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So you've got a species which we'll label "1". Within the species 1 there are slight variations, so you have 1a, 1b, and so on. Let's say in this species there are 10 1a and 10 1b. In a certain environment, 1a dies and 1b survives. 1b's mate with each other, until the species become soley 1b's. As the environment changes, so do the attributes needed for survival.
Dunno if that makes any sense, but it's hard to explain.
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