This difference is controlled in large part by the interaction of two proteins, the melanocortin-1-receptor (MC1R) and the agouti-signaling protein ( 12– 14). A key distinction in melanogenesis is between the production of eumelanin (brown or black pigment) and pheomelanin (yellow or red pigment). Approximately 80 genes have been identified that affect coat color in the laboratory mouse, and more than one-quarter of these have been molecularly characterized ( 11). This system is amenable to genetic analysis because the biochemistry, development, and genetics of the pigmentation process have been intensively studied in other mammals. This view is strengthened by the strong correlation between substrate color and coat color across 18 different populations in Arizona and New Mexico, ranging from very light to very dark ( 9). intermedius, and that differences in coat color are an adaptation for crypsis. Thus, it is likely that owls exert strong selection on coat color in C. Owls are common predators of these mice, and experiments by Dice ( 10) on deer mice showed that owls can effectively discriminate between light and dark mice even at night. The close match between the color of the mice and the color of the substrate on which they live is thought to be an adaptation against predation ( 8, 9).
Most of the lava flows are surrounded by light-colored substrate and are isolated from one another by hundreds of kilometers, raising the possibility that melanic mice have evolved independently on different lava flows. The mice from these lava sites are typically melanic, with dark-colored dorsal hairs and white underbellies. In several different regions, however, these mice are found on lava flows. In most places, these mice have a sandy dorsal pelage and white underbelly, and they inhabit light-colored rocks. Classic studies ( 8, 9) in the 1930s revealed a strong correlation between the color of the dorsal pelage and the color of the substrate on which C. This species is found in rocky habitats in southern Arizona, New Mexico, and in adjacent areas in northern Mexico.
The rock pocket mouse, Chaetodipus intermedius, provides a useful system for studying the genetics of adaptation. Most involve either biochemical polymorphisms ( 4– 6) or response to human disturbance, such as heavy metal tolerance in plants, insecticide resistance, warfarin resistance in rats, or antibiotic resistance ( 7), and in many cases, the specific nucleotide changes have not been identified. Because of these difficulties, the molecular basis for adaptation is known in only a handful of cases. Finally, many fitness-related traits are quantitative and are unlikely to have a simple genetic basis. Yet, after more than a half-century of study, the genes responsible for these color differences remain unknown ( 3). For example, one of the best known cases of adaptation involves color morphs of the peppered moth, Biston betularia. Second, phenotypic variation of ecological relevance has often been studied in species for which we have little genetic information, making the genetic basis of the traits difficult to analyze. First, it requires that we identify traits that are ecologically important and that we have some understanding of how these traits affect fitness in different environments. Finding the genes underlying adaptation has been difficult for a number of reasons. A key problem in evolutionary biology is to connect genotype with phenotype for fitness-related traits ( 1, 2).
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