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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