Darwin and Wallace's theory of organic evolution via natural selection is built on several key observations. First, organisms have an innate potential to produce vast numbers of offspring (Overproduction) . However, in nature, resources are limited, and as a result, population sizes generally remain stable over time (Constancy of population size) . This leads to a struggle for existence. Within any population, individuals exhibit heritable differences (Variations) . Those individuals with variations that provide a survival advantage in their specific environment are more likely to reproduce and pass those traits to the next generation, a process termed Natural Selection .