natural selection

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The process in nature by which, according to Darwin's theory of evolution, only the organisms best adapted to their environment tend to survive and transmit their genetic characteristics in increasing numbers to succeeding generations while those less adapted tend to be eliminated.

Wiktionary

1. n. 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.
2. n. 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.

GNU Webster's 1913

1. adj. (Biol.) the operation of natural laws analogous, in their operation and results, to designed selection in breeding plants and animals, and resulting in the survival of the fittest; the elimination over time of species unable to compete in specific environments with other species more adapted to survival; -- the essential mechanism of evolution. The principle of natural selection is neutral with respect to the mechanism by which inheritable changes occur in organisms (most commonly thought to be due to mutation of genes and reorganization of genomes), but proposes that those forms which have become so modified as to be better adapted to the existing environment have tended to survive and leave similarly adapted descendants, while those less perfectly adapted have tended to die out through lack of fitness for the environment, thus resulting in the survival of the fittest. See Darwinism.
2. n. (Biol.) See under Natural.

WordNet 3.0

1. n. a natural process resulting in the evolution of organisms best adapted to the environment