When Charles Darwin and Alfred Russell Wallace proposed 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 account 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 feature 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 allow us to develop testable hypotheses about cause-and-effect relationships between feature of the environment and presumed adaptations.
Natural selection immediately became a source for controversy, although the nature of the controversy, although the nature of the controversy has shifted over time. First there has been considerable debate about the definition of adaptation. 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 except 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 a 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 lineage.
A more telling criticism considers the application of cause-and-effect reasoning to the interpretable as adaptive because some are simply the by-products of other adaptations. Gould and Vrba(1982) argued that an association between a trait and a function does not demonstrate that the trait evolved for that function. Instead, trait could be coopted for new functions without further adaptation. Neither of these arguments represents a radical departure form Darwin’s original concept of adaptation. They apply instead to the difficulty of understanding cause and effect in retrospect.
An adaptation is thus a feature of an organism that evolved in response to an identifiable form of natural selection. The term adaptation of actually used in two fashions. First, it is the process of change in response to natural selection as a result of selection. Second, it is the change that is seen in organisms as a result of selection. In either case, the use of the word implies that natural selection has acted on or is currently acting on the trait. The goal of our empirical study of adaptation is thus to focus on defining these cause-and-effect relationship.
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