Here's why:

* Natural selection is the process where organisms with traits better suited to their environment tend to survive and reproduce more successfully than those with less suitable traits. This leads to the gradual accumulation of favorable traits in a population over time.

* Other mechanisms of evolution: While natural selection is the primary driving force, other mechanisms also contribute to evolution:

* Genetic drift: Random fluctuations in the frequencies of alleles (gene variants) within a population, especially in small populations.

* Gene flow: The movement of alleles between populations through migration.

* Mutation: Random changes in DNA sequences that can introduce new traits.

In essence: Natural selection is the mechanism that drives the adaptation of organisms to their environment, while the other mechanisms contribute to the diversity and variation within populations.