Why five fingers? Evolutionary constraints on digit numbers

Evolutionary changes in the number of digits and other limb elements appear to be severely constrained, probably as a result of a low level of modularity during limb development. Reduced limb structures typically develop through a process of construction followed by destruction and amniotes have evolved many digit-like structures rather than actual extra digits. In amniotes, limb development occurs during the crucial phylotypic stage, when many inductive interactions are occurring throughout the body. As a result, changes in limb development usually engender changes in other body parts. Thus, mutations that change the number of limb bones are expected to have many pleiotropic effects, which severely reduces the chance of such mutations being successful. In amphibians with aquatic larvae, limb development occurs after the phylotypic stage and limb development is decoupled from the interactivity of the phylotypic stage. The constraint of pleiotropic effects is, therefore, expected to be weaker. This expectation agrees with the larger variability in the number of hand and foot structures in amphibians, with frogs even occasionally possessing six toes. These facts once again emphasize the importance of pleiotropic effects as constraints to evolutionary change, including their role in the conservation of body plans.