Darwin’s theory is based on the notion of variation. It argues that the numerous traits and adaptations that differentiate species from each other also explain how species evolved over time and gradually diverged. Variations in organisms are apparent both within domesticated species and within species throughout the natural world. Variations in colors, structures, organs, and physical traits differentiate a multitude of species from one another. Heredity is the mechanism that perpetuates variations, Darwin argues, as traits are passed from parents to offspring. What is important about these variations to Darwin, though, is the way they allow species to adapt and survive in the natural world. He gives numerous examples of variations that illustrate the wondrous adaptations that allow species to survive in their natural environments: the beak that allows the woodpecker to gather insects, the wings that allow the bat to fly, the paddles that allow the porpoise to swim, and so on. Darwin hypothesizes that the minor variations we see within a single species—such as variations in size, shape, and color of organisms—are related to the more distinct variations seen across different species. His theory of evolution explains how variations cause the origin of species.

Natural selection is the key component of Darwin’s theory, as it explains the relationship between variation and the eventual evolution of a species. Borrowing from Thomas Malthus’s principle of exponential population growth, Darwin argues that the possibility of infinite growth of population sizes is checked by the limits of geography and natural resources, which will not allow an infinite number of beings to survive. As a result of limited food, water, shelter, and so on, species must engage in a “struggle for existence,” creating competition for survival. What decides, then, which species will survive and which will become extinct? Here is where “natural selection” comes in. Darwin argues that organisms exhibiting “advantageous variations”—variations that will allow them to adapt to their environment better than other organisms do—will be more likely to survive. Through heredity, these advantageous variations will be passed on to the organisms’ offspring. Eventually, natural selection will allow those species best adapted to their environments to survive and prosper, while species without these advantageous adaptations will lose the struggle for existence and become extinct.

Natural selection is the mechanism that leads to “descent with modification,” Darwin’s term for the process of evolution. Organisms will continually give birth to offspring that carry variations, some of which are advantageous and some of which are not. As advantageous variations are naturally selected and become perpetuated through successive generations, organisms carrying these advantageous variations will diverge from the original species, eventually becoming a species of their own. Continual modification and divergence, then, create a branching scheme of evolution, in which new species continually branch off from old ones. The “branches” help biologists link later species back to an original parent species, identifying the point at which different species are related to one another. Darwin notes that existing classification systems developed by naturalists already show these relationships between species. Darwin’s theory of descent with modification, then, simply provides an explanation for why many species seem so similar: Either they evolved from one another, or they both evolved from a common parent species.

After laying out the main principles of his theory in the early chapters of On the Origin of Species, Darwin devotes much of the rest of the book to defending his theory against criticisms and presenting detailed examples of how natural selection occurs. The geological record is a formidable impediment to Darwin’s theory, as the existing fossil record does not provide the “missing links” in the chains of descent that Darwin proposes. In response, Darwin argues that the geological record is imperfect and that many fossil remains have been destroyed by changes in the earth or have yet to be discovered.

Darwin also attempts to explain how variations occur in species, driving natural selection and the creation of new species. Geographical isolation is a key component of Darwin’s theory. Darwin hypothesizes that because all species originated from one or a few original beings, species needed modes of transportation to migrate between geographical areas throughout the world. Barriers such as oceans and mountain ranges restrict the ability of organisms to migrate, and the few that manage to do so play a large role in shaping the evolution of species on islands and in geographically isolated areas. Geographical isolation accounts for the plethora of unique species on islands, as well as the wider distribution of species across continents.

Darwin’s theory challenged not only the prevailing view of the independent creation of species but also larger claims of religion and science. Darwin explicitly denied the validity of natural theology, which posited that species’ adaptations to their environments was proof of their “intelligent design” by a creator. It was natural selection, not independent creation, that resulted in these adaptations, Darwin argued. Moreover, Darwin’s use of scientific methodology to prove his theory amounted to an explicit critique of naturalists who would attempt to ignore the scientific validity of his theory because of its controversial nature. While the text of The Origin of Species did leave room for religious theology, Darwin’s overall commitment to scientific rationale rather than theological reasoning pitted him against religious doctrine. Darwin’s text was controversial when it was published, and it remains controversial today. However, his theory of natural selection has stood the test of time in scientific circles, and it remains the leading scientific explanation for the origin of species.