Heterochrony and Evolutionary Processes

Recognising Heterochrony





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


The standard story is that Haeckel's Biogenetic Law was throughly discredited in the early twentieth century. According to this account, rigorous scientific empiricism defeated the unfounded metaphysics of Haeckel and the Naturaphilosophie biologists by piling up enough counter-examples to ontogenic recapitulation to finally bury the thing. But recapitulation does occur, and most of the immediate succesors to people like Beecher failed to recognise these cases. It seems as though the denouncers of the metaphysics of the Naturaphilosophes had their own ideological blinders on.

In any case, the maxim "ontogeny recapitulates phylogeny" survived the purported demise of the idea it expressed: it is still a familiar refrain and it may retain a certain heuristic value (for instance, as a starting point for discussing heterochrony). But before fully immersing ourselves in the exploration of thoroughly modern ideas on heterochrony, there remains one more bugbear to slay, and that is, why does human embryology seem to demonstrate the validity of those old Haeckelian ideas? The answer is that, although the retention of ancestral adult forms as juvenile stages of descendants is quite rare (though it does happen), the retention of ancestral embryological forms is quite common. Human embryos are quite similar to other vertebrate embryos, even from lineages which diverged from ours a substantial time ago.

Embryos of a dog, an alligator and a human. (Taken from http://www.carleton.ca/~tpatters/teaching/67.236/)

This is a result of "selection inertia". Timing is everything, and every gene expression depends on other genes being properly expressed before it. This has been described as a "cascading effect" where gene mutations affecting early developmental stages have far more profound morphological consequences than those affecting later stages, as every subsequent developmental stages build upon the early ones. Innovations that are preserved tend to occur in late developmental stages, because they are more likely to be selectively neutral (i.e. fitness neutral) and thus, less likely to be eliminated by the action of natural selection.

True Haeckelian recapitulation can occur through heterochrony. Ancestral adult forms will appear as the juvenile forms of the descendants if the onset of maturity is delayed: the descendant goes through all the stages of the ancestral ontogeny and then extends the effects of pre-existing developmental processes or adds innovations. This is peramorphosis, and it can also occur by accelerating the rate at which development occurs.

The converse process, paedomorphosis, has the descendant adults appearing like the ancestor juvenile forms. This can occur by precocious maturation, or by slowing down developmental rates. It seems as though paedomorphosis is actually the more common heterochronic phenomena.