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The Mystery of the Genome
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Figure 8a
Sources of phenotypic variation. Variation between individuals results from numerous factors – both genetic and non-genetic. Within the genetic component of phenotypic variation, there are also numerous components. Only one genetic component is “selectable”, and that is the “additive” genetic component. But this component is totally overshadowed by the other sources of phenotypic variation. The primary source of phenotypic variation is environmental variation (sector “1”). This variation is not heritable, and interferes with selection. The second major source of variation is the interaction of the environment with the genotype (2). This variation is also not heritable, and interferes with selection. Within the genetic component of variation, there is variation due to: epigenetics (3), epistasis (4), and dominance (5). None of these genetic components are heritable, and all of them interfere with true long-term selection. Lastly, there are “other” genetic components, which would otherwise be selectable, but are “neutralized”, either by homeostatic processes or such things as cyclic selection (6). All these non-heritable components account for the vast bulk of all phenotypic variation. This leaves additive genetic variation as a relatively insignificant component of phenotypic variation (7). For a very general phenotypic trait, such