Few Mutational Pathways Lead to Darwinian Evolution

first_imgMutations may not be as helpful for neo-Darwinian evolution as expected, say researchers from Harvard.  Let’s say five mutations need to occur for a bacterium to gain resistance to an antibiotic, and there are 120 ways to get these mutations.  They found that only about 10 of the pathways could be selected by natural selection.  Since natural selection would have to confer better fitness at each step, most of the pathways are dead ends.    Although the team studied only one particular kind of resistance, “this finding likely applies to most protein evolution,” they said.  “Although many mutational paths lead to favored variants, only a very small fraction are likely to result in continuously improved fitness and therefore be relevant to the process of natural selection.”    See also Science Daily, EurekAlert.  The original paper was published in Science.1  The abstract states:Five point mutations in a particular beta-lactamase allele jointly increase bacterial resistance to a clinically important antibiotic by a factor of 100,000.  In principle, evolution to this high-resistance beta-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 [i.e., one modification causing a cascade of effects] within the beta-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 [sic] may be largely reproducible and even predictable.   (Emphasis added in all quotes.)The last sentence about what this implies is purely a speculation.  They elaborate slightly in the last sentence of the paper: “It now appears that intramolecular interactions render many mutational trajectories selectively inaccessible, which implies that replaying the protein tape of life might be surprisingly repetitive.  It remains to be seen whether intermolecular interactions similarly constrain Darwinian evolution at larger scales of biological organization.”  If those larger scales are random and require multiple steps, it would seem the same principle applies.1Weinreich et al., “Darwinian Evolution Can Follow Only Very Few Mutational Paths to Fitter Proteins,” Science, 7 April 2006: Vol. 312. no. 5770, pp. 111-114, DOI: 10.1126/science.1123539.It wasn’t going to work anyway, so this just makes it harder.  They are’t talking about adding new genetic information or function, but rather losing function (susceptibility to the antiobiotic) in such a manner that each stage doesn’t kill all of the organisms in one fell swoop.  If this principle applies, as they suggested, to larger scales of biological organization, then the neo-Darwinian gig is, for all practical purposes, over.  Try getting a whale from a cow against these kinds of constraints.  This makes “the protein tape of life” predictable?  In whose Tinker Bell tale?(Visited 13 times, 1 visits today)FacebookTwitterPinterestSave分享0last_img

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