Written for Anthro 205, 4/13/1999
Natural Selection is defined by Charles Darwin on page 3 of his 1878 edition of Origin of the Species as when “many more individuals of each species are born than can possibly survive; and. . . there is a frequently recurring struggle for existence, any being, if it vary however slightly in any manner profitable to itself, will have a better chance of surviving, and thus be naturally selected.” He also refers to this principle by the term Herbert Spencer coined, survival of the fittest. Darwin considered change on a local level to be the most important cause of variation. Natural Selection to him was about individual adaptation to a changing local environment. He did not refer to the process as evolution because in the terms the word was understood in the Victorian era, evolution implied progress, which is not how Darwin saw Natural Selection. His theory was non-progressive; it was local adaptation, not a process toward a pre-determined end, the goal of a species.
It is mathematically impossible that all offspring of an organism should survive. If this were so, the earth would be overrun by the accumulation of species in a very short time, as is illustrated by Linnaeus’s calculations on annual plants, and Darwin’s on elephants. Therefore only the offspring who can survive the circumstances in which they live will propagate the next generation. The strength of the “hereditary tendency” ensures that these characteristics or variations which allowed the organism to survive will be passed on to its offspring, and they in turn will struggle for their own existence and develop variations which allow them to do so. Natural Selection, then, is the process by which useful variations (i.e. variations that aid in an organism’s struggle for existence) are preserved and accumulated and injurious or nonuseful variations are destroyed, or weeded out. It does not cause variability, it only preserves variations which arose on their own. Natural Selection, Darwin is careful to stress, does not imply Deity or a conscious choice on the part of the organism, it is merely a function of (non-personified) Nature. The well-designed nature of organisms does not prove God’s existence; this is just a side consequence of the struggle for reproductive success. Large species are more likely to produce variations than species of small numbers; rare species are less quickly modified because there are fewer organisms. They are therefore more sensitive to destruction. They become rarer and rarer as a more common species beats them to the punch, as it were, and replaces them and their function in Nature, and thus less well-adapted species become extinct through Natural Selection. Continued selection produces more pronounced variations and eventually leads to a new species. Hence Darwin likes to call variations “incipient species”.
The influences that led Darwin to develop his principle of Natural Selection can be found in his readings while in school at the University of Edinburgh. He was a great fan of Lamarck, who began the idea of change by accumulation, which Darwin incorporated into his theory of Natural Selection. Lyell, whom he quotes in Origin of the Species, was another major influence, and from him came the idea that all organisms experience severe competition for survival. Another underlying principle that came from Lyell’s Principles of Geology was that the factors that induce change have not themselves changed over time. John Herschel, who said that nature is governed by laws, not Deity, in his 1831 Preliminary Discourse on the Study of Natural Philosophy, was another influence. A number of scientists and scholars such as Adam Sedgwick, William Paley, Von Humboldt, and Captain Robert Henslow, all of whom had a theory on why studying nature and science was the pursuit of religion and God, also influenced Darwin greatly, who believed in Sedgwick’s statement that “no opinion is heretical” and Henslow’s “the study of nature is a divine quest”, and therefore did not consider his theory of Natural Selection as completely heretical. Darwin never read Gregor Mendel’s studies on inheritance in plants, which would probably have helped his theory a great deal, but he nevertheless had an understanding of inheritance, no doubt from his medical training at Edinburgh. Darwin was a philosophical materialist, believing that matter is the stuff of all existence. His evolutionary theory was the first that did not involve some idea of progress, or an underlying direction in history. He did not see variation as necessarily leading to an end in progress. Natural Selection was merely local adaptation, and there was no bigger picture.