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 modular ity during limb development. Reduced limb structures typically develop thro ugh a process of construction followed by destruction and amniotes have evo lved many digit-like structures rather than actual extra digits. In amniote s, limb development occurs during the crucial phylotypic stage, when many i nductive interactions are occurring throughout the body. As a result, chang es in limb development usually engender changes in other body parts. Thus, mutations that change the number of limb bones are expected to have many pl eiotropic effects, which severely reduces the chance of such mutations bein g successful. In amphibians with aquatic larvae, limb development occurs af ter the phylotypic stage and limb development is decoupled from the interac tivity of the phylotypic stage. The constraint of pleiotropic effects is, t herefore, expected to be weaker. This expectation agrees with the larger va riability in the number of hand and foot structures in amphibians, with fro gs 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.