Microevolution can be caused by natural or artificial selection, mutation, migration, non-random mating, or genetic drift. All of these processes cause a change in allele frequencies within a population over time. The time scale necessary to observe microevolution is relatively small, only a few generations.
It is possible for humans to observe microevolution in a population if the changes in allele frequencies cause major phenotypic divergence from the original population. For example, dog breeding, a type of artificial selection, has created major morphological and behavioral divergence in dogs. This has led to the large number and diverse set of dog breeds observed as pets.
Selective pressure that leads to microevolution can also be seen in nature. This is known as natural selection. Natural selection occurs when an individual or group of individuals inherit an allele that provides a survival advantage over the rest of the population. Since those individuals are more likely to live longer, they breed more, passing their advantageous alleles on to the next generation. Over several generations, the advantageous alleles become more prominent, or in extreme cases, become fixed.
Sexual selection, or non-random mating, can also lead to microevolution. An example of non-random mating in humans is inbreeding.
Random chance events can also cause changes in allele frequency within a population. Mutation, for example, can allow for new alleles to be created within a population. Migration, or the movement of individuals into the population, can also bring new alleles that were not present in the original population. Finally, genetic drift occurs when a population size is suddenly reduced, and by chance the remaining individuals possess a similar set of alleles.