conditional neutrality - senecio7の日記

Hermisson J & Wagner GP 2004 The population genetic theory of hidden variation and genetic robustness. Genetics 168:2271-2284.

we present a general model for the interaction of a major mutation or a novel environment with the additive genetic basis of a quantitative character under stabilizing selection
the release of hidden genetic variation is a generic property of models with epistasis or genotype-environment interaction, regardless of whether the wild-type genotype is canalized or not
the additive genetic variance increases upon a change in the environment or the genetic background even if the mutant character state is as robust as the wild-type character
the release of hidden genetic variance due to a major mutation or environmental stress does not demonstrate canalization of the wild-type genotype

there is indeed ample evidence for genetic variation that has no effect on the phenotype under normal conditions, but is expressed in mutants or in altered environments

the increase in variation can be considerable even if the character state under the changed conditions is no more—or even less—canalized than the wild type
there is no need to demonstrate canalization of the wild type in arguments about a potential evolutionary role of hidden variation
the observation of hidden variation is not sufficient to imply canalization of the wild type
the classic experiments demonstrating hidden variation do not provide convincing evidence for mutational robustness

this variation can be released if the genetic background or the environment changes, leading to an increase of the genetic variance directly after the change
the result does not require that the population has evolved genetic robustness (i.e., canalization) prior to the environmental change
the amount of hidden variation released depends on a number of factors, which are the same in both scenarios
the most important ones are population size and the magnitude and kind of the interaction effects
larger population size and larger interaction effects lead to more hidden genetic variation
for the latter, conditionally neutral alleles are particularly important, i.e.,, alleles that have no effect under the original conditions but are expressed in the new context

to distinguish the variable interaction scenario from genetic canalization, we need to know whether conditional neutrality is particular to wild populations under prevailing ecological conditions or rather is a generic phenomenon

while the release of hidden variation due to the introduction of a mutation is a general observation, the evidence for canalization is ambiguous

canalization is not necessary for hidden variation to accumulate
there is no need to explain the evolution of canalization in a theory that uses hidden variation
the only assumption needed is sufficient time for mutations to accumulate at conditionally neutral loci

the assignment of a privileged role to Hsp90 in the maintenance and release of hidden variation was recently challenged by a computer simulation study
in a model of a complex gene network, any knock-out mutation of a gene is likely to reveal hidden phenotypic variation
our result explains and generalizes their conclusion in an analytical framework
any genetic element that interacts with other genes or the environment will facilitate the accumulation of hidden genetic variation
no specific molecular mechanisms are needed for the existence and putative evolutionary role of hidden variation
our results further predict that hidden variation should be most prevalent in large populations

whether there is any important evolutionary role of hidden variation remains an open question
the main unresolved issue is the benefit of this type of variation for the adaptive process
these questions can and should be addressed separately from problems concerning the evolution of canalization mechanisms