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Science 7 April 2006:
Vol. 312. no. 5770, p. 15
DOI: 10.1126/science.312.5770.15k
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Genetic mutations are the substrate for evolution. Genes conferring fitness can accumulate multiple mutations during a period of selection. There are, of course, many potential evolutionary trajectories for the appearance of these mutations. However, it is likely that not all trajectories are available because the fitness of individual mutations may depend on the genetic background in which they appear. Weinreich et al. (p. 111) chart the available evolutionary trajectories for five mutations in beta-lactamase in Escherichia coli, which together confer a 100,000-fold increased resistance to the antibiotic cefotaxime. Only 18 of a potential 120 routes to high fitness are accessible to selection, due to pleiotropic effects of the mutations on the enzyme.




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