A Gene Mutation Which Changes a Behavior Pattern

It is tempting to assume that behavior patterns have evolved in the same manner as other features of animal populations, by the selection of inheritable differences. This idea is supported by those comparisons which have been made between the behavior patterns of closely related animals, one of the best examples of which is the review by Spieth (1952) of the courtship behavior of 101 species and subspecies of the genus Drosophila. Spieth compares the degree of morphological with that of behavioral divergence and he finds that the two roughly agree so that mating behavior confirms the validity of the existing classification as presented by Patterson and Wheeler (1949). Yet it is impossible to be certain, however strong the implication about the evolution of behavior, unless one can demonstrate the existence of 'genetic' variations in behavior upon which selection could act. It is true that some gene mutations are known which affect behavior in some way. Thus one mutation in Panaxia dominula has influenced its mating 'preferences' (Sheppard, 1952), the ebony mutant in Drosophila melanogaster mates more successfully in the dark than in the light (Rendel, 1951), and there is a mutation which affects tameness in rats (Keeler and King, 1942). But the differences in the courtship behavior of the Drosophila species studied by Spieth and also by Milani (1951a, b) are of a more specific nature. Courtship is a complex behavior pattern and the variations often occur only in the relative frequency of the elements concerned, some elements occurring more often in one species than another. Sometimes elements are omitted or new ones are added and sometimes they may change in form, but the most common difference between the most closely related species concerns this difference in frequency. The investigations reviewed by Moynihan (1953, 1955) on the differences between closely related gulls and the descriptions given by Clark, Aronson and Gordon ( 1954) on the differences between the platyfish (Xiphophorus (Platypoecilus) maculatus), and the swordtail (Xiphophorus helleri), suggest changes of the same nature, and yet no single gene mutation is known which alters a behavior pattern in this way. However since considerable detailed investigation is necessary to reveal differences in frequency or patterning, this is not so very surprising. Indeed some of the mutations listed above might well have such effects; they simply have not been investigated. It seemed worth while therefore to examine more closely one example of a gene mutation affecting behavior and to ask two questions, ( 1 ) how does it bring about its effect? (this is of considerable genetical interest), (2) what part might it play in evolution? (this is of considerable evolutionary interest).