Charles Darwin

Charles Darwin spent five years, from 1831 to 1836, as the ship's naturalist aboard HMS Beagle on a survey voyage around South America and the Pacific, collecting specimens and taking notes that would occupy him for the rest of his career. The finches and tortoises of the Galapagos Islands, varying subtly from island to island, became a famous part of the story he later told, though the idea of natural selection itself crystallized gradually over the following two decades of reading, breeding experiments with pigeons, and correspondence rather than in a single moment of insight on the islands.

The theory Darwin eventually published rests on a small number of observations that, taken together, have a striking logical force. Organisms produce more offspring than their environment can support, individuals vary from one another in heritable ways, and some variations make their bearers more likely to survive and reproduce than others. Given enough time, Darwin argued, this simple process would accumulate small advantages generation after generation, gradually transforming populations and, eventually, producing new species, all without any guiding intelligence or purpose. He delayed publishing for years, aware how the idea would be received, until he learned that Alfred Russel Wallace, working independently in the Malay Archipelago, had arrived at essentially the same theory, the two men's work was presented jointly to the Linnean Society in 1858, and Darwin published On the Origin of Species the following year.

The book's central claim, that species are not fixed but change over time through an unguided natural process, remains the organizing theory of all of biology, though Darwin himself knew nothing of genes or DNA, the mechanism by which variation arises and is inherited was worked out only in the following century, by scientists building on the rediscovered work of Gregor Mendel.

Darwin argued that natural selection was the main engine of evolutionary change, and it remains the best explanation for adaptation, the way an organism's features fit the demands of its environment: the shape of a beak, the camouflage of a moth, the flight muscles of a hawk. But the modern understanding of evolution, built on population genetics that did not exist in Darwin's lifetime, recognizes that not every change in a population's genetic makeup is the work of selection.

The clearest challenge to a purely selectionist picture came from the Japanese geneticist Motoo Kimura, who proposed in 1968 what is now called the neutral theory of molecular evolution. Kimura argued, and molecular data have since supported, that most changes at the level of DNA sequence are neither helpful nor harmful to the organism carrying them, and spread through a population or vanish from it by simple chance, a process called genetic drift, rather than because selection favored or disfavored them. This is not a challenge to natural selection as the explanation for adaptation, which Kimura never disputed, but a claim about which changes selection is actually responsible for at the molecular level, a distinction that would have been impossible to draw before DNA sequencing existed.

The current, still developing consensus in evolutionary biology is that selection and drift operate together, with their relative importance varying by the size of the population, the part of the genome in question, and how strongly a given trait affects survival and reproduction. Some biologists, working under the banner of an extended evolutionary synthesis, further argue that factors such as how an organism develops from embryo to adult, and even behavior itself, can shape the direction evolution takes in ways the mid twentieth century's modern synthesis did not fully capture. These are live debates within evolutionary biology rather than challenges to whether evolution happened, a distinction a nonspecialist reader can easily lose in headlines that flatten selection is not the whole story into Darwin was wrong.