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--> PART V PATTERNS Ten thousand human generations take 250,000 years and 10,000 fruit fly generations take 500 years. Ten thousand generations are but an instant of evolutionary time, but in humans and flies demand too much time for direct experimentation. Not so in the case of bacteria; 10,000 generations of Escherichia coli require "only" 4 years. Richard E. Lenski and Michael Travisano, in Chapter 13, show that in new but constant environments evolution occurs rapidly during the first 2000 generations, slowly during the following 3000, and not at all over the last 5000. They have 12 separate populations derived from identical ancestors and evolving in identical environments, but their trajectories are different in both morphology and fitness. The conclusion is inescapable that chance events play an important role in adaptive evolution. DNA polymorphisms along the chromosomes of Drosophila flies exhibit an unanticipated pattern. Where the incidence of genetic recombination is low, such as near the centromere and the tips, the level of polymorphism also is low. This is not a consequence of different mutation rates, concludes Richard R. Hudson in Chapter 14, because divergence between species is indifferent to incidence of recombination. The pattern can be explained by "hitchhiking"—that is, by selection of favorable mutations that carry along other mutations as they spread through a population; how much DNA will be carried along is determined by the incidence of recombination. The reciprocal of selection of favorable mutations is selection against unfavorable ones. This possibility, however, does not quite explain the observed pattern. 1944 was a propitious year for evolutionary studies. In addition to
OCR for page 252
--> Simpson's Tempo and Mode it saw the publication of a monograph by Theodosius Dobzhansky and Carl Epling (Contributions to the Genetics, Taxonomy, and Ecology of Drosophila pseudoobscura and Its Relatives ) that would usher in an interest in reconstructing phylogenetic history on the basis of genetic information. The method relies on the sequential composition of chromosomes, a premonition of the molecular methods that rely on the sequential composition of the DNA. The third chromosome of Drosophila pseudoobscura exhibits a rich polymorphism with more than 40 alternatives. One vexing problem is rooting the topological relationships, that is, identifying the ancestral element. Aleksandar Popadic and Wyatt W. Anderson, in Chapter 15, examine the nucleotide sequence of a DNA fragment included within the chromosomal elements and conclude that only two of the elements are possible ancestors, one of them ("Santa Cruz") with higher probability. The last chapter is a display of molecular biology virtuosity. Daniel L. Hartl and colleagues have produced a physical map of the chromosomes of the fruit fly Drosophila melanogaster, by ordering sequentially 2461 different DNA fragments, each about 80,000 nucleotides long. Eighty-five percent of all genes are included in these fragments. The methods are the same in use for mapping human chromosomes, and they can readily be extended to other Drosophila species, a possibility of genetic and evolutionary consequence.
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