Although the popular view of genes is that one gene has one function, this has long been recognised as simplistic and misleading. Genes have multiple functions. And genes interact: they have epistatic effects. The Neodarwinian synthesis invokes genetic mutations as the source of variation on which natural selection acts. The theory allows that the majority of these mutations are deleterious, but selection of the few favourable mutations is all that really matters. In the past this mechanism has been discussed without reference to epistatic effects. What relevance is this for mutations that do not appear to be deleterious?
Bershtein and co-workers have been looking at mutation in E. coli proteins. They were interested in exploring â€œthe common view that the vast majority of protein mutations are tolerated.â€ They devised ways to purge the proteins of detectable deleterious mutations and monitored the fitness with time. Epistasis was observed: â€œthe combined deleterious effects of mutations were, on average, larger than expected from the multiplication of their individual effects.â€ Although there appeared to be robustness in the mutated proteins, a threshold was reached leading to chronic failure. â€œOnce the stability threshold is exhausted, the deleterious effects of mutations become fully pronounced, thereby making proteins far less robust than generally assumed.â€
The results are very significant for the evolutionary mechanisms advanced by Neodarwinists. Epistasis effects constrain evolutionary trajectories associated with random mutations. Go past the threshold and the apparently â€œfitâ€ mutations are revealed as deleterious. For a long time, Darwinists have resisted any critical examination of their mechanism for evolution, but the evidence is getting stronger every year that â€˜mutations + natural selectionâ€™ will not deliver the transformations required.
Robustnessâ€“epistasis link shapes the fitness landscape of a randomly drifting protein
Shimon Bershtein, Michal Segal, Roy Bekerman, Nobuhiko Tokuriki and Dan S. Tawfik
Nature 444, 929-932 (14 December 2006) | doi:10.1038/nature05385
The distribution of fitness effects of protein mutations is still unknown1,2. Of particular interest is whether accumulating deleterious mutations interact, and how the resulting epistatic effects shape the protein's fitness landscape. Here we apply a model system in which bacterial fitness correlates with the enzymatic activity of TEM-1 -lactamase (antibiotic degradation). Subjecting TEM-1 to random mutational drift and purifying selection (to purge deleterious mutations) produced changes in its fitness landscape indicative of negative epistasis; that is, the combined deleterious effects of mutations were, on average, larger than expected from the multiplication of their individual effects. As observed in computational systems3,4,5, negative epistasis was tightly associated with higher tolerance to mutations (robustness). Thus, under a low selection pressure, a large fraction of mutations was initially tolerated (high robustness), but as mutations accumulated, their fitness toll increased, resulting in the observed negative epistasis. These findings, supported by FoldX stability computations of the mutational effects6, prompt a new model in which the mutational robustness (or neutrality) observed in proteins, and other biological systems, is due primarily to a stability margin, or threshold, that buffers the deleterious physico-chemical effects of mutations on fitness. Threshold robustness is inherently epistaticâ€”once the stability threshold is exhausted, the deleterious effects of mutations become fully pronounced, thereby making proteins far less robust than generally assumed.
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