When a new allele appears in a population, it has the potential to change the genetic make-up of successive generations. Harmful mutations will likely not persist because the affected individual will either not survive, or will have limited reproductive success. However, some mutations may be passed on to successive generations because an organism with that allele is better equipped to survive in its environment, that is, it has a selective advantage. Those individuals that produce a greater number of offspring that survive, are said to be more fit. Other mutations may have no effect on phenotype, and may persist simply by chance. It is the selective advantage that drives evolution, albeit momentarily, in one direction or another.

In 1859, Charles Darwin published The Origin of Species , based in large part on his observations of the striking differences in the beaks of 13 finch species that inhabit the Galapagos Islands off the coast of Ecuador. He noticed that each island's species had developed a unique beak that allowed the animals to access different food sources (insects, seeds and flower nectar) that were also unique to each island. Darwin speculated that the finches originated from a single mainland species, and that as they populated the islands, each was confronted with a different environment that subsequently conferred selective advantage to those birds with a slightly different beak. Continual selection from generation to generation resulted in the highly specialized and diverse island species, in a process called adaptive radiation .

From his observations and speculations, Darwin developed his theory of natural selection , which is a continual process through which new species emerge and old ones become extinct. Darwin noted that in this process, population growth is controlled through the natural struggle for survival and reproduction, and as a consequence only a few individuals will successfully contribute progeny to the next generation. Individuals show variation in heritable traits that influence their success in this struggle, and successive generations show a disproportionate number of individuals with beneficial traits. In Darwin's own words:

These laws, taken in the largest sense, being growth with reproduction; inheritance, which is almost implied by reproduction; variability from the indirect and direct action of the external conditions of life, and from use and disuse; a ratio of increase so high as to lead to a struggle for life, and as a consequence to Natural Selection, entailing divergence of character and the extinction of less-improved forms. Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved. - Charles Robert Darwin