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


Macrodontia Cervicornis-male.

These beauties (unfortunately not to scale: the beetle on the left is about twelve inches long) show striking sexual dimorphism as a result of heterochrony. Female cervicornis matures earlier than its male counterpart (i.e. females are paedomorphic relative to males). But since the size of the madibles does not vary linearly with respect to overall size (this kind of relation is known as positive allometry; increase in body size results in proportionally bigger increase in mandible size), the male mandibles attain a larger relative size.

Macrodontia Cervicornis-female.

The question here is what, if anything was the target of selection? Adjusting the relative timing of male and female development could have been selected for because large mandibles in males increased fitness; large size in males increased fitness; earlier onset of sexual maturity in females may have been favoured despite the resulting decrease in female mandible size. Of course all of these could have been selection targets, or maybe none of them were.