Recombination refers to the processes of reassortment and crossing over. These happen during separation of the chromosomes as gametes are produced by meiosis. Gene flow can increase genetic variability by bringing in alleles from other populations. There are several processes that decrease the genetic variability of a population. Natural selection retains relatively more of those alleles that are favorable and tends to reduce those alleles that are less favorable in a particular environment.
Genetic drift is the random loss of alleles.
We'll look at this later in the lecture as a process that happens to very small populations. Finally, you can loose alleles by gene flow. Migrating individuals can remove a non-representative set of alleles from a population. Certainly, evolution is concerned with these big events in biological history. These include the development of the kingdoms over more than 3 billion years.