The Mexican characid Astyanax mexicanus gave rise to a series of cavcrnicolous populations (“Anoptichthys”) not earlier than during the Pleistocene age.
When searching for food the blind cave fish swims at an angle of about 45° to the ground. The river fish, however, when having no visual information, stands vertically on its head. With the aid of infrarcd-videorecording and on the basis of crossing experiments it could be demonstrated that this difference in feeding behaviour is controlled by polygenes, although frequency distributions of the crossings suggest an apparent monofactorial inheritance.
The evolutionary processes of adaptive reduction of headstanding and of progressive development of the gustatory equipment are genetically independent and both traits have been achieved by small genetic steps.