Charles Darwin introduced the idea of evolution by natural selection in On the Origin of Species.
Ideas about the emergence and disappearance of species throughout Earth's history have been around since ancient Greece. Charles Darwin, an English naturalist born in the early 1800s, was raised in a scientific climate in which his contemporaries studied evidence that would contribute to the development of modern evolutionary thought. Geologist Charles Lyell examined evidence that Earth is old and in a state of constant change. Biologist Thomas Huxley focused on comparisons of the anatomy of various organisms. And economist Thomas Malthus observed that populations grow exponentially while resources grow arithmetically (slower than populations), resulting in competition for resources. A population is an interbreeding group of individuals of the same species. These ideas and others set the stage for Darwin's theory. As evolutionary biologist Theodosius Dobzhansky wrote in a famous essay more than 100 years after the publication of On the Origin of Species, "Nothing in biology makes sense except in light of evolution." Narrowly defined, evolution is a change in gene frequencies within a population from one generation to the next. Darwin traveled around the world on the HMS Beagle as a resident naturalist. During the five-year expedition (August 1831 to October 1836), Darwin took copious geological and biological notes and collected hundreds of samples of species from four continents. This gave him an opportunity to synthesize a huge collection of observations. One observation that Darwin noted repeatedly was that organisms tend to have traits allowing them to survive and reproduce in their environment. He began sharing his notes and early thoughts about his experience with other scientists upon his return. Personal illness and deaths in his family, however, slowed Darwin's progress toward publication of his work. Twenty years went by as Darwin worked on his theory. When a younger scientist named Alfred Russel Wallace wrote to Darwin with a description of his thoughts on evolution and its mechanisms, it was so close to Darwin's own ideas that he decided to publish some notes along with Wallace's paper. A few years later, Darwin published On the Origin of Species. In this book, he detailed two major ideas that would create the field of evolutionary biology: (1) Populations evolve over generations through a process called natural selection. (2) All life descended from a common ancestor. Natural selection is a mechanism of evolution in which individuals that are better adapted to their environment survive and reproduce more successfully than less well adapted individuals do.