i'll add.  you have replicating mechanism - dna. the most successful , most adaptive organism, organism with the most food survives and replicates.  also in organisms with separate sexes, like animals and humans, there are other selective mechanisms of quick evolution. > females select the most successful , tallest, with the most money , with the most food, house etc. > if let's say we have a planetary catastrophe and need to survive in caves. > the most successful will be shorter but muscular, with better night vision to stay warm and find food etc. they will be selected by females for reproduction.
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