9629787 RIDDLE The recent appearance of extensive regions of aridlands in southwestern North America has led many to assume that pattersn of relationships among the plants and animals adapted to these regions are also recent. Yet many of the species are ancient, having persisted in scattered dry sites for many millions of years. In collaborative investigations, Brett R. Riddle and David J. Hafner, together with their collaborators, will investigate the possibility of ancient structure among the regional deserts and reconstruct the sequence of divergence of these deserts. They will examine, by means of sequence divergences in mitochondrial DNA, the genetic relationships among four groups of rodents. These rodents are currently restricted to regional warm deserts and neighboring chaparral or thornscrub habitats, and have fossil records that indicate their presence in the study area during the early development of the regional deserts, up to 5.5 million years ago. Relationships among these rodents will serve as indicators of relationships among other animals and plants in the deserts. Dr. RiddleOs portion of the collaboration will involve examination of the mitochondrial DNA of the groups of rodents, which include white-footed and pocket mice, antelope ground squirrels, and kangaroo rats. These animals will be collected during field work in the Chihuahuan, Great Basin, San Joaquin, Sonoran, and Peninsular deserts of the southwestern United States and northern Mexico, as well as the neighboring California chaparral and Sinaloan thornscrub regions, and six islands off Baja California. The island samples are expected to provide estimates of the geographic distribution of the rodents on the Baja California peninsular mainland ca. 5,000 years ago, when the islands were separated from the peninsula. Gene sequences isolated from the mitochondrial DNAs will be sequenced, and the sequences used to construct phylogenetic trees that will test the degree of relationships among the animals. The re search is expected to contribute to human understanding of how species, and the distribution of species, change over time, and especially how such changes in species may be influenced by changes in the topology and climate of the regions in which the species live.
