Answer: Microevolution is simply a change in gene frequency within a population. Evolution at this scale can be observed over short periods of time — for example, between one generation and the next, the frequency of a gene for brown coloration in a population of beetles increases. Such a change might come about because natural selection favored the gene, because the population received new immigrants carrying the gene, because of mutation, or because of random genetic drift from one generation to the next.
House sparrows were introduced to North America in 1852. Since that time, the sparrows have evolved different characteristics in different locations. Sparrow populations in the north are larger-bodied than sparrow populations in the south. This divergence in populations is probably at least partly a result of natural selection: larger-bodied birds can often survive lower temperatures than smaller-bodied birds can. Colder weather in the north probably selects for larger-bodied birds.