When Charles Darwin and Alfred Russell Wallace proposed their theory of evolution by natural selection, the concepts of evolution and speciation were not new. Darwin introduced The Origin with “An Historical Sketch,” in which he summarized the work of 34 previous authors who had speculated on evolution and the origin of species. What was new about Darwin and Wallace’s proposition was natural selection as the mechanism of evolutionary change. Darwin further proposed that natural selection was a unifying process that accounts for adaptation, for speciation, and hence for the diversity of life on earth. Darwin and Wallace proposed natural selection as a process that caused evolution. Adaptations are features of organisms that were shaped by this process. The modern version of Darwin and Wallace’s theory allows for other agents of evolution, such as genetic drift, migration, and mutation, but adaptation remains a product of natural selection alone. The virtue of their proposal is that it allows us to develop testable hypotheses about cause-and-effect relationships between features of the environment and presumed adaptations. Natural selection immediately became a source of controversy, although the nature of the controversy has shifted over time. First, there has been considerable debate about the definition of adaptation (e.g., Reeve and Sherman 1993). We do not wish to add to or summarize this debate because we feel that Darwin got it right the first time. Besides defining a cause-and-effect relationship between selection and adaptation, Darwin emphasized that we should not expect organisms to be perfectly adapted to their environment. In fact, this emphasis was a large component of his argument against divine creation. For example, Darwin recognized, through his experience with artificial selection, that different aspects of morphology were in some way “tied” to one another so that selection on one trait would cause correlated changes in others that were not necessarily adaptive. He also recognized that organisms were subject to constraints that might limit their ability to adapt. Finally, he argued that how organisms evolved was a function of their history, so that the response to selection on the same trait would vary among lineages. A more telling criticism considers the application of cause-and-effect reasoning to the interpretation of features of organisms as adaptations, and hence to the empirical study of adaptation.
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