Why does evolution depend on variation, explain?

Evolution depends on variation; to put this into simpler terms, you could say that every animal has physical characteristics that are benefital to its lifestyles. this type of trait that varies throughout a population is called natural selection; examples of this would include, a short necked giraffe compared to a long necked one.( Which one would survive?) the answer would be the long necked giraffe because it can reach taller leaves and is more fitter( survival of the fittest). You could say that these animals who are born differently, have a difficult life; while trying to achieve normal activities. the animals who have more benefital traits make more offspring than the other, in which allows its helpful trait to pass on to future generations.
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The answer would be sand and gravel