Ape to Man
Out of context: Reply #179
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JazX - i refer you to the work of Stuart Kauffman:
http://home.wxs.nl/~gkorthof/kor…
http://en.wikipedia.org/wiki/Stu…
http://www.iscid.org/stuartkauff…
I think there is some great work being done on the scientific side emerging from the field of Complexity that bagins to explain alot of the processes at work behind the Cambrian explosion...
Here's an excerept taken from here:
http://www.culturaleconomics.atf…
"The Cambrian Explosion
From the first chapter of this book I have regaled you with images of the Cambrian explosion and the profound asymmetry of that burst of biological creativity compared with that following the later Permian extinction. In the Cambrian, over a relatively short period of time, according to most workers in the field, a vast diversity of fundamentally different morphological forms appeared. Since Linnean taxonomy has been with us, we have categorized organisms hierarchically. The highest categories, kingdoms and phyla, capture the most general features of a very large group of organisms. Thus the phylum of vertebrates - fish, fowl, and human - all have a vertebral column forming an internal skeleton. There are 32 phyla today, the same phyla that have been around since the Ordovician, the period after the Cambrian. But the best accounts of the Cambrian suggest that as many as 100 phyla may have existed then, most of which rapidly became extinct. And as we have seen, the accepted view is that during the Cambrian, the higher taxonomic groups filled in from the top down: the species that founded phyla emerged first. These radically different creatures then branched into daughter species, which were slightly more similar to one another yet distinct enough to become founders of what we now call classes. These in turn branched into daughter species, which were somewhat more similar to one another yet distinct enough to warrant classifying them as founders of orders. They in turn branched and gave off daughter species distinct enough to warrant being called founders of families, which branched to found genera. So the early pattern in the Cambrian shows explosive differences among the species that branch early in the process, and successively less dramatic variation in the successive branchings.
But in the Permian extinction some 245 million years ago, about 300 million years after the Cambrian, a very different progression unfolded. About 96 percent of all species became extinct, although members of all phyla and many lower taxa survived. In the vast rebound of diversity that followed, very many new genera and many new families were founded, as was one new order. But no new classes or phyla were formed. The higher taxa filled in from the bottom up. The puzzle is to
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account for the vast explosion of diversity in the Cambrian, and the profound asymmetry between the Cambrian and the Permian.
A related general phenomenon is this: during postextinction rebounds, it appears to be the case that most of the major diversification occurs early in the branching process of speciation. Paleontologists call such a branching lineage a clade. They speak of “bottom-heavy” clades, which are bushy at the base, or oldest time, and note that genera typically diverge early in the history of their families, while families diverge early in the history of their orders. In short, the record seems to indicate that during postextinction rebounds most of the diversity arises rather rapidly, and then slows. Thus while the Cambrian filled in from the top down and the Permian from the bottom up, in both cases the greatest diversification came first, followed by more conservative experimentation.
Might it be the case that the general features of rugged fitness landscapes shed light on these apparent features of the past 550 million years of evolution? As I have suggested, the probable existence of three time scales in adaptive evolution on correlated rugged landscapes, summarized in Figure 9.3, sounds a lot like the Cambrian explosion. Early on in the branching process, we find a variety of long-jump mutations that differ from the stem and from one another quite dramatically. These species have sufficient morphological differences to be categorized as founders of distinct phyla. These founders also branch, but do so via slightly closer long-jump variants, yielding branches from each founder of a phylum to dissimilar daughter species, the founders of classes. As the process continues, fitter variants are found in progressively more nearby neighborhoods, so founders of orders, families, and genera emerge in succession.
But why, then, was the flowering after the Permian extinction so different from the explosion during the Cambrian? Can our understanding of landscapes afford any possible insight? Perhaps. A few more biological ideas are needed. Biologists think of development from the fertilized egg to the adult as a process somewhat akin to building a cathedral. If one gets the foundations wrong, everything else will be a mess. Thus there is a common, and probably correct, view that mutants affecting early stages of development disrupt development more than do mutants affecting late stages of development. A mutation disrupting formation of the spinal column and cord is more likely to be lethal than one affecting the number of fingers that form. Suppose this common view is correct. Another way of saying this is that mutants affecting early development are adapting on a more rugged landscape than mutants affecting late development. If so, then the fraction of fitter neighbors
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dwindles faster for mutants affecting early development than those affecting late development. Thus it becomes hard to find mutants altering early development sooner in the evolutionary process than to find mutants affecting late development. Hence if this is correct, early development tends to “lock in” before late development. But alterations in early development are just the ones that would cause sufficient morphological change to count as change at the phylum or class level. Thus as the evolutionary process continues and early development locks in, the most rapid response to ecological opportunity after a mass-extinction event should be a rebound with massive speciation and radiation, but the mutations should affect late development. If this is true, no new phyla or classes will be found. The radiation that occurs will be at the genus and family level, corresponding to minor changes that result from mutations affecting late development. Then the higher taxa should fill in from the bottom up.
In short, if we imagine that by the Permian early development in the organisms of most phyla and classes was well locked in, then after 96 percent go extinct, only traits that were more minor, presumably those caused by mutations affecting later stages of an organism’s ontogeny, could be found and improved rapidly.
If these views are correct, then major features of the record, including wide radiation that fills taxa from the top down in the Cambrian, and the asymmetry seen in the Permian, may find natural explanations as simple consequences of the structure of fitness landscapes. In the same vein, notice that bushy radiation should generally yield the greatest morphological variation early in the process. Thus one might expect that during postextinction rebounds, genera would arise early in the history of their families and families would arise early in the history of their orders. Such bottom-heavy clades are just what is observed repeatedly in the evolutionary record."
- end.