Natural Selection, or the Survival of the Fittest

Abstract In this chapter, Darwin emphasizes that natural selection works slowly and over extremely long periods of time, favoring variations that provide advantages in the struggle for nutrients and space, against predators and physical stresses, and even in the struggle for mates. Such “sexual selection” explains why individuals of one sex (usually the males) sometimes look so different from individuals of the other sex of the same species. Natural selection also works not just on adults but also on all other stages of development—eggs, seeds, embryos, larvae, and juveniles—and can act to either increase or decrease the complexity of various organs.

In all cases, the best-adapted individuals will typically survive and propagate their offspring more successfully to the next generation, and those offspring will inherit the characteristics that made their parents so successful. Darwin also discusses the importance of outbreeding (matings among unrelated individuals) in promoting the vigor of offspring and the negative impacts of self-fertilization and inbreeding (matings between close relatives). He also describes the evolutionary advantages of “divergence in character” over time: individuals that differ in certain ways from their fellows can exploit new niches and reduce competition with others of their own species, eventually driving intermediate forms to extinction.

Finally, he provides a detailed example of natural selection in action, creating new species, new genera, and even new families and orders from a few ancestral species in a single genus over thousands of generations. A great many species go extinct because of their inability to compete with newer species that are better adapted to their conditions of life.