As the strongest evolutionary force, selection has the ability to retain or eliminate alleles in a population. Favorable alleles spread. Unfavorable alleles disappear.
For example, a new mutation or a neutral trait that already exists becomes advantageous under environmental conditions and it enables individuals to be more successful than those without it. Humans are generalists and can thrive in a wide variety of conditions, but all organisms are influenced by changing environmental conditions, some are just more vulnerable or sensitive than others.
Selection favors individuals with the "best" traits, but if conditions change, traits that are neutral or even adverse one day could be advantageous the next. Because of the variation maintained by meiosis, recessive traits, mutation, and gene flow and because of the chance retention of traits due to genetic drift, neutral and undesirable traits will always persist in populations. If conditions change dramatically or quickly, this variation will save a species because natural selection will have more to work with before that species is lost to extinction. --
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