The Origin of Species

Darwin now focuses on natural selection, returning to his discussion of a breeder selecting desired characteristics for animals. If breeders can select characteristics they wish to breed in their animals and perpetuate these characteristics in their domestic populations, is it possible for nature to act in the same way? Darwin believes it is. Variations that give one species an advantage, no matter how slight, over another species, allow organisms a better chance of surviving and therefore of leaving more offspring. While breeders can select variations that are beneficial for them, omniscient nature has the power to develop and select variations that are imperceptible to man and beneficial to species in ways that man might never have considered. Nature can make any slight advantage increase an organism's likelihood of surviving over another, and since these advantageous variations are heritable, they can be perpetuated in future generations. Conversely, nature can also limit species by not bestowing advantageous characteristics on them, therefore making the species less likely to survive and putting them at risk of extinction.

Darwin gives a number of examples of variations that could be advantageous to an individual organism, passed along to subsequent generations, and preserved in the future of the species. For instance, a wolf with slimmer features might be able to run faster and therefore to escape from its predators. A bear might be born with the instinct to pursue a more abundant type of prey, which will be easier to obtain than less abundant types. In a more complex case, a plant might contain a sweet nectar or colorful petals that attract more insects than other plants attract, which will spread the plant's pollen more frequently. Similarly, an insect might inherit a smaller body type, allowing it to obtain food more easily from a plant.

Adding to the concept of natural selection, Darwin briefly outlines the concept of sexual selection, which is another factor in species evolution. Because having offspring is key to species' survival, male individuals with variations that allow them to successfully attract females are more likely to reproduce and therefore to produce more offspring than others. These advantageous sexual characteristics are also variations that would be passed on to offspring. Darwin gives examples of tactics that species of male birds use to attract females: singing to their potential mates, displaying various brightly colored feathers, and performing antics to attract attention. If any variation in these characteristics were to give male birds a better chance of mating, that variation would likely spread to the birds' offspring and, as a result, would become predominant in the population in subsequent generations. Variations of this sort, such as color, aptitude for singing, or ornamentation, may have no direct impact on the ability of an individual organism to survive in nature; rather, the variations' utility in helping an organism reproduce is what allows these variations to be selected and perpetuated in a species.

Darwin goes on to explain the geographical requirements for the propagation of advantageous traits, noting that isolation is key to natural selection, as it creates a situation in which few individuals live in a specific area in which no new organisms will be introduced. In isolated areas, variations that allow organisms to survive in that particular environment win out over disadvantageous ones. In time, these variations spread throughout the species, making the organisms appear perfectly adapted to their environment. The workings of natural selection in geographical isolation explain why floral and animal life is unique in island environments. On the other hand, large geographical areas in which plant and animal populations are not isolated allow species to spread and propagate widely, increasing the species' numbers and its overall chances of survival in a range of environments.

Finally, Darwin relates the concept of natural selection to divergences of character and the gradual creation of new species, thus explaining the origin of species. As particular variations are selected over others, these variations lead to a divergence in characteristics from the parent species. Additional variations then lead to more divergences, and as some variations propagate and expand, others die out as a result of the struggle for existence and the limits of population growth. Darwin provides a chart to illustrate this process, demonstrating how these divergences create “branches” of related species. Some branches end as species become extinct, while other branches multiply as more subspecies develop and a new species becomes a parent species. Eventually the divergences become so great that new species come about. Darwin says this process can take tens of thousands of years.

Chapter IV is perhaps the most important chapter in The Origin of Species, because it lays out the principle of natural selection, on which Darwin's theory of evolution is based. Darwin does not answer every question about the origin of species in his theory. For example, he does not explain how variations actually occur in a species population. He simply assumes that new variations in organisms must be proven to be advantageous and that, over time, they change the characteristics of the species. How these variations occur remained a mystery until Hugo de Vries discovered genetic mutation in 1903. Additionally, Darwin's explanation of how variation leads to the creation of species is not based on clear evidence, because the distinction between varieties and species had not yet been defined. The holes in his theory, however, do not take away from the brilliance of his identification of natural selection as the mechanism for evolution. Significantly, Darwin answers the question of how species become so well adapted to their environment without relying on environmental adaptation. With natural selection, a species' adaptation to its environment occurs through a hereditary process—a concept that would eventually connect Darwin's evolutionary theory to discoveries in genetics and genetic mutation in the twentieth century.

Darwin's emphasis on the gradual process of natural selection proved to be a double-edged sword, although it did provide a persuasive argument for his greater theory of evolution. On the one hand, the idea of gradual change enabled Darwin to fight critics who argued that scientists' inability to observe evolution in current populations means that it is not occurring. If change is gradual, a scientist might not observe any change in his lifetime, even though change is constantly occurring. In defense of this concept, Darwin borrows from Lyell's Principles of Geology (1833), which suggests that geological change occurs gradually over time. He relates Lyell's notion of geological evolution to his own theory of species evolution: Just as geological change cannot be seen by observers in one lifetime, species change is also unobservable in one's lifetime, even as it is gradually happening.

However, at the time of the publication of The Origin of Species, Darwin's ideas about the gradual nature of evolution also opened his theory to criticism from the scientific community. The notion that evolution happens gradually would later be proven incorrect. The fossil record showed that species seemed to have appeared and become extinct rapidly, with long periods of continuity between years of rapid change. The rediscovery of Mendelian genetics and mutation theory at the turn of the twentieth century would help resolve this, as Gregor Mendel (often called the father of modern genetics) discovered the patterns of inheritance that explain how traits are passed from parents to offspring. Hugo de Vries later discovered how genes can mutate, introducing new genes into a population that would cause the variations necessary for natural selection to take hold. The discovery of genetic mutations proved that species change occurrs rapidly once a mutation appears.

Darwin explicates natural selection by using metaphors that relate it to human society, a tactic that unknowingly paved the way for the Social Darwinist movement that would spring up years later. In discussing geographical isolation, Darwin uses the term immigrants to describe the introduction of new species into a geographic area. In the late nineteenth and early twentieth century, the rapid increase in immigration (and its effects on society) became a major concern of policymakers in the United States. Some of those calling for limitations on immigration applied Darwin's theory of population change to human society, arguing that the increase of immigrants in the United States was leading to the degeneration of society. Darwin's explanation of sexual selection was influenced by, and had an influence on, human society. By arguing that male organisms contain the variations to attract females, Darwin perpetuates the notion that males are the pursuers of females, with the female playing no active role in the reproductive relationship. Darwin also suggests that males, not females, pass on characteristics to their offspring. Darwin did not consider that females might also contain variations, such as color and shape, that could better attract males and be sexually selected. He therefore implies that males, not females, are the founders of society.

In his discussion of natural selection, Darwin revisits the concept of the glory of nature, which contrasts with the “good versus evil” portrayal of the struggle for existence. Darwin's description of natural selection characterizes nature as a wise force: Because advantageous characteristics are selected and perpetuated, species become perfectly adapted to their environments, minimizing nature's potential to destroy. Natural selection suggests an advantageous and therefore positive end—the perfect adaptation of species to their environment—rather than a struggle to survive, with extinction as an end. Although destruction will happen in the process of natural selection, the overall result is one of survival. Darwin concludes that nature is wise in its selection—even wiser, Darwin notes, than man himself, who cannot always detect advantageous characteristics in his own attempts to breed animal populations. This portrayal of nature complicates Darwin's previous discussion of nature's capacity for both good and harm. By placing man beneath nature in a hierarchy of wisdom and perfection, it also complicates man's relationship to nature itself.