Evolution by Natural Selection

 The central question still remained: if evolution occurred, by what means did it occur? In 1838 Darwin read a book called Essay on the Principle of Population by a British economist, Thomas Malthus (1776-1834). Malthus stated that a human population growing unchecked would double every 25 years. Resources such as food, air and water cannot increase at the same rate, Malthus argued. Thus human beings are involved in an intense "struggle for existence," competing for the limited resources. This idea helped Darwin uncover the mechanism he needed.

Combining the idea of competition with his other observations, Darwin explained how evolution could occur. First, he stated that variation exists among individuals of a species. Second, he stated that scarcity of resources in a burgeoning population would lead to competition between individuals of the same species because all use the same limited resources. Such competition would lead to the death of some individuals, while others would survive. From this reasoning Darwin concluded that individuals having advantageous variations are more likely to survive and reproduce than those without the advantageous variations.

Darwin coined the term natural selection to describe the process by which organisms with favorable variations survive and reproduce at a higher rate. An inherited variation that increases an organism's chance of survival in a particular environment is called an adaptation. Over many generations, an adaptation could spread throughout the entire species. In this way, according to Darwin, evolution by natural selection would occur.

As an example Darwin noted that the ptarmigan turns white in winter. This color change, he inferred, helped protect it from predators, which would have a hard time spotting the bird in snow. Ptarmigans that didn't change color in winter would be spotted easily and eaten. In this way, Darwin implied, ptarmigans that turned white in winter would be more likely to survive, reproduce, and pass this adaptation to future generations.

The Origin of Species

Darwin compiled evidence for evolution by natural selection for about 20 years. Between 1842 and 1844 he wrote a 230-page essay summarizing his theory and the evidence for it. In the 1850s he began working on a detailed, multivolume book to present his theory to the scientific community. Darwin might never have completed the book if another British scientist, Alfred Russel Wallace (1823-1913), had not come up with the same idea in 1858. While living in the Malay Archipelago in the Pacific Ocean, Wallace formulated his theory and wrote it in an essay, which he sent to Darwin. Darwin's fellow scientists persuaded him to let them present his theory and Wallace's essay jointly at a scientific meeting. The presentation excited very little attention, according to the modest Darwin.

However, the publication of Darwin's book The Origin of Species in 1859 changed biology forever. The first printing of the book sold out in one day. Darwin clearly and logically presented the idea that natural selection is the mechanism of evolution. In Darwin's own lifetime many scientists became convinced that evolution occurs by means of natural selection. Today this theory is the unifying one for all biology.