The proportions of variance attributable to each component obviously will differ depending on 1) the variability of the environment and 2) the range of genotypes in the population under consideration. If all individuals are clones, then all variation is environmental. If environmental variation is relatively low, such as in Scandinavian societies compared to the USA, then the proportion of variation from genetic factors will increase. Interactions between genetic and environmental factors are, of course, important. In short, it is senseless to estimate the heritability of a trait; heritability reflects the proportion of variation in a trait that can be attributed to genetic variations given a particular environment.
Less recognized is the possibility that there may be many different routes to a disorder, so the responsible factors may be mainly genetic in one individual and mainly environmental in another. One person may get atherosclerosis or depression because of a strong family predisposition in the absence of special environmental risk factors, while another with no predisposition may get the condition mainly because of environmental exposures. An evolutionary approach fosters simultaneous consideration of the many factors that may explain individual variation in a trait.
Some sources of individual differences include:
- Additive genetic differences among individuals that result in phenotypic differences (in this environment). The differences are additive if the simultaneous effects of multiple genes on a trait can be predicted by adding the effects of each individual genes. The “narrow sense heritability” refers to effects of additive genetic effects and is estimated by h2 = Vadditive genetic effects/Vphenotype
- Non-additive variation resulting from Gene x Environment interactions
- Non-additive variation resulting from Gene x Gene interactions
- Assortative mating—nonrandom mating can increase trait variance
- Random factors in development, such as the stochastic nature of neuron migration or different exposure to trace elements in the diet or happenstance social experiences
- Effects of cues that influence development via facultative mechanisms shaped by natural selection to a trajectory suited to the particular environment, such as early heat exposure influencing the number of adult sweat glands
- Effects of trauma, toxins and other environmental exposures outside the range of “normal” that damage the organism or distort its development
- Effects of environmental factors that influence the organism “top-down” via perceptual experience without resulting in damage or acting via a specific facultative mechanism.
- Effects of environmental factors that influence the organism from the “bottom-up” that are neither damaging nor mediated via facultative adaptations.
- Effects of individual learning that facilitate flexible coping with current aspects of the environment.
- Experiences shared within a culture and whose effects are incorporated into values and emotional proclivities that may be difficult to change later (such as values or attitudes about sex)
- Experiences shared within a culture whose effects account for variation, but these effects are not mediated by enduring influences on individuals.