This work provides a unified theory that addresses the important problem of the origin and maintenance of genetic variation in natural populations. With modern molecular techniques, variation is found in all species, sometimes at astonishingly high levels. Yet, despite these observations, the forces that maintain variation within and between species have been difficult subjects of study. Because they act very weakly and operate over vast time scales, scientists must rely on indirect inferences and speculative mathematical models. However, despite these obstacles, many advances have been made....