This is Microevolution , Systematic changes in the frequency of homologous alleles, chromosome segments, or chromosomes in a local population. Changes in allele frequencies can be attributed to 4 factors. These are the primary factors in Evolution. Almost all mutations are harmful. Accumulation of dominance modifiers takes time.
Most mutations have a minor affect.
Mutation rates are under genetic control Drosophila example Chr. Dispersal - Movement of individuals, spores, gametes, etc. If know the allele frequencies and the migration rate, you can easily calculate the expected new allele frequencies of the population. Idea of a panmictic breeding unit vs. a neighborhood. The differential and non-random reproduction of different alternative alleles in a population.