Heterochrony

Heterochrony, broadly defined, refers to evolutionary change in the rate or timing of development. The concept has long been central to evolutionary developmental biology and remains actively investigated, but both the meaning of the word and the evolutionary significance of heterochrony have changed more than once since the word was first introduced in Haeckel’s Naturliche Schopfungsgeschichte (Berlin: Georg Reime, 1868). Gould’s Ontogeny and Phylogeny (Cambridge, MA: Belknap Press of Harvard University Press) is the foundational work that reinvigorated the study of heterochrony; the first half of that book is a historical overview of the subject; the second half is about the significance of heterochrony (and of development) for evolution. Heterochrony has been a major focus of evolutionary developmental biology, but the definition of the term and its biological significance have repeatedly changed. There are at least four distinct concepts of heterochrony, two of which are nearly mutually exclusive, and the others partially overlap both. Since the 2010s, that conceptual incoherence has decreased, but heterochrony remains a complex concept because of the disparate views about what it means as a word, and what follows from that: the biological significance of heterochrony and the methods for detecting it empirically. The concept remains a focus of evolutionary developmental biology because the core of ideas central to most concepts of heterochrony are now central to the field: the related concepts of “dissociability,” “modularity,” and “integration,” on which definitions (and theories) of heterochrony are grounded. Dissociability is the ability to alter individual developmental processes or stages independently of others, and it is dissociability that allows for shifts in (relative) developmental timing or rate. That concept links heterochrony to theories about developmental integration and modularity and thus to theories of intrinsic constraints. The major objective of this article is to clarify the conceptual frameworks associated with each definition of heterochrony and to highlight major themes in the literature, keeping semantic issues to a minimum.