Developmental constraints and the evolution of vertebrate digit patterns.

The skeletal makeup of the digits of 145 hands and feet from species from the four classes of tetrapod vertebrates is analysed. The analysis leads to the conclusion that developmental constraints are very influential in the evolution of vertebrate digit patterns. Furthermore, 98% of the patterns analysed fall within the specific constraints of the Stock & Bryant (1981) version of the polar coordinate model for the formation of digit patterns during development and pattern regulation. Three cases of apparently forbidden morphology are discussed in terms of the phenomenon of differential growth during development.