Here's why:

* Variation: Organisms within a population exhibit natural variations in their traits. This variation is due to mutations and sexual reproduction.

* Inheritance: These variations are heritable, meaning they can be passed down from parents to offspring.

* Differential survival and reproduction: In a given environment, some variations may be more advantageous than others, leading to individuals with those advantageous traits being more likely to survive and reproduce.

* Adaptation: Over time, the frequency of advantageous traits increases in the population, leading to adaptation, where organisms become better suited to their environment.

Natural selection is the driving force behind evolution. It is the process by which organisms with traits better suited to their environment are more likely to survive and reproduce, passing on those advantageous traits to their offspring.

Other factors that contribute to evolution include:

* Genetic drift: Random changes in gene frequencies within a population.

* Gene flow: The movement of genes between populations.

* Mutation: The ultimate source of genetic variation.

However, natural selection is the key component that shapes the course of evolution by determining which traits become more prevalent within a population over time.