Centripetal Selection and the House Sparrow

Grant, P. R. (Dept. of Biology, McGill Univ., Montreal, Canada) 1972. Centripetal selection and the House Sparrow. Syst. Zool. , 21:23-30. -In 1898 H. C. Bumpus compared morphological measurements of House Sparrows which survived a severe storm and those which did not. He observed that the group of birds which survived had fewer extreme measurements than did those which died, and suggested that selection had eliminated extreme individuals. The data, given in full by Bumpus, have been analyzed statistically in the present study. The results support the suggestion made by Bumpus. It is found that among females the survivors varied significantly less than did those which died, in four measurements; weight, length of humerus, length of tibiotarsus and length of sternum. There are no significant differences in variation in any measurement among the males. Femalessuffered proportionately heavier mortality than the males. It is not possible to identify the target of selection because the four dimensions involved are strongly correlated with each other. It is suggested that females suffered particularly heavy mortality because they were at least partly denied access to food by the socially dominant males. It is also suggested that the extreme individuals were selected against in the storm because the largest birds were unable to mobilize energy reserves rapidly enough, and because the smallest birds exhausted their energy reserves -and were unable to replace them. [Selection; House Sparrows; Bumpus. ] The House Sparrow, or English Sparrow (Passer domesticus), has undergone adaptive change in North America. Following the original introduction from Europe in 1851 and additional introductions thereafter (Calhoun, 1947), it spread rapidly throughout most of North America, and has become geographically differentiated (Calhoun, 1947; Johnston & Selander, 1964, 1971; Lack, 1940; Packard, 1967, 1968; Selander & Johnston, 1967). In several size and colour characteristics it shows a pattern of variation which can be related to environmental variables. The colour shifts may be purely phenotypic responses, since it is known that feather pigment is under the direct influence of such environmental factors as humidity (Beebe, 1907). Likewise the size of some exposed parts is influenced by temperature (Allee and Lutherman, 1940). But certain changes in the proportions of exposed parts, and hence shape, cannot so easily be explained as phenotypic responses, and it is therefore reasonable to conclude, as did Johnston & Selander (1964, 1971), that genetic changes have occurred in the North American environment as a result of directional selection. WVith the exception of Calhoun (1947), these authors have paid little attention to the fact that more than 70 years ago Bumpus (1899) presented evidence in support of the process of natural selection acting on this species. There has been debate on the validity of this evidence (e.g., Calhoun, 1947; Harris, 1911; Robson and Richards, 1936). In view of the current interest in the patterns of geographical variation in this species which might have been produced by selection, it is appropriate to re-examine the evidence given by Bumpus, making 'use of statistical procedures not available to. 'him.