Variation - Background
Additive genetic variation is important in determining the strength of natural selection. Populations with a large amount of additive genetic variation can adapt quicker to changing environments. Populations with little additive genetic variation adapt more slowly, and are more susceptible to extinction.
Natural selection may also affect the amount of variation in a population. For example, persistent selection in one direction (such as that seen to the right) can reduce the amount of variation in a population. This variation can only be replenished by mutation, which is the source of all new variation. Variance is a descriptive statistic which is a measurement of the amount of variation in a population.