Why do populations have variation in certain traits

Because of adaptation

Explanation:

adaptation helps make different varieties of a species. when an species is in a different environment it has to adapt giving it different traits

c.more precipitation,but only when emission levels.

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.