genetic architecture - senecio7の日記

Rice SH 2008 Theoretical approaches to the evolution of development and genetic architecture. Ann NY Acad Sci 1133:67-86.

most heritable phenotypic variation is translated through development from genetic variation
a complete theory of phenotypic evolution must be ready to incorporate all the complexities of development and genetic architecture
one of the motivations for the emergence of evolutionary developmental biology as a distinct field

much of the early interest in "evo-devo" came from developmental biologists who saw the relevance of comparative studies based on phylogenetic relationships
basically "devo" with a little bit of "evo"
a truly synthetic theory needs to incorporate development and genetic architecture into the formal mathematical theory of evolution and then connect this theory back to developmental data

the very properties of quantitative genetic models that make them particularly useful for studying phenotypic evolution make them ill suited to the study of the evolution of genetic architecture
focusing only on additive contributions of genes to phenotype precludes most of the study of the evolution of genetic architecture
particular attention has been paid to genetic covariance since early studies suggested that this evolves faster than additive genetic variance
understanding exactly why genetic covariance changes has been difficult using only the machinery of quantitative genetics
understanding the variation in genetic covariance seen in experimental studies will require explicit models of the specific gene interactions underlying quantitative traits
when gene products interact nonadditively, genetic covariance changes in response to selection on phenotype and that this change is often not what we would expect from an additive model

the simplest genetic models focus on variation resulting from mutation
in fact variation resulting directly from mutation is probably less relevant to developmental evolution than are other sources of variation, such as recombination, migration, and environmental fluctuation
the response of a population to selection to reduce variation is likely to be a rapid change in the mutation-selection equilibrium rather than a restructuring of genetic architecture

Siegal & Bergman (2002) define stabilizing selection as "selection for a particular optimum gene expression pattern"
they interpret their results as showing that canalization can evolve without stabilizing selection
this definition of stabilizing selection is consistent with that given in many introductory texts, but it is not the same as the definition used in most of the evolutionary genetics literature, where stabilizing selection is defined as selection to reduce phenotypic variance
under this definition, Siegal & Bergman's simulations imposed stabilizing selection within each lineage, so their result is exactly in line with prior predictions

the phenomenon of a system that is buffered against one source of underlying variation also being buffered against other sources of underlying variation has been called congruence
congruence is usually discussed in the context of comparing environmental canalization with mutational canalization and has been seen as important because of the limitations on mutational canalization
mutation is not the only source of genetic variation
in addition to recombination, mutation, and environmental variation, the phenotypic (and genetic) variation within a local population is influenced by migration

one observation that was long thought to provide evidence for canalization is that populations often express increased heritable variation after being perturbed either genetically or environmentally
in fact, though, several processes can cause variation to increase after perturbations even if the trait was not initially canalized
although this expression of variation may facilitate adaptive evolution, it does not follow that these systems have evolved, wholly or in part, from selection for evolvability
the expression of variation is expected when a canalized system is disrupted and is a common occurrence even in noncanalized systems that involve substantial epistasis

along with canalization, modularity (especially as it relates to evolvability) is among the principal motivations for research into the evolution of genetic architecture
modularity is sometimes taken to imply that different sets of genes influence different modules
this inference is not necessarily true, though, because there can be evolutionarily independent modules that are influenced by many of the same genes
supporting this claim, Hansen (2003) found that in simulations, evolvability was not maximized in systems with minimal pleiotropy

most of the data used in traditional evo-devo research comes directly from developmental biology
it is thus largely concerned with specific genes that have large effects when mutated in the lab and has little to say about the distribution of genetic variation that actually contributes to phenotypic variation in natural populations
the details of the distribution of underlying genetic and environmental variation are even more important to understanding how the development of a trait evolves than they are to understanding the evolution of trait itself
empirical research in evo-devo may thus have to expand out of the lab and into the field more than it has