In order for natural selection to operate, genetic variation must be present. Genetic variation leads to heterogeneity between generations; if the environment favors one variant, then this advantageous variant will increase in frequency over generational time. If environmental conditions change, individuals migrate, or new predators or pathogens are introduced, then genetic variation may allow a new variant to do better in these new conditions. Therefore, genetic variation is "insurance" for organisms against changing conditions because it helps to insure that some of their offspring will survive if conditions change.