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Shapes of Time
Many popular books on evolution ignore or downplay the role of growth and development, of ontogeny. But in Shapes of Time Kenneth McNamara's focus is on heterochrony, on evolutionary changes in the timing of developmental features and in rates of growth. As he puts it:
"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.
You might like to check out Danny's other evolution, developmental biology, and popular science reviews. You can purchase Shapes of Time from bn.com. Slashdot welcomes readers' book reviews -- to see your own review here, read the book review guidelines, then visit the submission page.
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if I had any karma points to give, I would give them to you :)
Are you serious? Tell me you have something better to do w/ your time. Why would you even take time to get a user name on this site if you feel this way.
Seems he got very drunk one night and fell out of his guard tower!!! LOL ROFL mod up +5 FUNNY
It's the new business-model proposed by the free software foundation.
1: Write free software.
2: ?
3: Say "Shapes of Time" ten times in a row fast.
4: Profit!
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He old and stupid
I like Rasta
It make me smile"
"I don't like George-Pa
I glad He die
I like ganja
It make me high""I
Castrate that war criminal Ariel Sharon. I wish the Palestinians would push the Jews into the sea.
This great book can also be ordered online from Barnes and Nobels, for those of you who are still angry at Amazon for their one-click patent.
funniest damn reply to an idiotic troll in months, thanks for the laugh!
Parent is goatse.cx link. I saw the informative +1 tag, and the [amazon.com] thing and had to see that ghastly scene yet another time. Yuck.
/. would require multiple votes to raise or lower a score, based on the number of potential voters, such things would be harder to slip through. Instead of having 1 person collaborating to raise the score of this thing, you'd need maybe ten. And then you'd have ten IP's to ban for a period of time.
/. experience. If one has a significant contributing factor to a thing, is it not logical to design such factors with a bit of care?
If
It works the other way too. The stupid stuff, the karma whoring, would require more votes as well. And with the reduced number of moderation points available to make change, people would use them more thoughtfully.
I know CT doesn't like it when people talk like this but it will become unavoidable eventually. The moderation system is a big part of the
Maybe I'm mistaken and the moderation system recieved more thought than what I imagine. But as a tool it seems overburdoned with the complexity of the dynamics it seeks to modulate. and it is a complex system to deal with. And the author of the moderation system could not have foresaw the types of behavior that would have grown around the yet to be implemented system. But sufficient time has passed to see some of the shortcomings and to make corrections.
It's an interesting problem to design a voting system for an environment like this. I would assume the goal would be to bring forth the sorts of things people like to read. Is that happening?
Though maybe the above post doesn't deserve a mod up, I don't think it deserved to modded down. Though I disagree with the post, opinions ought not be modded down just because the moderator disagrees.
Great, I've been modded down. I guess people dont know who John Koza IS...
It's Christmas everyday with BitTorrent.