VARIATION HERITABILITY ADAPTATION

Variation Heritability Adaptation

Variation Heritability Adaptation

Introduction

Phenotypic variation is the central concept linking studies of evolution, development, and population genetics. The integration of evolutionary and developmental biology has been guided by two central questions about variation: (1) is the variation that leads to evolutionary change continuous or discrete; and (2) how does the structure of development bias or constrain the production of variation and hence the course of evolution? Both questions have deep roots. The former can be traced from the correspondence between Huxley and Darwin, through Bateson (1894), and Goldschmidt (1940) to recent approaches grounded in developmental genetics (Stern, 2000; Haag and True, 2001). The latter can be traced to von Baer's and Haeckel's interests in the relationship between ontogeny and phylogeny in the early nineteenth century to questions about the role of developmental timing in evolutionary change (Gould, 1977) and interest in the developmental basis for evolutionary change.

Interestingly, the study of variation as a subject in itself has remained peripheral to the ongoing synthesis of evolutionary and developmental biology. By variation as a subject, I mean the question of how variability—or the tendency to vary (Wagner et al., 1997)—is patterned among traits, populations, species, and higher taxa. Perhaps this is because, as Van Valen (1974) points out, the additional ingredients of ecology and morphometrics must be added to the mix of evolution, development, and genetics to study and appreciate the significance of patterns of variability. Alternatively, as argued by Hall (1999a), the lack of interest in patterns of phenotypic variability may reflect the focus of evolutionary developmental biologists on a few model organisms reared under controlled laboratory conditions.

Discussion

Components of Phenotypic Variation

The variance of any aspect of the phenotype is the result of genetic and environmental effects. The genetic component of phenotypic variance can be broken down into the additive effects of genes and the deviations from simple additive effects due to dominance and epistatic interactions among genes. Although only additive effects contribute to heritability in the narrow sense, dominance and epistasis probably contribute significantly to genetic variance in most cases; see Falconer and Mackay (1996) or Roff (1997) for fuller discussions.

The environmental component of phenotypic variance can be partitioned in various ways depending on research aims. In human genetic studies, for instance, it is often useful to divide the environmental variance into familial and nonfamilial effects (Rice and Borecki, 1999). A useful distinction in evolutionary contexts is between environmental factors that contribute to differences among individuals and those that contribute to variation within individuals. This distinction defines the boundary between canalization and developmental stability. Canalization refers to the reduction of variation among individuals, whereas developmental stability refers to the minimization of variation within individuals (Clarke, 1998).

A final variance component is true developmental noise. Waddington (1957) regarded developmental noise as imprecision at the molecular level of developmental processes. Such variation is not of environmental origin as it is intrinsic to the material substrate of development. Unfortunately, true developmental noise is very difficult to ...