Here's why:

* Natural selection acts on existing variation: Natural selection doesn't create new traits, it selects for traits that already exist within a population. This variation can be caused by mutations, gene flow, or sexual reproduction.

* Variation provides the raw material for selection: If all individuals in a population were identical, there would be no basis for natural selection to operate. It's the differences between individuals that allow some to be better suited to their environment than others.

* Variation leads to differential survival and reproduction: Individuals with traits that make them better adapted to their environment are more likely to survive and reproduce, passing on those traits to their offspring. This leads to the gradual accumulation of favorable traits within a population over time.

Therefore, variation is essential for natural selection to occur and drive evolution.