Heterochrony and Evolutionary Processes

Introduction Glossary 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 References.
	It is not necessary to go to the fossil record to find examples of heterochrony. Changing developmental timing can have morphological effects even within a species -- not only between an ancestor and descendant. The onset of maturity in some species of salamander changes as a result of environmental conditions.
	If population densities are high, the onset of maturity will be delayed, and salamanders will move out of an aquatic environment to procreate. If population densities are low, maturity will occur precociously (i.e. paedomorphically) and salamanders will reproduce while in a tadpole stage.
The phenomena of sexual dimorphism is fairly ubiquitous -- it describes the occurance of morphological differences between female and male members of a species beyond simply the differentiation of sexual organs (plumage in birds is an example). Many cases of sexual dimorphism occur as a result of heterochronic processes where the onset of maturity or the rate of development varies across sex, creating morphological differences.

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