Populations of individuals have the capacity to produce more offspring than the environment is able to support, so individuals must compete for resources.

Individuals of a population vary in their traits. The variant forms of a trait may be more or less adaptive under prevailing conditions.

When a form of a trait is adaptive under prevailing conditions, and when it has a heritable basis, its bearers tend to survive and reproduce more frequently than individuals with less adaptive forms of the trait. Over generations, the adaptive version becomes more common in the population.

Natural selection is the result of differences in survival and reproduction among individuals of a population that differ from one another in one or more traits.

Natural selection results in modifications of traits within a line of descent. Over time it may bring about the evolution of a new species, with an array of traits uniquely its own.