"evolution is not only about genetics and natural selection. Just as crucial are the changes in the timing and rate of development, with the three, genetics, heterochrony, and natural selection, forming an interdependent evolutionary triumvirate."Heterochrony constrains natural selection; it also provides it with raw material, allowing small genetic changes to have big phenotypic effects.
Ideas about the relationship between ontogeny and phylogeny (evolutionary history) have changed over the last few centuries, with notions of recapitulation and paedomorphosis going in and out of fashion. McNamara's outline of this covers Ernst Haeckel, Karl Ernst von Baer, and Walter Garstang, ending with Stephen Jay Gould, from whose Ontogeny and Phylogeny he takes the terminology for different kinds of heterochrony. The basic division is into paedomorphosis (less growth) and peramorphosis (more growth). These can each take three forms: paedomorphosis can be the result of progenesis (finishing early), neoteny (slower growth rate), and postdisplacement (starting late), while peramorphosis can result from predisplacement (starting early), acceleration (greater growth rate), and hypermorphosis (finishing late).
That's a lot of technical terms, but don't let them scare you away — the bulk of Shapes of Time consists of lively and engaging examples of heterochrony, taken from across the animal kingdom, from dogs and humans to invertebrates (McNamara is an invertebrate paleontologist), which help both to explain those terms and to fix them in the memory. But first McNamara presents a little bit of developmental biology, covering the stages of neofertilization, differentiation and growth, touching on Hox genes and morphogens, and mechanisms of organ and appendage formation. This is enough background for the higher level (zoological and ecological and paleontological) survey that follows, but may be frustratingly slender for those after more, after a better understanding of the developmental biology behind heterochrony.
McNamara begins his tour of heterochrony with dog varieties — even looking at paedomorphosis in depictions of Snoopy in Peanuts cartoons — and examples from insects and salamanders. Heterochrony is "all-pervasive" in the generation of sexual dimorphism, from simple size differences to extreme cases with males that are little more than "parasitic" sperm sacs. And heterochrony can play a key role in speciation, often combining with environmental gradients to separate populations; examples include Darwin's finches, brachiopods, and bushbucks.
Are some forms of heterochrony more common than others in particular lineages? In some cases paedomorphism seems unusually common, notably among the amphibians (axolotls are paedomorphic salamanders, for example); McNamara also looks at paedomorphism in lungfish, cats, and various invertebrates and at connections with genome and cell size. In other cases peramorphosis seems to dominate: a dramatic example is the combination of hypermorphosis and acceleration that produced increasing size in dinosaur lineages, but Cope's rule suggests that size tends to increase more generally. More common is the mixing of peramorphism and paedomorphism, acting on different features and subject to "trade-offs": examples here come from the evolution of wings (and of flightlessness) and tetrapod limbs, with a brief glance at the origin of turtle shells.
Heterochronic mechanisms enable the adaptation of life cycles to different environments: hypermorphosis and neoteny are more common in stable environments ("K-selected") and progenesis and acceleration in unpredictable ones ("r-selected"). Heterochronic changes can be driven by biological "arms-races", with a clear example in the evolution of sea urchins in response to predation by cassids (marine snails). And heterochrony has played a key role in human evolution, where McNamara highlights peramorphic features against a tradition which has stressed paedomorphism.
McNamara sometimes appears to reduce the significance of ontogeny in evolution to heterochrony, when it is actually considerably broader. There are ontogenetic constraints and processes other than those of timing and rate: biophysical and biochemical limits, ways in which novel proteins or cell types arise, and self-assembly and exploration allowing "adaptive" development, to list just a few. If there is a "triumvirate" that rules evolution it has to be "genetics, ontogeny, and natural selection". Still, there's no doubting that heterochrony is one of the key links between ontogeny and phylogeny — at least not after reading Shapes of Time.
November 2002
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