Differential reproduction is a crucial component of Charles Darwin's theory of natural selection. It states that individuals within a species compete with each other for mates, and those that are best equipped to succeed in this struggle will produce the most offspring. More generally, those individuals who are best equipped to survive the longest will produce more offspring than individuals who have shorter lifespans. And while Darwin was very hazy on his understanding of heredity (so was everyone else at the time besides for Gregor Mendel), he did recognize that individuals tend to resemble their parents. In this way, evolution occurs when the fittest individuals produce the most offspring- and over time the most descendants- and the least fit individual produce the fewest. Eventually, most or all of a population comes to resemble its fittest members.