By selecting thos organisms that will become reproductively successful, natural selection controls the future frequency of a population's genes. The appearance of new mutations in a population together with the change in gene frequency results in evolution. The spatial isolation of sub-populations from a main population is also an important condition for evolution. If a remote sub-population cannot trade genes to the main population because of isolation, natural selection acts differently in each population resulting in divergent evolution . Differences in gene frequency emerge between the two populations because no two patches of habitat are absolutely identical, and because new adaptations enter the gene pool through mutations at the level of the individual.
Isolation can be due to distance, mountain barriers, a river, etc.
The best known example of natural selection operating in a modern species is the development of pesticide resistance in many insect species. Prior to the extensive use of pesticides that began in the 1940s, crop pest insect species only contained a small amount of genetic variability for resistance to these chemicals. Natural selection in the absence of pesticides could not lead to changes in the frequencies of genes causing resistance to chemical pesticides. However, once the spraying of pesticides started, individuals that happened to possess resistant genes became much more frequent as they survived the applications. Further, they were able to pass a greater percentage of their genes on to the next generation's gene pool.