How does natural selection affect animals

It affects animals by gradually (over generation) "selecting" or rather favoring those animals that seem to be more adapted to their environment. In effect also, those animals that survive in a certain environment seem to be very adapted it. 

One very important point we have to understand when talking about natural selection is that this is a very slow and gradual process, it doesn't happen over night but rather takes very long and this means generations and generations of animals. In this sense, you won't see natural selection having drastic effects in only one night for example. 

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