Natural Selection is such a deceptively simple idea that it should come as no surprise that one biologist developed it independently of Darwin, or that others may have some limited claim to credit for the theory.  The following are capsule biographies of Charles Darwin, Alfred Russel Wallace, and three 19th century biologists who have been named as having some claim to developing the theory of natural selection.

Edward Blyth (1810-1873) was an active researcher in various areas of natural history, especially ornithology.  After his efforts in business failed, he left England in 1841 to take the post of curator at the Museum of the Royal Asiatic Society of Bengal (India).  Blyth's connection with natural selection is based largely on two papers he published in the Magazine of Natural History in 1835 and 1837.  These papers appear to discuss the concept of natural selection, but purely in terms of a mechanism by which the sick, old, and "unfit" are removed from the population.  In other words, Blyth saw selection as a mechanism for the maintenance of the status quo.  In fact, Blyth was very traditional in terms of his theological and philosophical views, and he was describing the process by which, as he saw it, the characteristics of a species remained fixed.  Several authors, most notably the naturalist Loren Eiseley (1979) have implied that Darwin was influenced by Blyth.  However, both the context of Blyth's argument and the discovery of Darwin's notebooks for the period do not support this charge (Mayr, 1982; Gould, 1987).

Charles Darwin (1809-1882).  So much has been written about Darwin's life and work that any short biography is superficial.  However, he came from an upper middle class family, and, after attempts to be first a physician and then an Anglican clergyman, he was able to secure the position as a member of the expedition of the H. M. S. Beagle (1831-1836).  This gave him the opportunity to pursue his interest in natural history on a full time basis.  After his return to England, his inheritance plus the income of his wife Emma Wedgewood (she was a member of the Wedgewood family whose name is still connected with fine china and pottery) was sufficient to enable him to pursue his research and writing on a full time basis.  Even if he had never published The Origin of Species his work in other areas of biology and geology would have secured him a substantial place in the history of science.  In fact, one Darwin scholar (Michael T. Ghiselin) has argued that the entire body of Darwin's work is an integrated framework which explains and illustrates both the evolutionary process and the hypothetico-deductive method in operation
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Patrick Matthew (1790-1874) was a wealthy landowner in Scotland.  In 1831, he published a book, On Naval Timber and Aboriculture, to which he appended a set of notes which showed clearly that he understood both the concept of natural selection and its implications for evolutionary change, including the formation of new species.  Matthew himself publicly claimed that he had anticipated Darwin, but he also credited Darwin with developing the idea in more detail.  In fact, Matthew felt that natural selection was such a self-evident process that he assumed that everyone recognized its importance.  There is no evidence that Darwin was aware of Matthew's work prior to 1860, and this is not surprising given the obscure way in which it was presented
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Alfred Russel Wallace (1823-1913) is universally recognized for his formulation of the theory of natural selection, independently of Darwin, and for pursuing the ramifications of the process to their logical conclusion.  Wallace was an active field naturalist, first in England, then South America, and finally in the Malay Archipelago, where he developed the concept of natural selection.  He was convinced of the fact of evolution by 1845, and part of his motivation for research in the tropics was that he believed the process of evolution could best be appreciated in the incredible biological diversity of the tropics.  Wallace published a paper in 1855 which detailed his ideas about the process of speciation.  In 1858, not knowing of Darwin's 15 year record of unpublished work on evolution and natural selection, Wallace sent his manuscript on natural selection to Darwin for comment.  Darwin and his scientific colleagues, realizing the delicate nature of questions relating to priority and originality, decided that the best solution was the joint presentation (at a meeting of Linnaean Society of London on July 1, 1858) of Wallace's paper and excerpts of an unpublished essay that Darwin had circulated among his friends in 1844.  Wallace was not consulted about this arrangement (communications with Malaya being slow to say the least), but, in spite of the later arguments that he was denied the recognition he deserved (McKinney, 1972; Brackman, 1980), he never complained about the arrangement, and always stated both publicly and privately that Darwin deserved the lion's share of the credit.  Ironically, while Wallace was even stronger than Darwin in his belief in the relative importance of natural selection in the evolutionary process, he felt that the existence of human consciousness and moral sense were beyond the ability of any materialistic process and had been developed by some higher power - i. e., God.  While Wallace may seem to reside in Darwin's shadow when it comes to evolutionary biology, it is clear that he is one of the founders of Biogeography.  His book, The Geographical Distribution of Animals (1876) is one of the classic works in this field
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William C. Wells (1757-1817) was an English doctor who lived in South Carolina for a time.  In an 1818 work on skin color variation in humans (that was published posthumously), he described a concept like natural selection, but it was applied purely to the evolution of human skin color and resistance to tropical diseases (Gould, 1983; 1985).

Darwin appended an Historical Sketch to subsequent editions of The Origin of Species which briefly traced the history of evolution and natural selection.  It mentions the works of Blyth, Matthew, Wells, and, of course, Wallace.  However, it is clear today that, for the evidence he gathered, the clarity with which he described natural selection, and his realization of the implications of the evolutionary process, the major credit should be given to Darwin.