Darwin, Fisher, and a theory of the evolution of the sex ratio

I wrote the first version of this note before I was aware of a passage in Bul-mer’s (1994) book that points out the main fact that I discuss. The remaining content of the note is a report of the results of my search for the reason Darwin changed his mind. Summary: A theory of the evolution of the sex ratio, universally attributed to Fisher (The genetical theory of natural selection, 1930) is contained in the first edition of Darwin’s Descent of man (1871). I raise the question of why Darwin eliminated the theory from the second edition. In his book The genetical theory of natural selection, R. A. Fisher proposes a simple, and now well known, theory of the evolution of the sex ratio (1930, 141–143), a theory that is the foundation of much subsequent work. Before presenting the theory, he quotes a paragraph from Darwin’s Descent of man in which Darwin writes that he cannot see how a 1:1 sex ratio could be the result of natural selection. Darwin’s paragraph concludes “I formerly thought that when a tendency to produce the two sexes in equal numbers was advantageous to the species, it would follow from natural selection, but I now see that the whole problem is so intricate that it is safer to leave its solution for the future. ” As Bulmer (1994, 207–208) has pointed out, the paragraph occurs in the second edition of Darwin’s book (1874, 252–256). Neither Fisher, nor any scholar prior to Bulmer, appears to have noticed that the section in which it occurs replaces a section in the first edition that contains the essence of Fisher’s theory!2 I quote from Darwin (1871, 316): 1I am grateful to Vijay Krishna for encouragement and helpful discussions, to John Maynard Smith for encouragement and illuminating correspondence (including a pointer to Bulmer’s book), and to the Social Sciences and Humanities Research Council of Canada fo