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ReportsDarwinian Evolution Can Follow Only Very Few Mutational Paths to Fitter Proteins
Five point mutations in a particular ß-lactamase allele jointly increase bacterial resistance to a clinically important antibiotic by a factor of Department of Organismic and Evolutionary Biology, Harvard University, 16 Divinity Avenue, Cambridge, MA 02138, USA.
* To whom correspondence should be addressed. E-mail: dmw{at}post.harvard.edu
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100,000. In principle, evolution to this high-resistance ß-lactamase might follow any of the 120 mutational trajectories linking these alleles. However, we demonstrate that 102 trajectories are inaccessible to Darwinian selection and that many of the remaining trajectories have negligible probabilities of realization, because four of these five mutations fail to increase drug resistance in some combinations. Pervasive biophysical pleiotropy within the ß-lactamase seems to be responsible, and because such pleiotropy appears to be a general property of missense mutations, we conclude that much protein evolution will be similarly constrained. This implies that the protein tape of life may be largely reproducible and even predictable. 
Present address: Integrative Oceanography Division, Scripps Institute of Oceanography, 9500 Gilman Drive, La Jolla, CA 92037, USA. 
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