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Phyletic Gradualism Versus Punctuated Equilibrium
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Darwin visualized cladogenesis as a gradual process, which we refer to as phyletic gradualism. However, an alternative view arose in 1972, when N. Eldredge and S. J. Gould suggested that speciation itself, and the morphological changes accompanying speciation, occur rapidly, separated by long periods of time when little change occurs (stasis). They called their model punctuated equilibrium (periods of stasis punctuated by rapid evolutionary change). Although gure 21.7 presents what appear to be two clear alternatives, in practice the models are very hard to tell apart. They both start with the same ancestral species and predict the same number of modern species. Allopatric, parapatric, and sympatric speciation mechanisms apply to both punctuated equilibrium and phyletic gradualism. The only major difference between the models is the rate of change, and this can only be discovered from an almost complete fossil record. The punctuated equilibrium model has brought much excitement to modern evolutionary biology. We await a time in the near future when we can decide which model has predominated in evolutionary history.
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The Gal pagos Archipelago is located about 700 miles west of Ecuador. This isolated chain of islands is a natural laboratory for the study of evolutionary processes.
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Stephen J. Gould (1943 ).
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(Courtesy of Dr. Stephen J. Gould and the Harvard University News Of ce.)
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(From David Lack, Darwin s Finches. Copyright 1947 by Cambridge University Press, New York, NY. Reprinted by permission.)
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Tamarin: Principles of Genetics, Seventh Edition
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IV. Quantitative and Evolutionary Genetics
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21. Evolution and Speciation
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The McGraw Hill Companies, 2001
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Evolution and Speciation
Species of Darwin s nches. These birds apparently evolved from a single group of migrants from the South American mainland. Isolated on the different islands, the birds evolved to ll many vacant niches.
Diagrammatic interpretation of cladogenesis. (a) Phyletic gradualism is depicted as a gradual divergence over time. (b) Punctuated equilibrium is depicted as a rapid divergence of two groups after long periods of no change. The horizontal axis is some arbitrary measure of species differences.
Tamarin: Principles of Genetics, Seventh Edition
IV. Quantitative and Evolutionary Genetics
21. Evolution and Speciation
The McGraw Hill Companies, 2001
Twenty-One
Evolution and Speciation
G E N E T I C VA R I A T I O N
Darwinian evolution depends on the variation within a population. E. B. Ford, a British evolutionary biologist, applied the term genetic polymorphism to the occurrence of more than one allele at a given locus. Usually, we consider a locus polymorphic if a second allele occurs in the population at a frequency of 5% or more. Before the mid-1960s, the general belief was that only a few loci were polymorphic in any individual or any population. In 1966, two researchers found a way to sample the genome in what they perceived was a random manner. R. C. Lewontin and J. L. Hubby used acrylamide-gel electrophoresis (see chapter 5) to investigate variability in a fruit y species, Drosophila pseudoobscura. (H. Harris reported independent, similar work with human DNA.) Lewontin and Hubby reasoned that choosing enzymes and general proteins that are amenable to separation by electrophoresis, is, in fact, choosing a random sample of the genome of the fruit y. If this is the case, then the degree of polymorphism found by electrophoretic sampling would provide an estimate of the amount of variability occurring in the individual organism and in the population. Their results were startling. Lewontin and Hubby found that the species was polymorphic at 39% of eighteen loci examined, the average population was polymorphic at 30% of its loci, and the average individual was heterozygous at 12% of its loci. The high rate of polymorphism sparked two interrelated controversies. The first was whether electrophoresis does, in fact, randomly sample the genome. The second was whether most electrophoretic alleles are maintained in the population by natural selection. Let us return to the arguments after looking at ways in which genetic polymorphisms could be maintained in natural populations.
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