Biology and Philosophy 13: 359–391, 1998. c 1998 Kluwer Academic Publishers. Printed in the Netherlands. The Tragedy of a priori Selectionism: Dennett and Gould on Adaptationism JEREMY C. AHOUSE Howard Hughes Medical Institute University of Wisconsin R.M. Bock Laboratories 1525 Linden Drive Madison, WI 53706 U.S.A. Abstract. In his recent book on Darwinism, Daniel Dennett has offered up a species of a priori selectionism that he calls algorithmic. He used this view to challenge a number of positions advocated by Stephen J. Gould. I examine his algorithmic conception, review his unqualified enthusiasm for the a priori selectionist position, challenge Dennett’s main metaphors (cranes vs. skyhooks and a design space), examine ways in which his position has lead him to misun- derstand or misrepresent Gould (spandrels, exaptation, punctuated equilibrium, contingency and disparity), and discuss recent results in developmental biology that suggest that an a priori position does not fill the demands of an evolutionary biology. I conclude by insisting that evolutionary biology is many leveled, complicated, and is carried on an ever shifting and expanding empirical base that when disregarded results in caricature. Key words: adaptation, algorithm, atavism, contingency, deep homology, Dennett, develop- ment, disparity, epicurean selectionism, evolution, exaptation, Gould, metaphors, punctuated equilibrium, selectionism, spandrels From the early days of selectionist explanations there have been those who found selectionist reasoning obvious. Patrick Matthew who is credited by Darwin as anticipating his own and Wallace’s view, writes; To me the conception of this Law of nature came intuitively as a self- evident fact, almost without effort of concentrated thought. Mr. Darwin here seems to have more merit in the discovery than I have had; to me it did not appear a discovery. He seems to have worked it out by inductive reason, slowly with due caution to have made his way synthetically from fact to fact onwards; while with me it was by a general glance at the scheme of Nature that I estimated this select production of species as an a priori recognizable fact – an axiom requiring only to be pointed out to be admitted by unprejudiced minds of sufficient grasp. 1 Recently, Daniel Dennett has written an account of evolutionary theory that is part of this tradition (Dennett 1995). He calls his book Darwin’s Dangerous