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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.
Repeat after me: There is no such thing as "macroevolution." Biological evolution is change in the gene pool of a population over time. That's all that it is. That this happens is an undisputable matter of fact. The only difference between what fundamentalists call "microevolution" and "macroevolution" is the amount of time that evolutionary effects take to manifest themselves. The physical process itself is the same.
If I stand on one side of my living room and walk heel-to-toe, I will eventually end up at the other side of my living room. If I stand in San Francisco and walk heel-to-toe in an easterly direction, I will eventually end up in New York. Fundamentalists would call the former activity "microwalking" and the latter "macrowalking", but it should be perfectly obvious that the process in both cases is exactly the same; the difference is in the scale of magnitude.
"Macroevolution" is a fictional concept invented by fundamentalists who have discovered (to their chagrin) that biological evolution cannot be honestly denied, but cannot be allowed to be the explanation for the biodiversity on life on Earth. It's just too bad for the fundamentalists that their basic premise (that evolution is incompatible with religion) is one of the most outlandish and potentially destructive lies ever told.
Two problems with this contentin that I can see right off the bat:
Helium balloons want to be free.
You may need to read a little deeper into it. I've always like the description "the boundry between what is, and what is not".
The rough edges where all the action goes on in the universe. The energy potentials dancing around each other until they equalize. In their wake are impressions left in the universe.
Wintertime is a good time to see this. All the trees without their leaves stand contrasetd against the sky. Elements in the air and sky are no longer sufficiently reactive to induce change when left by themselves. So they formed trees to maintain the steady march downwards towards equilibrium.
This time of year you can literally see this fuzz of entropy that branches out from what is to what is not.
Again, this is a false dichotomy. Macroevolutionary changes are only detectable after the fact. They are microevoultionary changes, physically. There is no "jump" over an ocean.
Humans determine when a "macroevoltuionary" event has occured in hindsight. Nature does not care whether we deem it a "micro" or "macro" evolution. As far as the physical process is concerned, there is nothing but microevolitionary changes or, in your analogy "footsteps".
There are no oceans seperating a mutated child from it's parent. The only way oceans occur is after the fact, when ancestors of the new orgamism die out.
The fossil record is VERY incomplete and is by far the weakest evidence for evolution. The large gaps in it go no where to show that chasms between certain evolutionary paths exist or not. It's in how you interpret the gaps in the record. It is hardly irrational to look at microbiology and the gaps in the fossil record and then think maybe, just maybe those gaps were never filled.
Marxists believe that cooperation is more efficient than competition in the long term and that Darwinistic tendancies like those above lead to more wasted energy and in turn there is less energy in the system to profit from. The Marxists don't question the evidence or case for evolution in the same manner as the religious fanatics. They just don't believe evolution by competition is the better way to achieve goals.