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Concept
Robustness (evolution)
Summary
In evolutionary biology, robustness of a biological system (also called biological or genetic robustness) is the persistence of a certain characteristic or trait in a system under perturbations or conditions of uncertainty. Robustness in development is known as canalization. According to the kind of perturbation involved, robustness can be classified as mutational, environmental, recombinational, or behavioral robustness etc. Robustness is achieved through the combination of many genetic and molecular mechanisms and can evolve by either direct or indirect selection. Several model systems have been developed to experimentally study robustness and its evolutionary consequences.
Mutational robustness (also called mutation tolerance) describes the extent to which an organism's phenotype remains constant in spite of mutation. Robustness can be empirically measured for several genomes and individual genes by inducing mutations and measuring what proportion of mutants retain the same phenotype, function or fitness. More generally robustness corresponds to the neutral band in the distribution of fitness effects of mutation (i.e. the frequencies of different fitnesses of mutants). Proteins so far investigated have shown a tolerance to mutations of roughly 66% (i.e. two thirds of mutations are neutral).
Conversely, measured mutational robustnesses of organisms vary widely. For example, >95% of point mutations in C. elegans have no detectable effect and even 90% of single gene knockouts in E. coli are non-lethal. Viruses, however, only tolerate 20-40% of mutations and hence are much more sensitive to mutation.
Biological processes at the molecular scale are inherently stochastic. They emerge from a combination of stochastic events that happen given the physico-chemical properties of molecules. For instance, gene expression is intrinsically noisy. This means that two cells in exactly identical regulatory states will exhibit different mRNA contents.
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