HETEROCHRONY AND HETEROTOPY - STABILITY AND INNOVATION IN THE EVOLUTION OF FORM

Heterochrony, change in developmental rate and timing, is widely recog nized as an agent of evolutionary change. Heterotopy, evolutionary cha nge in spatial patterning of development, is less widely known or unde rstood. Although Haeckel coined the term as a complement to heterochro ny in 1866, few studies have detected heterotopy or even considered th e possibility that it might play a role in morphological evolution. We here review the roles of heterochrony and heterotopy in evolution and discuss how they can be detected. Heterochrony is of interest in part because it can produce novelties constrained along ancestral ontogeni es, and hence result in parallelism between ontogeny and phylogeny. He terotopy can produce new morphologies along trajectories different fro m those that generated the forms of ancestors. We argue that the study of heterochrony has been bound to an analytical formalism that virtua lly precludes the recognition of heterotopy, so we provide a new frame work for the construction of ontogenetic trajectories and illustrate t heir analysis in a phylogenetic context. The study of development of f orm needs tools that capture not only rates of development but the spa ce in which the changes are manifest. The framework outlined here prov ides tools applicable to both. When appropriate tools are used and the necessary steps are taken, a more comprehensive interpretation of evo lutionary change in development becomes possible. We suspect that ther e will be very few cases of change solely in developmental rate and ti ming or change solely in spatial patterning; most ontogenies evolve by changes of spatiotemporal pattern.