Changes that occur within a particular species are classified as microevolution. It represents evolution on a small scale driven by mutations, environmental factors and natural selection that does not result in the creation of a new species.
Microevolution occurs within a species whenever the frequency of any genes change. The increased resistance of Culex mosquitoes along the Mediterranean coast of France to pesticides over an interval of several years is an example of microevolution. The mosquitoes with genes that protect them from the insecticide survive pesticide application. Areas left pesticide-free experienced no increase in the number of mosquitoes carrying the gene while the amount of gene-carrying mosquitoes in the sprayed areas increased annually.