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 (as shown below), because the population received new immigrants carrying the gene, because of mutation, or because of random genetic drift from one generation to the next.