Final answer:
The fourth part of natural selection is variation, the inherited differences among individuals that contribute to the adaptability of a population to its environment, leading to evolution through 'survival of the fittest'.
Step-by-step explanation:
The fourth part of natural selection that complements overproduction, selection, and adaption is variation. Variation refers to the differences among individuals in a population. These variations can be inherited and greatly influence an organism's adaptability to the environment.
Because of these variations, those individuals with traits that are favorable in their environment are more likely to survive and reproduce, passing those advantageous traits to the next generation, a concept known as survival of the fittest.
Over time, this process leads to populations evolving to better suit their environment. Ultimately, natural selection is the force of evolution that enables the population to become better adapted to its local environment through the ongoing and cumulative changes to inherited traits.