Environmental and Genetic Variation

An animal's phenotype is determined by a collaboration of its genes and the effects of the environment in which it was reared. Using the symbol V to represent the variation of the phenotype, e to represent variation due to the environment, and g to represent variation due to genetic effects, we can generate a simple equation which describes the relationship among phenotype, environent, and genes:

Vp = Vg +Ve

In other words, total phenotypic variation is equal to variation due to genetic factors plus variation due to environmental factors. Another way of looking at this relationship is to divide the genetic variation by the phenotypic variation. This gives the percentage of the phenotypic variation that can be attributed to genetic influences, which is called the broad-sense heritability (hb^2):

hb^2 = Vg/Vp

Genetic variation can be divided in additive variation--variation caused by differences among genes--and non-additive variation--differences caused by interactions between genes:

Vg= Va + Vn

This allows the calculation of narrow-sense heritability, h^2:

h2 = Va/Vp

By now, of course, you are wondering what all this is good for, and why algebra has any relevance to animal behavior. Heritability is useful in two ways:

It can be used to predict how animals will respond to artificial or natural selection. If a trait has a high heritability, selection or controlled breeding can change that trait. Narrow sense heritability is particularly useful in making this type of prediction.
Broad sense heritability is used as a measure of the magnitude of genetic influences on a trait.

Both of these approaches can be applied to behavioral traits.

The most common way of calculating heritability of a trait is by using a statistical technique called regression analysis. By applying regression analysis to measures taken from family members, such as parents and their offspring, heritability can be estimated.

In theory, strong natural selection should eliminate all additive genetic variation, because the alleles favored by selection will be the only ones remaining in the population. Without additive genetic variation, the narrow-sense heritability is 0 and no further evolutionary change in the trait would be expected. In practice, in some cases the theoretical prediction is supported by data, while in others there is substantial heritability of traits which seem to have been under strong selection. Click here for examples of systems in which heritability of behavioral traits has been studied.