Phase Fixation

a type of phylogenetic change in the organs of animals in which one of the phases in the functioning of an animal’s organ becomes the only phase in the animal’s descendants. Phase fixation leads to the morphological change of the organ.

An example of phase fixation is the change from plantigrade locomotion to digitigrade locomotion in mammals. In plantigrades, such as bears, support on the digits is the final phase of support during walking and the only phase of support during running. Digitigrades, such as dogs and cats, are supported by the digits alone in both walking and running. As a result, the supporting surface of the extremities was diminished and each extremity gained an extra lever. Both factors promoted an increase in the velocity of locomotion of the animals.