Essays on Evolution. III. On the Origin of Interspecific Sterility

III. ON THE ORIGIN OF INTERSPECIFIC STERILITY BY A. H. STURTEVANT W. G. Kerckhoff Laboratories of the Biological Sciences, California Institute of Technology, Pasadena, Calif. T HE problem of the origin of interspecific sterility has long presented the greatest difficulty in the way of constructing a complete theory of the evolution of species. This difficulty was felt by Darwin, and has, if anything, become more striking with the development of modern genetical theory. One solution, has, it is true, been developed. It now seems clear that, in certain cases, allopolyploidy may be effective in producing a new fertile form that is cross-sterile with its parents. This solution is, however, certainly a special one, applicable only to hermaphroditic organisms. There is no recognized general scheme, applicable to separate-sexed forms, that can be pictured in detail. It is the purpose of the present note to suggest such a general scheme. It was pointed out by Fisher (I930) that, if a species is supposed to be broken up into two more or less isolated groups, and the hybrids between these two groups are relatively infertile, then any genes tending to decrease the cross-mating between the two groups will be selected. This is because cross-mating, since it leads to the production of less fertile offspring, will lead to a decrease in the potential ultimate reproductive value of the cross-mated individuals, as compared to those that mate within their own group. In other words, intra-group matings will, if we consider generations later than the first one, produce more descendants than will inter-group matings-and selection will therefore decrease the proportion of the latter. On this basis, sterility of the F1 comes first, and leads to an accumulation of genes that prevent cross-mating. The alternative assumption, more often made, is that cross-mating is prevented in some manner, and sterility of the F1 follows as a more or less incidental consequence. This latter view has always remained rather vague, and presents serious logical difficulties when one attempts to make it more precise. There seems to be no good reason why the hybrid should become sterile, yet sterility of the hybrid is one of the most wide-spread characteristics of distinct species. There is also a logical difficulty about the view that sterility of the F1 is primary. For, in this case, there must have been a fertile F1 present in the beginning. If A and B give a sterile hybrid, how can B be derived from A, unless self-fertilization be assumed? There are two ways of escaping this difficulty: One may suppose the hybrid not to be completely sterile (in which case, as the sterility is decreased, it becomes easier for the condition to become established, but its effectiveness in leading to failure of cross-mating decreases), or one may assume that the sterility arose by two steps. In the latter case, one may assume two pairs of complementary genes, Ss and Tt, such that Ss Tt is sterile, but SS tt, Ss tt. ss TT. ss Tt. and ss tt are