From the glossary of this site:
quote:
Natural selection: Differential reproduction or survival of replicating organisms caused by agencies that are not directed by humans (See Artificial selection). Since such differential selective effects are widely prevalent, and often act on hereditary (genetic) variations, natural selection is a common major cause for a change in the gene frequencies of a population that leads to a new distinctive genetic constitution (evolution).
Think of a tournament in which all contestants pair off and compete, then the winners of all those competitions pair off, and so on. The traditional arrow-shaped diagram used to keep track of tournament standings is a useful way to visualize the process. There's no reason that any one of the possible pathways through the diagram should be the
a priori correct one, but one contestant will eventually emerge as the winner.
Natural selection is just such a filter, except that the lines of the diagram also radiate from each nexus as the winners advance. In other words, the winners of each round reproduce more contestants, and the process becomes a cumulative one. The inherent variation in any population is therefore subjected to successive rounds of testing, and each winning variation both narrows down the field (by eliminating other variants) and tries to expand it to its advantage (by reproducing variations on its successful genetic formula).
regards,
Esteban Hambre