Here's why:

* Variation: Differences in traits among individuals within a population. These variations can be in physical characteristics, behaviors, or even at the genetic level.

* Natural Selection: The process where organisms with traits better suited to their environment survive and reproduce more successfully, passing on those beneficial traits to their offspring.

Variation is the fuel for natural selection. Without variation, there would be no differences for natural selection to act upon. The more variation there is in a population, the more likely it is that some individuals will possess traits that give them an advantage in a changing environment.