Alfred Russel Wallace : biography
In 1889, Wallace wrote the book Darwinism, which explained and defended natural selection. In it, he proposed the hypothesis that natural selection could drive the reproductive isolation of two varieties by encouraging the development of barriers against hybridisation. Thus it might contribute to the development of new species. He suggested the following scenario. When two populations of a species had diverged beyond a certain point, each adapted to particular conditions, hybrid offspring would be less well-adapted than either parent form and, at that point, natural selection will tend to eliminate the hybrids. Furthermore, under such conditions, natural selection would favour the development of barriers to hybridisation, as individuals that avoided hybrid matings would tend to have more fit offspring, and thus contribute to the reproductive isolation of the two incipient species. This idea came to be known as the Wallace effect.Slotten pp. 413–15. Wallace had suggested to Darwin that natural selection could play a role in preventing hybridization in private correspondence as early as 1868, but had not worked it out to this level of detail.Slotten p. 404. It continues to be a topic of research in evolutionary biology today, with both computer simulation and empirical results supporting its validity.
Application of theory to man, and role of teleology in evolution
In 1864, Wallace published a paper, "The Origin of Human Races and the Antiquity of Man Deduced from the Theory of ‘Natural Selection’", applying the theory to humankind. Darwin had not yet publicly addressed the subject, although Thomas Huxley had in Evidence as to Man’s Place in Nature. He explained the apparent stability of the human stock by pointing to the vast gap in cranial capacities between humans and the great apes. Unlike some other Darwinists, including Darwin himself, he did not "regard modern primitives as almost filling the gap between man and ape".
He saw the evolution of man in two stages: achieving a bipedal posture freeing the hands to carry out the dictates of the brain, and the "recognition of the human brain as a totally new factor in the history of life. Wallace was apparently the first evolutionist to recognize clearly that...with the emergence of that bodily specialization which constitutes the human brain, bodily specialization itself might be said to be outmoded." For this paper he won Darwin's praise.
Shortly afterwards, Wallace became a spiritualist. At about the same time, he began to maintain that natural selection cannot account for mathematical, artistic, or musical genius, as well as metaphysical musings, and wit and humour. He eventually said that something in "the unseen universe of Spirit" had interceded at least three times in history. The first was the creation of life from inorganic matter. The second was the introduction of consciousness in the higher animals. And the third was the generation of the higher mental faculties in mankind. He also believed that the raison d’être of the universe was the development of the human spirit.Wallace Darwinism p. 477. These views greatly disturbed Darwin, who argued that spiritual appeals were not necessary and that sexual selection could easily explain apparently non-adaptive mental phenomena. While some historians have concluded that Wallace’s belief that natural selection was insufficient to explain the development of consciousness and the human mind was directly caused by his adoption of spiritualism, other Wallace scholars have disagreed, and some maintain that Wallace never believed natural selection applied to those areas.Shermer pp. 157–60. Reaction to Wallace’s ideas on this topic among leading naturalists at the time varied. Charles Lyell endorsed Wallace’s views on human evolution rather than Darwin’s.Larson p. 100.Shermer p. 160. Wallace’s belief that human consciousness could not be entirely a product of purely material causes was shared by a number of prominent intellectuals in the late 19th and early 20th centuries.Shermer pp. 231–33. However, many, including Huxley, Hooker, and Darwin himself, were critical of Wallace.Slotten pp. 280–96. As the historian of science Michael Shermer has stated, Wallace’s views in this area were at odds with two major tenets of the emerging Darwinian philosophy, which were that evolution was not teleological (purpose driven) and that it was not anthropocentric (human centered).Shermer pp. 208–09. Much later in his life Wallace returned to these themes, that evolution suggested that the universe might have a purpose and that certain aspects of living organisms might not be explainable in terms of purely materialistic processes, in a 1909 magazine article entitled The World of Life, which he later expanded into a book of the same name; a work that Shermer said anticipated some ideas about design in nature and directed evolution that would arise from various religious traditions throughout the 20th century.