Natural selection is the process that enables some organisms or individuals to live and reproduce while others do not survive. Those who reproduce pass their genetic characteristics on to their offspring.
Natural selection is the force that drives Darwinian evolution. For example, certain individuals within a population might possess a genetic characteristic that provides resistance to a local disease. As a result, those individuals tend to survive longer and to produce more offspring than the other members of the population. Moreover, their children who inherit the favorable characteristic will likewise tend to live longer and leave more descendants. Over time, individuals who possess the favorable trait will tend to outnumber those who do not, and the gene frequencies of the population will have changed.
As a result of natural selection, a population that lives in a certain area for many generations tends to exhibit distinctive genetic traits or clusters of traits. Scientists have shown that differences in skin color, body build, and other physical characteristics represent adaptations to environmental factors.
Excerpt from the "Races, Human" article, The World Book Encyclopedia © 1999