natural selection

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The process in nature by which, according to Darwin's theory of evolution, organisms that are better adapted to their environment tend to survive longer and transmit more of their genetic characteristics to succeeding generations than do those that are less well adapted.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun evolutionary biology A process by which heritable traits conferring survival and reproductive advantage to individuals, or related individuals, tend to be passed on to succeeding generations and become more frequent in a population, whereas other less favourable traits tend to become eliminated.
• noun quantitative genetics A process in which individual organisms or phenotypes that possess favourable traits are more likely to survive and reproduce: the differential survival and reproduction of phenotypes.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a natural process resulting in the evolution of organisms best adapted to the environment