Darwin's theory of natural selection:

Charles Darwin was a British naturalist who proposed the theory of biological evolution by natural selection.

Darwin defined evolution as

 "descent with modification," the idea that species change over time, give rise to new species, and share a common ancestor.

Purpose of natural selection:

The mechanism that Darwin proposed for evolution is natural selection. Because resources are limited in nature, organisms with heritable traits that favor survival and reproduction will tend to leave more offspring than their peers, causing the traits to increase in frequency over generation.

Natural selection:

This mechanism explains that:

How populations could evolve (undergo descent with modification) in such a way that they became better suited to their environments over time.

Key observations:

Darwin's concept of natural selection was based   on several key observations:    

Traits are often heritable.

“In living organisms, many characteristics are inherited, or passed from parent to offspring.”

More offspring are produced than can survive.

Organisms are capable of producing more offspring than their environments can support. Thus, there is competition for limited resources in each generation.

Offspring vary in their heritable traits.

The offspring in any generation will be slightly different from one another in their traits (color, size, shape, etc.), and many of these features will be heritable.

Conclusion from above observations

In a population, some individuals will have inherited traits that help them survive and reproduce (given the conditions of the environment, such as the predators and food sources present). The individuals with the helpful traits will leave more offspring in the next generation than their peers, since the traits make them more effective at surviving and reproducing.

Traits are heritable:

Because the helpful traits are heritable, and because organisms with these traits leave more offspring, the traits will tend to become more common (present in a larger fraction of the population) in the next generation

Population’s adaptations to environment:

Over generations, the population will become adapted to its environment (as individuals with traits helpful in that environment have consistently greater reproductive success than their peers).

Hair color is inherited trait.

Natural selection depends on the environment

 Natural selection doesn't favor traits that are somehow inherently superior. Instead, it favors traits that are beneficial (that is, help an organism survive and reproduce more effectively than its peers) in a specific environment. Traits that are helpful in one environment might actually be harmful in another.

Natural selection acts on existing heritable variation

Natural selection needs some starting material, and that starting material is heritable variation. For natural selection to act on a feature, there must already be variation (differences among individuals) for that feature. Also, the differences have to be heritable, determined by the organisms' genes.