AN EVOLUTIONARY STUDY OF ISLAND FAUNAS IN THE GULF OF CALIFORNIA, MEXICO, WITH A METHOD FOR COMPARATIVE ANALYSIS

Abstract

The potentiality for rapid divergent evolution in small populations has been much discussed in the several recent and well-known works treating evolutionary mechanisms, viz., by Th. Dobzhansky, E. Mayr, J. Huxley, G. G. Simpson, G. L. Stebbins, and others. Mayr (1942: 234237) has synthesized data for both the invertebrates and vertebrates in which correlations have been established between the size of isolated islands and the variation and differentiation of the island populations. More divergent and usually more homogeneous populations are known to occur on certain smaller islands, and elsewhere where the size of the effective breeding population is relatively small. A theoretical basis for this has been shown by Wright (1931, 1948 and elsewhere), and others, in the relationship of random and directed gene changes to the size of the effective breeding population. too small a population there is nearly complete fixation, little variation, little effect of selection and thus a static condition, modified occasionally by chance fixation of rare mutations, leading inevitably to degeneration and extinction (Wright, 1931). The herpetofaunae and mammalian faunae of Isla Tiburon and its smaller satellite islands in the Gulf of California has become sufficiently well known to permit a preliminary evolutionary analysis and to warrant certain speculations. In figure 1 (map) the spatial relations of Isla Tiburon, Isla de San Esteban, and Isla Turners 1 are shown. The present analysis is restricted to that of the reptilian and mammalian land species occurring on these three islands; additional comments are made on the lesser known avifauna of this area. The numbers of endemic species and subspecies of the reptiles and mammals of these islands as presently known are given in table 1. For the mouse Peromyscus eremicus tiburonensis Mearns, the opinion of Osgood (1909) rather than that of Burt (1938) has been followed, and this population has not been recognized herein as a subspecies distinct from mainland populations of Sonora. For the remainder of the mammals treated the systematic conclusions of Burt (op. cit.) have been followed with full realization that one or more of these species may in reality be especially well marked sub-