Fig. 4

A consistent pattern of terminal addition in the evolution of ontogenetic sequences, as illustrated here for four species in abstract terms, leads to ontogenetic recapitulation. Each species is characterized by a common developmental trajectory consisting of the first step, M1→M1. A new feature is added at the end of the sequence. In this ideal case, species “D”, the one with the most specialized condition, contains in its ontogeny the sequence of evolutionary transformations. But there can be deviations from the recapitulatory pattern. Features in the sequence can move around, one or more of them can be deleted, or a whole new feature can appear. When those changes are of great evolutionary significance, they are thought of as an evolutionary innovation, as in the origin of hair in mammals or feathers in dinosaurs. Among the different kinds of deviations from recapitulation are heterochrony—changes in timing—and heterotopy—changes in spatial position in a structure. Modified from Sánchez-Villagra (2012), based on Wägele (2005)