ORIGINAL SCIENTIFIC ARTICLE The Concept of Co-option: Why Evolution Often Looks Miraculous Deborah A. McLennan Published online: 24 June 2008 # Springer Science + Business Media, LLC 2008 Abstract Darwin believed that evolution generally occurred through a series of small, gradual changes. This proposal was counter-intuitive to many people because it seemed likely that “transitional” forms would not survive. Darwin, and later Cuènot, recognized that this problem was easily solved if characters that had evolved for one reason changed their function at a later time with little to no concurrent structural modification, at least initially. In other words, traits that had evolved under one set of conditions were co-opted to serve a different function under a second set of conditions. This meant that organisms carried with them in the structures of their genes, proteins, morphological, physiological, and behavioral characters the potential for rapid evolutionary change, so rapid, indeed, that the process looked miraculous and Lamarckian. In this paper, I discuss some of the paradigm examples of co-option, from genes to behavior. Keywords Co-option . Preadaptation . Exaptation . Evolution Darwin believed that evolution generally occurred through a series of small, gradual changes. Oftentimes, though, it appears that evolution has leapt over many of the intermediate forms to arrive rapidly at a later, fully formed stage. These leaps look almost miraculous in retrospect, attributable more to Lamarckian than Darwinian mecha- nisms (e.g., animals needed jaws, so they grew them). In the following essay, I shall discuss how various researchers have tried to solve the problem of these evolutionary leaps within a Darwinian framework. This shall involve traveling a long road beginning with Darwin himself and continuing through to the present day. Along the way we will encounter three terms: preadaptation, exaptation, and co- option. No other terminology is needed for the journey. The History of the Concept in Biology In chapter VI of The Origin of Species (Darwin 1872), Darwin answered objections raised by various authors to his theory of natural selection. One of the major criticisms revolved around the perceived absence of transitional stages (of characters) in either the fossil record or in living species. If natural selection really does operate through a series of small, gradual changes, then the world should be awash in transitional stages. We don’t see this in either the fossil record or in the living creatures around us, so where are they? Darwin formulated numerous answers to this critical question, one of which involved changing the function of an already existing structure. In some cases, this occurred when two organs performed the same function, thus setting the stage for selection to increase the efficiency of one, then modify the second, and now superfluous organ, for a different function. Darwin believed that this type of dynamic was “an extremely important means of transition” (Darwin 1872, p. 175) in evolution. In other cases, an organ that served a major and minor function was modified to serve the latter at the expense of the former. In both of the preceding processes, the starting conditions might be obscured and the “transitional stage” missing. So for example, who other than a good functional morphologist and ichthyologist would think that lungs started their evolutionary journey as a gas bladder, that organ deluxe Evo Edu Outreach (2008) 1:247–258 DOI 10.1007/s12052-008-0053-8 D. A. McLennan (*) The Department of Ecology and Evolutionary Biology, University of Toronto, 25 Harbord St., Toronto, ON M5S 1A1, Canada e-mail: mclennan@eeb.utoronto.ca