What must be present first in order for natural selection to occur?

In order for natural selection to occur there must be a change in the environment (I. e. natural disasters, predators, food source change, etc.) and then one of two things must happen: either all

Of the organisms die out in that population, or some survive and reproduce and all of the organisms have better suited traits for their environment

Four conditions are needed for natural selection to occur: reproduction, heredity, variation in fitness or organisms, variation in individual characters among members of the population. If they are met, natural selection automatically results.

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