Zelditch, M.L. and W.L. Fink

Heterochrony, change in developmental rates and timing, is widely recognized as an agent of evolutionary change. Heterotopy, evolutionary change in spatial patterning of development, is less widely known or understood. Although Haeckel coined the term as a complement to heterochrony in 1866, few studies have detected heterotopy nor even considered the 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 ontogenies, and hence result in parallelism between ontogeny and phylogeny. Heterotopy can produce new morphologies along trajectories different from those that generated the forms of ancestors. We argue that the study of heterochrony has been bound to an analytical formalism that virtually precludes the recognition of heterotopy, so we provide a new framework for the construction of ontogenetic trajectories, and illustrate their analysis in a phylogenetic context. The study of development of form needs tools that capture not only rates of development, but the space in which the changes are manifest. The framework outlined here provides tools applicable to both. When appropriate tools are used and the necessary steps are taken, a more comprehensive interpretation of evolutionary change in development becomes possible. We suspect that there will be very few cases of change solely in developmental rate and timing or change solely in spatial patterning; most ontogenies evolve by changes of spatiotemporal pattern.