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OCR for page 85
--> PART II MACROEVOLUTION The majority of skeletonized (readily preserved) animal phyla appear in the early Cambrian, in an exuberant burst of diversity long known as the Cambrian explosion. The evolution of so numerous and diverse body plans would seem to call for long time spans. At the time of the publication of Tempo and Mode and for the next two decades, the scarcity of the fossil record prior to the Cambrian was seen as the "explanation" of the Cambrian explosion. The explosion was only apparent. The evolution of the major body plans had come to be gradually, but the record was lacking. The eventual discovery of the Ediacaran faunas and of many Precambrian fossils failed to show a record of gradual emergence of the phyla. The evolution of the metazoan body plans and subplans happened rapidly, James W. Valentine tells us in Chapter 5. He combines fossil evidence from the Precambrian and early Cambrian with genetic and cell biology analysis of living forms, to reconstruct the evolutionary burst that created so much novelty, more than would ever appear at any other time. Species extinction was long a neglected, if not totally ignored, subject of investigation for twentieth-century evolutionists. This is surprising, says David M. Raup in Chapter 6, because Darwin attached considerable significance to extinction, and because species extinctions have of necessity been just about as common as originations, living species representing the small surplus cumulated over millions of years. Raup concludes that mass extinctions have been of great consequence in restructuring the biosphere, because successful groups become eliminated, thereby empowering previously constrained groups to expand
OCR for page 86
--> and diversify. He also shows that species extinctions for the most part are not caused by natural selection. Stephen Jay Gould, in Chapter 7, takes issue with Simpson's conclusion that paleontological processes can be accounted for by microevolutionary causes. Two major domains exist, he argues, where distinctive macroevolutionary theories are needed. One concerns nongradual transitions, such as punctuated equilibrium and mass extinction; the other calls for an expansion of the theory of natural selection to levels both below and above organisms. Whence the topological configuration of vascular land plants? Physics, geometry, and computer simulations allow Karl J. Niklas to explore, in Chapter 8, the rules and significance of morphological variations. The more complex the functions that an organism must perform in order to grow, survive, and reproduce, the greater the diversity of morphological types that will satisfy the requirements. Unexpected is the additional conclusion that the number and accessibility of fitness optima also increase with the complexity of functions.
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