Natural selection causes what to occur?

Natural selection causes populations to become adapted, or increasingly well-suited, to their environments over time. Natural selection depends on the environment and requires existing heritable variation in a group.

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Explanation:

(I'm not sure if this is what you mean) It causes reproduction, heredity, variation in physical characteristics and variation in number of offspring per individual to occur.

Explanation:

If the four conditions (reproduction, heredity, variation in physical characteristics and variation in number of offspring per individual) change, then a natural selection occurs.