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More organisms are produced than can survive because of limited resources.

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Organisms struggle for the necessities of life; there is competition for resources.

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Individuals within a population vary in their traits; some of these traits are heritable -- passed on to offspring.

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Some variants are better adapted to survive and reproduce under local conditions than others.

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Better-adapted individuals (the "fit enough") are more likely to survive and reproduce, thereby passing on copies of their genes to the next generation.

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Species whose individuals are best adapted survive; others become extinct.