We've been talking a lot about alleles. We really needed to get through those discussion for you to appreciate how evolution works. Focus now on evolution as a change in the frequency of alleles in a population. What we need to look at is how these frequencies change. There are several processes that increase the genetic variability of a population.
Mutations increase variability.
This is not always good change, as you've seen before. The point is, however, that the frequency of the alleles increases when there are mutations. Recombination also increases variability. Recombination refers to the processes of reassortment and crossing over. These happen during separation of the chromosomes as gametes are produced by meiosis.