8.4: Observing geographic similarities and differences among fossils and living plants and animals, Darwin developed a theory of evolution.

Darwin didn’t have a great “Eureka!” moment about evolution while on the Beagle. The wheels were turning in his head after his voyage, however, and inspiration struck when he was reading “for amusement” Essay on the Principle of Population, by the economist Thomas Malthus. Malthus prophesied doom and gloom for populations, including humans, based on his calculations that populations had the potential to grow much faster than food supplies could. Darwin speculated that, rather than the future holding certain catastrophe for all, maybe the best individuals would “win” in the ensuing struggle for existence, and the worst would “lose.” If so, he suddenly saw that “favourable variations would tend to be preserved, and unfavourable ones to be destroyed.”

In 1842, Darwin prepared a draft of his ideas in a 35-page paper, written in pencil, and he fleshed it out over the next couple of years. He knew that his idea was important—so important that he wrote a letter to his wife instructing her, in the case of his sudden death (an odd request, given that he was only 35 years old), to give his “sketch” to a competent person, along with about $1,000 and all of his books, so that this person could complete it for him.

He also had an inkling that his ideas would rock the world. In a letter to a close friend, he wrote: “At last gleams of light have come, and I am almost convinced (quite contrary to the opinion I started with) that species are not (it is like confessing to murder) immutable.” But “confessing to murder” was apparently more than Darwin was ready for. Inexplicably, he put his sketch into a drawer, where it remained for 14 years—even as his friends, including Charles Lyell, warned him that he should publish his ideas in case someone else came up with the same ideas independently. In 1858, Darwin found that his friends’ warnings had “come true with a vengeance.”

In a letter to Darwin, Alfred Russel Wallace (FIGURE 8-7), a young British biologist in the throes of malaria-induced mania in Malaysia, laid out a clear description of the process of evolution by natural selection, after having read Malthus’s book. He asked Darwin to “publish it if you think it is worthy.” Although he may have been inspired by his fever to put his thoughts in writing, they were careful and precise, and resulted from many years of thought and exposure to the writings of many of the same scientists who influenced Darwin. Crushed at having been scooped, Darwin wrote that “all my originality will be smashed.” He had numerous friends among the most prominent scientists of the day, however, and they arranged for a joint presentation of Wallace’s and Darwin’s work to the Linnaean Society of London. As a result, both Darwin and Wallace are credited for the first description of evolution by natural selection.

Darwin then sprang into action, rapidly putting together his thoughts and observations and completing, 16 months later, a full book. In 1859, he published The Origin of Species (its full title is On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life).

The book was an instant hit, selling out on its first day, provoking public discussion and debate and ultimately causing a wholesale change in the scientific understanding of natural selection and many other important evolutionary ideas. Where once the worldview was of a young earth, populated by unchanging species all created at one time, with no additions or extinctions, now there was a new dynamic view of life on earth: descent with modification—species could and did change over time, and as some species split into new species, others became extinct (FIGURE 8-8).

Darwin’s theory has proved to be among the most important and enduring contributions in all of science. It has stimulated an unprecedented diversity of theoretical and applied research, and it has withstood repeated experimental and observational testing. With the background of Darwin’s elegant idea in hand, we can now examine its details.