Endler_1980 - E volution, 34(1), 1980, p p i 6-91 N ATURAL...

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Evolution, 34(1), 1980, pp i6-91 NATURAL SELECTION ON COLOR PATTERNS IN POECILIA RETICULATA JOHN A. ENDLER' Departmeizt of Biology, Priizceton University, Princeton, S.J. 08.540 Received September 14, 1978. Revised March 28, 1979 All too often in evolutionary biology we are led to speculate or infer the mode of action of natural selection; we usually do not know why some individuals are more adaptive than others. Very often attempts to measure natural selection are unsuc- cessful, leading to heated arguments about the relative importance of selection, ge- netic drift, and epistasis in evolution (Le- wontin, 1974). Until we know more about how and why natural selection occurs, at- tempts to measure it are quixotic, and dis- cussions of its importance are theandric. It is no coincidence that most of the suc- cessful studies of natural selection have dealt with animal color patterns; it should be obvious which color patterns are more adaptive in the presence of visually hunt- ing predators. The adaptive significance of warning coloration and mimicry of dis- tasteful species has been worked out (Cott, 1940; Wickler, 1968; Edmunds, 1974; Rothschild, 1975; Turner, 1977). But most species are neither distasteful nor mimetic; most have inconspicuous or cryptic color patterns in their natural habitats (Poulton, 1890; Thayer, 1909; Cott, 1940; Endler, 1978). Most field and experimental studies have shown that the overall color or tone of inconspicuous species matches or ap- proximates the background (DiCesnola, 1904; Sumner, 1934, 1935; Isley, 1938; Popham, 1942; Dice, 1947; Kettlewell, 1956, 1973; Turner, 1961; Kaufman, 1974; Wicklund, 1975; Curio, 1976), but they treated species with solid colors or ' Present address: Department of Biology, Uni- versity of Utah, Salt Lake City, Utah 84112. have ignored color pattern. A color pat- tern can be regarded as a mosaic of col- ored spots or patches of various sizes, colors and shapes. Some striking examples of color pattern polymorphisms remain largely unexplained, for example in Par- tula and Cepaea (Clarke and Murray, 1971; Jones et al., 1977). In three fish species, the frequency of a color pattern element is correlated with the presence or absence of visually directed predation: Gasterosteus aculeatus (McPhail, 1969; Semler, 1971; Moodie, 1972), Nothobran- chius guntheri (Haas, 1976a, b), and Cichlasoma citrinellum (Barlow, 1976; Barlow and Ballin, 1976; McKaye and Barlow, 1976). But these simple pattern polymorphisms tell us very little about the factors which determine color patterns as a whole. The purpose of this paper is to show how various aspects of color pat- terns are moulded by natural selection Guppies (Poecilia reticulata Peters) show a complex color pattern polymor- phism which varies with predation pres- sure, and are excellent for a study of nat- ural selection on a complex character (Endler, 1978). They are native to the mountain forest streams of northeastern Venezuela, Margarita, Trinidad, and To- bago (Rosen and Bailey, 1963; Endler, 1978). Natural populations are highly polymorphic to the extent that no two in- dividuals are alike. The patterns consist of a mosaic of spots or patches varying in color, size, position, and reflectivity, and are controlled by many X and 'k' linked genes. The color genes are expressed only
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Endler_1980 - E volution, 34(1), 1980, p p i 6-91 N ATURAL...

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