Heterochrony and Evolutionary Processes





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



Phylogeny: The evolutionary history (i.e. descent) of an organism.

Ontogeny: The developmental history of an organism.

It was (and sometimes still is) thought that the two are related by...

Recapitulation: During development, and organism passes through ("recapitulates") the adult forms of its ancestors. Ernst Haeckel held this to be a law of nature, which was referred to as the Biogenetic Law.

Though not common, recapitulation does occur as a result of a more general process:

Heterochrony: A process whereby morphological change is effectuated by relative changes in timing of developmental events, including the onset of maturity.

As a result, descendants may not only recapitulate the adult forms of their ancestors, but the descendant adult may resemble the juvenile form of its ancestor. This is:

Paedomorphosis: The retention of ancestral juvenile characters in the descendant. Descendants may as a whole appear like juvenile ancestors, or perhaps only a specific feature may appear like the analogous ancestral juvenile trait.

This is, in essence, the converse of recapitulation, which is now referred to as:

Peramorphosis: The occurance of ancestral adult characters in descendant juveniles.

As usual, there is a tendency to get carried away with terminology (jargon?). Don't worry about it: hopefully, the ideas behind studying heterochrony can be examined without having to wallow in technicalities.

Continue navigating with the sidebar, or click here to continue with a foray into the historical antecedants of ideas about heterochrony.