmented skin, having a better olfactory sense by having taste buds all over its head, and by being able to store four times more energy as fat, allowing it to deal with irregular food supplies more effectively. Darwin said of sightless fish: Modern genetics has made clear that the lack of use does not, in itself, necessitate a feature's disappearance. In this context, the positive genetic benefits have to be considered, i.e., what advantages are obtained by cave-dwelling tetras by losing their eyes? Possible explanations include: Not developing eyes allows the individual more energy for growth but not egg production. However the species does use other methods to locate food and detect danger, which also consume energy that would be conserved if it had eyes or transparent eyelids. There remains less chance of accidental damage and infection, since the previously useless and exposed organ is sealed with a flap of protective skin. It is unknown why this species did not develop transparent skin or eyelids instead, as some species of reptiles did. The lack of eyes disables the "body clock", which is controlled