Here's why:

* Individuals within a population vary: No two individuals are exactly alike. This variation can be in traits like size, color, behavior, or resistance to disease.

* Variation is heritable: Some of these variations are passed down from parents to offspring.

* The environment selects for traits: The environment (including resources, predators, and climate) favors certain traits that help individuals survive and reproduce. Those with advantageous traits are more likely to survive and pass on those traits to their offspring.

* Differential reproductive success: Individuals with advantageous traits have a higher chance of surviving and reproducing, passing on those traits to the next generation. Over time, these traits become more common in the population.

In essence, natural selection is a process of "survival of the fittest," but "fittest" doesn't mean strongest or fastest. It means best adapted to the environment.

Here's a simple analogy: imagine a population of beetles. Some are brown, some are green. The brown beetles blend in better with the forest floor, making them harder for birds to see and eat. Over time, the brown beetles are more likely to survive and reproduce, passing on their brown color genes to their offspring. This leads to a greater proportion of brown beetles in the population, illustrating how natural selection works.