Heterochrony and Evolutionary Processes

Sexual Dimorphism




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


Dynastes hercules-male

An even more striking example of the profound effects that heterochrony can have on morphology is shown by Dynastes hercules, another charming beetle (this time the male is about 13.5 inches long). Positive allometries and peramorphic (relative to the female) development has caused the male to sport a massive cephalic horn. Again, the target of selection is in doubt. It seems "obvious" that there has been selection for the horn in the male, but the selection target may have simply been for large size, with the horn growth as a selection-neutral (or even at a cost to fitness?) consequence of allometric growth.


Dynastes hercules-female

Of course selection may not be operating on these beetles (Dynastes as well as Macrodontia) at all. Changes in developmental timing, producing heterochrony, can be triggered by a very small number (one?) mutation in the genes controlling timing. Now, while small gene changes in structural genes affecting development early in ontogeny often lead to a decrease in fitness, a small change in genes controlling developmental timing can lead to profound morphological differences that may be selectively neutral. This is because these changes occur along pre-existing developmental pathways -- tried and true recipes for building a viable organism. If this is possible, then the striking morphological differences between the male and female beetles seen here may be the result of genetic drift and thus be of no adaptive or functional signifigance.