Genetic variations: Are the differences in performance trait that due to alter gene activity or protein function (inherited trait) can introduce different trait forms in a population to vary from one another. Then, selection was done based on this genetic variation.
The natural selection: only traits that can be selected are those found within the narrow range of variation genetic variation present in the population and helps the individual to reproduce; the genetic variation is more likely to be passed to the next generation. Then, evolution causes a decrease in genetic variation.