Chapter 5 - Heterochrony

Abstract

This chapter discusses heterochrony, which is an ancestor–descendant difference in the relative timing of events during development. This phylogenetic pattern frequently has been confused with ecological and evolutionary processes that result in diverse larval forms that also vary in the timing of developmental events. Presently, heterochrony is considered as a process affecting the origin and evolution of larval forms. Discussions of heterochrony rarely distinguish patterns from processes, and heterochronic patterns are defined as paedomorphosis, truncation of development of a trait relative to development of the trait in an ancestor; or peramorphosis, extension of development relative to an ancestor. Heterochronic processes can produce these two patterns: deceleration and acceleration, the decrease or increase in k, the rate of development of the trait; hypomorphosis and hypermorphosis, the decrease or increase in β, the age at termination of development of the trait; and postdisplacement and predisplacement, the increase or decrease in α, the age at initiation of development of the trait. A combination of these heterochronic processes can result in a heterochronic pattern in which ancestor and descendant have the same phenotype, a pattern called isomorphosis.