Heterochrony and Evolutionary Processes

Recognising Heterochrony




Historical Antecedants

Recognising Heterochrony

Modern examples: Sexual Dimorphism

Cambrian trilobites

Cope's Rule

K- and r- selection: Tertiary echinoids

Consequences for debates on adaptation, constraints and evolutionary dynamics


A difficulty arises when trying to identify the existence of heterochronic relationships between species and their putative ancestors. It is crucial to be able to distinguish between the juvenile and adult forms of a species in question. Often, it is difficult to establish what stage of ontogenic development a given specimen represents. For certain species, growth lines (on shells, e.g.) indicate the age of the specimen, and thus whether the specimen is relatively juvenile or adult. Below are a few examples, from Jones (1988), of using growth lines as ontogenic metrics.

Banding in fish scales.

Growth lines in mountain goat horns.

Root annulations on black bear teeth.

In many cases, developing a suitable proxy for the developmental stage a specimen represents is a fairly involved process. Given a population of the same species, size can be used in order to determine a more widely applicable metric.