Heritable variation is one of the four most important factors that affect whether natural selection can occur. These factors include 1) Heritable variation 2) Descent with modification 3) Struggle for survival, and necessarily if the others are satisfied 4) Survival of the fittest. Heritable variation allows for possibly beneficial, negative, or even neutral mutations to get filtered during each generation by natural selection--weeding out negative mutations, passing on positive and neutral ones. Without heritable variation, species would quickly fall victim to parasites who take advantage of identical genetic material in a population and evolution would not occur.