Over the holidays I read a Review article entitled “From genotype to phenotype: buffering mechanisms and the storage of genetic information” by S.L. Rutherford.
Most models of evolution focus on DNA variation, either between individuals or between populations. However, reality is a bit more complicated, as most natural selection acts on variation among phenotypes rather than genotypes. The mapping from phenotype to genotype is complicated by the fact that genetic buffering allows for the buildup and storage of genetic variation in phenotypically normal populations.
“A more realistic picture of genetic networks will emerge from combining the perspectives of molecular and developmental genetics with those of population and evolutionary genetics. Understanding how specific genes are modulated relative to genetic and environmental variation is essential to understanding the course of evolution.”
Rutherford describes a model in which each genotype can specify a number of different phenotypes, depending on the given environment. For example, systems which act as all-or-none switches integrate continuous information from a variety of sources to produce discrete streotypic outputs. Variation can influence the underlying distribution (see Figure 1) or the threshold at which a particular phenotype is observed. Consequently cryptic genetic variation can be uncovered in mutant genetic backgrounds or by delivering specific stresses to wild-type animals. In a given environment, a probability function determines the mapping from genotype to a set of phenotypes. This function describes the results of environmental influence, developmental noise and stochastic events.
This sort of model can then be used to clarify the concept of canalization. According to Rutherford, Waddington’s description of canalization is essentially evolved constraint. Canalization implies that potentially functional genetic variation could exist, but that the phenotype in question is buffered from its expression. Consequently, if a trait varies more in a mutant background then canalization of the trait is inferred. Therefore genetic buffering mechanisms evolve as an adaptive consequence of stabilizing selection.
An alternative source for genetic buffering is simply biological system organization. Network structure can contribute to robustness when the connectivity of the network results in behaviors which are robust to perturbation of individual components. For example, functionally redundant pathways allow for multiple alternative pathways to reach the same metabolic, regulatory or developmental state. With redundancy, variation can change the underlying architecture even while the phenotype is maintained by strong selective pressure.
“The shift from the deterministic world view engendered by the molecular genetic dissection of specific pathways in genetically uniform laboratory strains to a population-based view of genetic variation and gene networks in the context of entire genomes changes our perpsective of life’s origins and organization. Genetic variation is pervasive but its expression as phenotypic variation is context dependent and heavily buffered.”
Rutherford, S.L. (2000). From genotype to phenotype: buffering mechanisms and the storage of genetic information. BioEssays, 22(12), 1095-1105.
Thirst for Science 
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