cryptic genetic variation
Schlichting CD 2008 Hidden reaction norms, cryptic genetic variation, and evolvability. ANN NY Acad Sci 1133:187-203.
- Stebbins and Hartl (1988) proposed hidden variation as one answer to the big question
- "given the multiplicity of constraints that organisms confront, one might ask how constraints are ever overcome ... ?"
- Hartl and Dykhuizen (1981) had noticed that some lines of Escherichia coli with different enzyme alleles that had equal fitness in glucose medium were significantly different in gluconate medium
- Hermisson and Wagner (2004) correctly point out that canalization need not be a prerequisite for hiding variation because any genetic system with epistatic relationships among loci will store variation
- sign epistasis
- sign epistasis is the name given to the conditionality of epistatic effects
- many studies are documenting such effects
- Wagner (2003) raised the issue of whether CGV that has been expressed in experiments is the sort of variation that can lead to adaptive evolutionary change
- this is an important question because the evolutionary significance of CGV, just as mutational variation, is proportional to the fraction that can be converted to expressed, functional phenotypes
- from another perspective, Wagner's query may at best be rhetorical:
- how well can we, under any circumstances, predict whether a given mutation will be "useful" in the future?
- neutral networks derive their evolvability from the condition dependence of the neutral effects
- all recognize that the freedom of movement within this neutral/nearly neutral envelope significantly increases evolvability by increasing the potential for accessing distant phenotypic space via small genetic changes
