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Suggested Citation:"Rates." National Academy of Sciences. 1995. Tempo and Mode in Evolution: Genetics and Paleontology 50 Years After Simpson. Washington, DC: The National Academies Press. doi: 10.17226/4910.
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PART IV
RATES

The chloroplast is an essential organelle derived from a cyanobacteria-like organism that was acquired as an endosymbiont by a remote ancestor of modern plants. The chloroplast's genome is a DNA molecule consisting of 150 kilobase pairs that encode 100 gene products. It has been completely sequenced in six very diverse plants and investigated for various purposes in several score species, yielding a tremendous wealth of information available for comparative evolutionary investigations. Clegg and his colleagues, in Chapter 11, uncover a complex evolutionary pattern. Some noncoding regions include hot spots for insertions and deletions and exhibit complex recombinational features. Selective drives in codon utilization have changed over evolutionary time. Patterns of amino acid replacements reflect functional constraints imposed by natural selection on protein configuration. Rates of evolution are quite variable from one order to another, although much of the variation can be accounted for by differences in generation time.

The constancy of evolutionary rates is the subject of Chapter 12. The Cu, Zn superoxide dismutase (SOD) seems to behave like a very erratic clock: the rate of amino acid replacement is 5 times faster among mammals than between fungi and animals. Walter M. Fitch and Francisco J. Ayala (19) analyze the amino acid sequences of several score species and show that SOD behaves like a fairly accurate clock by assuming a complex pattern in which different sets of amino acids have different probabilities of change that are nevertheless constant through time. The model for constancy requires that a set of only 28 amino acids

Suggested Citation:"Rates." National Academy of Sciences. 1995. Tempo and Mode in Evolution: Genetics and Paleontology 50 Years After Simpson. Washington, DC: The National Academies Press. doi: 10.17226/4910.
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be replaceable at any one time. Although the elements of the set vary from time to time and from lineage to lineage, a total of 44 amino acids are permanently unreplaceable. Moreover, the number of different amino acids that can occur at any particular variable site is very small, limited to 2–4 alternatives. The conclusion is that molecular clocks have complex features that must be ascertained before drawing out inferences about the topology and timing of historical relationships.

Suggested Citation:"Rates." National Academy of Sciences. 1995. Tempo and Mode in Evolution: Genetics and Paleontology 50 Years After Simpson. Washington, DC: The National Academies Press. doi: 10.17226/4910.
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Page 213
Suggested Citation:"Rates." National Academy of Sciences. 1995. Tempo and Mode in Evolution: Genetics and Paleontology 50 Years After Simpson. Washington, DC: The National Academies Press. doi: 10.17226/4910.
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Page 214
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Since George Gaylord Simpson published Tempo and Mode in Evolution in 1944, discoveries in paleontology and genetics have abounded. This volume brings together the findings and insights of today's leading experts in the study of evolution, including Ayala, W. Ford Doolittle, and Stephen Jay Gould.

The volume examines early cellular evolution, explores changes in the tempo of evolution between the Precambrian and Phanerozoic periods, and reconstructs the Cambrian evolutionary burst. Long-neglected despite Darwin's interest in it, species extinction is discussed in detail.

Although the absence of data kept Simpson from exploring human evolution in his book, the current volume covers morphological and genetic changes in human populations, contradicting the popular claim that all modern humans descend from a single woman.

This book discusses the role of molecular clocks, the results of evolution in 12 populations of Escherichia coli propagated for 10,000 generations, a physical map of Drosophila chromosomes, and evidence for "hitchhiking" by mutations.

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