Heterochrony and Evolutionary Processes

Cope's Rule




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


E. D. Cope proposed that, in general, organisms tend to larger sizes as a lineage's evolution proceeds. There are a number of possible explanations for why this regularity is observed (particularly in vertebrate groups). It is supposed, for instance, that new lineages arise at a small size relative to the ecologically optimal size. Recognition of the importance of heterochrony allows for a deeper explanation.


E.D. Cope - The Man with the Whiskers.

This graph shows the size range of North American rodents and illustrates the shift to a wider range of sizes, particularly large ones. Length of the lower first molar used as a proxy for size. From Benton (1990).

Heterochrony may be the mechanism that drives Cope's rule-type changes along lineages: peramorphosis tends to increase the size of descendants. Moreover, size increases tend to occur extremely fast, which supports the possibility of heterochrony being the active process: by exploiting pre-existing evolutionary pathways, extremely fast evolutionary change can result. The question then becomes: what causes peramorphosis? One suggestion is that size increases occur as a result of K-selective regimes : the subject of the next page.