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


Throughout this "display", it has been alluded that heterochronic processes may complicate the evolutionary story for a ceratin structure or for a species' morphology. But before returning to points made in passing elsewhere, the question arises of how common heterochrony is, in fact. Despite the difficulty in being able to establish heterochronic relations in the fossil record (even just determining ancestor-descendant relationships can be fraught with difficultites), in particular the difficulties in assesing the ontogenetic stage of a specimen ("Recognising heterochrony"), McNamara (1986), in a review of the literature covering 1976 to 1985 indentified over 200 cases of heterochrony, this process occuring in all major groups and taxa. This, of course, represents a lower bound, because of the difficulty in discerning these sorts of relations from a fragmentary record but more importantly, because of the fact that most researchers reviewed weren't concerned with describing various developmental stages. In short, they weren't looking for it.

But heterochrony is more important that merely as a result of its ubiquity. Examining heterochrony entails examining the processes by which evolutionary change may come about. Beyond "why does evolution occur?", questions are raised concerning how evolutionary change does occur. As a result the kinds of explanation we can offer for evolutionary change are deepened. They are also made complicated.

In particular, the role of natural selection in driving evolution is complicated. As a result of changes in developmental timing, many profound morphological changes can result. Consequently, it is not clear that every such change was "selected-for". Selection may have targeted generation times (as in McKinney [1986]'s echinoid suite, "K and r selection: tertiary echinoids"), and organism size may have been affected as a consequence. This change ( a "by-product" of selection) is not necessarily adaptive, it could have even led to a decrease in fitness. Similarily, in the case of the beetles' mandibles, it isn't clear whether selection targeted the size of mandibles in the male or the fact of early sexual maturity in the female ("Modern examples: sexual dimorphism"). Just because something looks like an adaptation, it doesn't mean that it is.

If small changes to developmental timing can lead to striking morphological effects (eg. Olenellidae trilobites), then this has further implications for the possibility of non-selectionist evolution. Stochastic changes in gene frequencies (genetic drift) can thus have large effects, if genetic drift affects the "regulatory" genes controlling development. Because changes to developmental timing exploit pre-existing developmental pathways (a cephalic horn is extended as in the Dynastes beetles), fast and striking evolutionary change can occur without undermining a population's biological fitness.

In short, examining processes that can mediate evolutionary change has the welcome effect of diversifying and deepening the kinds of explanations we can offer to account for evolutionary change.




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