Charles Darwin was not the first to come up with the concept of evolution. Many scientists, including Darwin’s grandfather, had accepted that plants and animals on Earth had changed form over time. Already in 1809, Jean-Baptiste Lamarck had argued that modern animals, including humans, had descended from other species. Lamarck promoted the idea of “inheritance of acquired characteristics” as the mechanism governing this evolution. For example, Lamarck argued that the neck of a giraffe stretching for food would eventually lengthen slightly, a physical characteristic that would be passed to its offspring.

In On the Origin of Species by Means of Natural Selection, better known as On the Origin of Species, Darwin achieves two things that Lamarck does not. He gathers detailed evidence to support evolution and offers a different, and ultimately workable, mechanism to govern it. This mechanism, natural selection, has been supported by later research, whereas Lamarck’s idea of inheritance of acquired characteristics has not. Natural selection is believed to be the major (but not only) force driving species evolution.

Natural selection is a simple concept. Consider animals. First, individuals of any species vary in size, coloration, behavior, and other characteristics. Second, these variations tend to be passed on to an individual’s offspring. Third, more offspring are born than those surviving to reproduce. Fourth, any characteristic (such as coloration) that allows one individual to survive better than others will help that individual reproduce more. Fifth, this characteristic will increase in frequency in the species over time.

Natural selection has been observed in many species. The finches of the Galapagos Islands provide a good example. The Galapagos finches are a group of closely related bird species that helped Darwin discover natural selection. These birds differ primarily in behavior and in beak size and shape. In dry years, heavier-beaked finches survive and reproduce better because they can open bigger seeds. Average beak size can change dramatically over a single breeding season. This illustrates natural selection.

Small changes within a species represent “microevolution,” although Darwin did not use this term in On the Origin of Species. Microevolutionary changes that continue over time can lead to new species, especially if physical barriers (such as isolation on different islands in, for example, the Galapagos) separate subgroups of the original species. This is “macroevolution,” which also has been observed in the laboratory, although with relatively small changes.

On the Origin of Species is a long work, but Darwin, in the book’s introduction, describes it as an “abstract.” He had been working on a longer book, but decided to publish his core thoughts after Alfred Russell Wallace independently discovered natural selection in 1858. Darwin had worked on the concept since 1837 and had written an unpublished 230-page report on it by 1844. Ultimately, he produced six editions, and historians are undecided as to whether the first edition or the sixth edition should be considered definitive. The primary information is similar in all editions, but the sixth contains “An Historical Sketch,” which discusses evolutionary thinking before the work was published.

On the Origin of Species is one long argument for evolution and for natural selection as the primary force behind it. Chapter 1 discusses variation among domesticated plants and animals and shows how humans have used this variation to produce specialized subvarieties. Darwin illustrates his concepts through a study of domesticated pigeons, which have been selected and bred by humans for certain desirable characteristics. Darwin calls this selected breeding artificial selection.

In chapter 2, Darwin examines the tremendous variability seen among plants and animals in nature. Even naturalists disagree on how to divide species and subspecies because of individual variability within each species. Darwin shows that common species with wide geographical ranges vary more than localized species. Chapter 3 focuses on the “struggle for existence.” From reading Thomas Malthus’s An Essay on the Principle of Population as It Affects the Future Improvement of Society (1798) and from personal observations, Darwin came to realize that far more offspring are born in a species than ever reach adulthood. This causes a ceaseless struggle for natural resources. Competition is most fierce among members of the same species.

Chapter 4 fully describes and defines natural selection. Darwin shows how the principle must act across all species and how it could explain extinction of previously living forms as well as the wide divergence of forms seen in the modern world. He also introduces the idea of sexual selection, which involves competition for mates, especially among male animals. Sexual selection favors characteristics (for example, horns) that help males defeat other males or (as in the elaborate plumage of male birds) help attract females.

In chapter 5, Darwin explores potential causes for natural variation. He admits not knowing the causes, but he argues that “use and disuse,” a Lamarckian concept, could play some role. For example, he suggests that flightless birds had become flightless for failing, over time, to use their wings. From a modern perspective, this chapter can be considered relatively weak because Darwin knew nothing of genetics, which later resolved the issue of natural variation in favor of natural selection over the concept of use and disuse.

Scientists should point out potential flaws in their own theories, as Darwin does in chapters 6, 7, 8, and 9. Chapter 6 acknowledges that good transitional fossils are rare in the fossil record (although many such fossils have since been discovered). Darwin also discusses organs (such as eyes) that seem too complicated to have evolved through natural selection, and he examines the difficulties in imagining dramatic changes, such as land mammals transforming gradually into aquatic ones. However, he gives numerous examples of intermediate characteristics in living animals that could be worked on by natural selection. For example, he describes modern species with simple eyes that are still adaptive.

Chapter 7 points out that complex instincts, such as those governing honeycomb-building by bees, pose some difficulty for natural selection. However, Darwin shows that instinctive behavior varies within individuals of a species and that this variability could be acted on by natural selection. Chapter 8 deals with hybridism, the crossing of closely related species, which usually results in sterile offspring. Knowledge of genetics would have helped Darwin here.

Chapter 9 discusses the imperfection of the fossil record. From reading Sir Charles Lyell’s masterwork The Principles of Geology (1830-1833; 3 vols.), Darwin understood the earth’s vast age and how it changed over time. He understood that fossils rarely form, and that many existing ones are buried and irretrievable. Others are destroyed by upheavals of the earth or by erosion. Chapter 10 reemphasizes the imperfection of the record but argues that available fossils do support natural selection. Darwin discusses the role of extinction, and points out that recent fossils resemble still-living creatures more closely than do earlier fossils.

Chapters 11 and 12 focus on the geographical distribution of plants and animals. Darwin shows how similar species live in related geographical areas, and he shows the importance of natural barriers in creating new species. He focuses on islands, like the Galapagos, which illustrate his points. Chapter 13 discusses how humans naturally organize species by their similarities and how these similarities reflect real biological relationships. Darwin also covers anatomical similarities, embryonic similarities, and the existence of vestigial organs such as rudimentary wings in some insects. Chapter 14 summarizes and restates the work’s major points.

On the Origin of Species is one of the most influential scientific books in history. It revolutionized biology and has greatly influenced modern psychology and philosophy, among other fields. Darwin’s concepts of natural and sexual selection remain mainstays of evolutionary thinking, but most of his original ideas have been modified and extended by the field of genetics. Darwin’s book remains controversial, but the controversy is more cultural and political than it is scientific.