![]() DeCoursey for commentary on the original activity records. ![]() Figures have been provided by Miss Cheryl Hughes. S.) and the Research Committee of the University of Wisconsin Graduate School. Species of Perognathus have long been considered nocturnal however, the mechanism for exact color matching, as opposed to grayness matching, in a 1We are grateful for support provided by the National Science Foundation (Grant No. Dice (1930), Bradt (1932), and Benson (1933) have suggested that matching of pelage color to that of the substrate upon which the populations reside has evolved as a consequence of differential predation and natural selection. intermedius, occurs on light-colored rocky outcrops marginal to the valley and but a few miles distant from the lava-bed population. ater, is found, among other localities, on the black Malpais Lava Beds in the Tularosa Basin, New Mexico, while the paler form, and more usual color variety, P. The black subspecies (or color variety), P. This species has also been of particular interest to ecologists because of two principal color forms. This species is exposed regularly to great daily fluctuation in temperature, thus suggesting the possibility that temperature cycles might contribute to or replace the regulatory action of light. The question is here examined in the rock pocket mouse, Perognathus intermedius, a small heteromyid rodent restricted to the extreme arid regions of southwestern United States. In striking contrast, experimental thermal cycles have not been effective in the entrainment of endogenous circadian rhythms to the natural day. Many experiments have shown the natural 24-hr cyclic fluctuation in light intensity to be the primary factor maintaining the endogenous rhythm of an animal in proper phase with its environment (reviews in Pittendrigh, 1960, 1965). Synchronization of the internal rhythm with the external world is brought about in the animal by utilization of periodically repeated environmental factors called entraining agents, clues, synchronizers, or Zeitgeber (DeCoursey, 1960 Aschoff, 1963 Enright, 1965 Pittendrigh, 1965). Individual rhythmic frequency usually differs from the natural day cycle and must be entrained to the 24-hr cycle of the physical environment if the animal is to operate successfully in nature. Such rhythms are believed to arise in the animal endogenously without external periodic stimuli. Together, the results suggest that color variation can evolve very rapidly over small geographic scales and that gene flow can both hinder and promote local adaptation.read more read lessĪbstract: T HAS been shown that many rodent species behave rhythmically even in a constant environment (summaries or extensive examples in Rawson, 1959 Justice, 1960 Pittendrigh, 1960 DeCoursey, 1961, 1964 Aschoff, 1963 Hoffmann, 1965). Finally, we raise the possibility that, in some cases, migration between populations of pocket mice inhabiting different lava flows may be responsible for similar melanic phenotypes in different populations. At a finer geographical scale, high levels of gene flow between neighboring melanic and light populations suggest the selection acting on color must be quite strong to maintain habitat-specific phenotypic distributions. Using Mantel tests, we show that there is no correlation between color variation and mtDNA phylogeny, suggesting that pelage coloration has evolved rapidly. Analyses of mtDNA sequences from these same individuals revealed strong population structure in this species across its range, where most variation (63%) was partitioned between five geographic regions. First, we quantified variation in pelage color (n=107 mice) and habitat color (n=51 rocks) using a spectrophotometer, and showed that there was a correlation between pelage color and habitat color across 14 sampled populations (R2=0.43). Here, we investigate whether phenotypic variation in color is correlated with local environmental conditions or with phylogenetic history. Rock pocket mice, Chaeotdipus intermedius, are an ideal system in which to study intraspecific phenotypic divergence because of the extensive color variation observed within this species. Abstract: Elucidating the causes of population divergence is a central goal of evolutionary biology.
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