Here's why:

* Natural selection acts on existing variation: It favors individuals with traits that are better suited to their environment, allowing those individuals to survive and reproduce more successfully. This means the variation must already be present in the population.

* Sources of variation: The primary sources of variation within species are:

* Mutations: Random changes in the DNA sequence. These changes can introduce new traits into a population.

* Recombination: The shuffling of genes during sexual reproduction. This creates new combinations of existing genes.

* Gene flow: The movement of genes between populations. This can introduce new alleles into a population.

Natural selection is a filter: It acts on the variation already present in a population, preserving beneficial traits and reducing the frequency of less advantageous traits. It doesn't create new variation itself.

In summary: Natural selection is the mechanism that drives evolution by selecting for advantageous traits, but it relies on the pre-existing variation within a species generated by other processes like mutation, recombination, and gene flow.