Why is differential success among organisms with different variants of a trait necessary for natural selection?

This is because the organisms  of high variant will have   high  resistance  to selective pressures  and therefore   higher reproductive success  and   more selective advantage due to natural selection  to survive than others with  less  successful variant and low resistant to selective pressures.

Explanation:

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the wetland preservation movement began in the u.s. in earnest with the clean water act (cwa) of 1972. although not specifically written to protect wetlands, the cwa is comprised of a series of regulations designed to prevent pollutants from contaminating u.s. waters.