Natural Selection occurs in any situation in which more individuals are born than can survive, (the struggle for existence), there is natural heritable variation, ( variation and adaptation), and there is variable fitness among individuals, (survival of the fittest)
The Struggle for Existence, after Darwin read Malthus, he realized that if more individuals were being produced than can survive members of a population must compete for food, living space, and water and other things needed to survive.
Variation and adaptation, Darwin knew that individuals have natural variations among their heritable traits. He hypothesized that some of those variants are better suited for life in their environment than others. Any heritable characteristics that increases an organism's ability to survive and reproduce in its environment is called an adaptation.
Survival of the Fittest, Darwin understood that there is a connection between the way the organism makes a living, and its environment it lives in. Fitness is how well an organism can survive and reproduce in its environment. Individuals with adaptations that are very well suited for their environment can survive and reproduce and are said to have high fitness. Individuals with characteristics that are not well suited for their environment either die without leaving offspring or leave very few offspring and have very low fitness
Artificial Selection, the intentional reproduction of individuals in a population that have desirable traits. In organisms that reproduce sexually, two adults that possess a desired trait — such as two parent plants that are tall — are bred together.