Heterochrony

Abstract

Heterochrony describes evolution in the timing of developmental traits and has been an important concept for explanations of phenotypic diversity. Heterochronic processes have the potential to produce tremendous diversification of forms by varying the ontogenetic timing and rate of trait development. When suites of traits are subject to the same heterochronic process, then disparate forms can be produced rapidly. The language of heterochrony has been complicated by several ad hoc additions and revisions of the original terms to suit different types of developmental patterns and processes. Furthermore, heterochronic terms have been applied to both individual traits and more vaguely to whole organisms, which has overshadowed the intricate uses of the concept and led to confused definitions. The analysis of heterochronic patterns must be well grounded in a phylogenetic or paleontological context, as interpretations are contingent upon understanding the conditions of both the descendants and the ancestors. The concept of heterochrony has been primarily used in morphology, even though these principles are broadly applicable to behavioral, ecological, and physiological traits. The recognition of heterochronic patterns and processes across disciplines will more broadly integrate these concepts into biology and provide further insights into the evolution of eukaryotes. This chapter provides definitions for the various types of heterochronic patterns with examples from salamanders, which exhibit extensive ontogenetic evolution. The challenges of analyzing ontogenetic data are also discussed. The evolution of developmental timing can provide a central framework for understanding the ecological drivers and developmental mechanisms that shape phenotypic diversity.