Natural Selection in a Population of Darwin's Finches Author(s): B. R. Grant and P. R. Grant Source: The American Naturalist , Mar., 1989, Vol. 133, No. 3 (Mar., 1989), pp. 377-393 Published by: The University of Chicago Press for The American Society of Naturalists Stable URL: https://www.jstor.org/stable/2462126 JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at https://about.jstor.org/terms The American Society of Naturalists and The University of Chicago Press are collaborating with JSTOR to digitize, preserve and extend access to The American Naturalist This content downloaded from 134.124.189.112 on Mon, 05 Feb 2024 18:58:39 +00:00 All use subject to https://about.jstor.org/terms Vol. 133, No. 3 The American Naturalist March 1989 NATURAL SELECTION IN A POPULATION OF DARWIN'S FINCHES B. R. GRANT AND P. R. GRANT Department of Biology, Princeton University, Princeton, New Jersey 08544 Submitted December 21, 1987; Accepted June 26, 1988 After surveying a large number of studies of natural selection, Endler (1986) found few in which the reason for selection was known. In this paper, we describe episodes of natural selection under natural and changing environmental conditions. Moreover, we combine functional and ecological aspects of trait variation in a population of Darwin's finches to demonstrate an interpretable fitness advantage in terms of survival to individuals with certain beak shapes. The focus of our study is a population of the large cactus finch, Geospiza conirostris, on Isla Genovesa, Galapagos. Beak size and shape were chosen for consideration because they vary substantially. Moreover, these characters have been identified as important in the evolu- tionary diversification of Darwin's ground finches, since species differ adaptively from each other in beak dimensions, whereas other traits, such as plumage coloration and behavioral characteristics associated with reproduction, are similar or identical (Orr 1945; Lack 1947; Ratcliffe and Grant 1983; P. Grant 1986). Mechanical analyses have shown that the efficiency of the beak in performing different tasks is related to its shape (Bowman 1961). A relatively short and deep beak allows a powerful crushing force to be applied at the base of the bill, whereas a relatively long and shallow bill is better suited to probing. Therefore, fitness is probably related to bill size and shape (P. Grant et al. 1976; B. Grant 1985). A correspondence between beak size and shape on the one hand and diet on the other has been shown by analysis of stomach contents (Snodgrass 1902; Bowman 1961; Downhower 1976; P. Grant and Grant 1980) and by quantitative observations in the field on the Galapagos Islands, where 13 of the species live (P. Grant et al. 1976; Abbott et al. 1977; Smith et al. 1978; P. Grant and Grant 1980; P. Grant 1981; B. Grant and Grant 1982; Schluter 1982; Schluter and Grant 1982, 1984a,b; Boag and Grant 1984; B. Grant 1985; Price 1987), and on Cocos Island, where the fourteenth species occurs (Smith and Sweatman 1976; Werner and Sherry 1987). Investigating selection requires variation in the character. Coefficients of variation for the three major bill dimensions of G. conirostris on Genovesa are relatively high, in the range of 6.1%-7.7% of the mean (P. Grant et al. 1985), and are reliably estimated because measurement errors are low (P. Grant 1983). Typical coefficients for continental passerine birds range from 3% to 5% (P. Grant Am. Nat. 1989. Vol. 133, pp. 377-393. ? 1989 by The University of Chicago. 0003-0147/89/3303-0005$02.00. All rights reserved. This content downloaded from 134.124.189.112 on Mon, 05 Feb 2024 18:58:39 +00:00 All use subject to https://about.jstor.org/terms This content downloaded from 134.124.189.112 on Mon, 05 Feb 2024 18:58:39 +00:00 All use subject to https://about.jstor.org/terms This content downloaded from 134.124.189.112 on Mon, 05 Feb 2024 18:58:39 +00:00 All use subject to https://about.jstor.org/terms This content downloaded from 134.124.189.112 on Mon, 05 Feb 2024 18:58:39 +00:00 All use subject to https://about.jstor.org/terms This content downloaded from 134.124.189.112 on Mon, 05 Feb 2024 18:58:39 +00:00 All use subject to https://about.jstor.org/terms This content downloaded from 134.124.189.112 on Mon, 05 Feb 2024 18:58:39 +00:00 All use subject to https://about.jstor.org/terms This content downloaded from 134.124.189.112 on Mon, 05 Feb 2024 18:58:39 +00:00 All use subject to https://about.jstor.org/terms This content downloaded from 134.124.189.112 on Mon, 05 Feb 2024 18:58:39 +00:00 All use subject to https://about.jstor.org/terms This content downloaded from 134.124.189.112 on Mon, 05 Feb 2024 18:58:39 +00:00 All use subject to https://about.jstor.org/terms This content downloaded from 134.124.189.112 on Mon, 05 Feb 2024 18:58:39 +00:00 All use subject to https://about.jstor.org/terms This content downloaded from 134.124.189.112 on Mon, 05 Feb 2024 18:58:39 +00:00 All use subject to https://about.jstor.org/terms This content downloaded from 134.124.189.112 on Mon, 05 Feb 2024 18:58:39 +00:00 All use subject to https://about.jstor.org/terms This content downloaded from 134.124.189.112 on Mon, 05 Feb 2024 18:58:39 +00:00 All use subject to https://about.jstor.org/terms This content downloaded from 134.124.189.112 on Mon, 05 Feb 2024 18:58:39 +00:00 All use subject to https://about.jstor.org/terms This content downloaded from 134.124.189.112 on Mon, 05 Feb 2024 18:58:39 +00:00 All use subject to https://about.jstor.org/terms This content downloaded from 134.124.189.112 on Mon, 05 Feb 2024 18:58:39 +00:00 All use subject to https://about.jstor.org/terms NATURAL SELECTION AND DARWIN'S FINCHES 393 Lefebvre, L., and B. Palameta. 1988. Mechanisms, ecology, and population diffusion of socially learned, food-finding behavior in feral pigeons. In T. R. Zentall and B. G. Galaf, Jr., eds. Social learning: psychological and biological approaches. L. Erlbaum, Hillsdale, N.J. (in press). Mitchell-Olds, T., and R. G. Shaw. 1987. Regression analysis of natural selection: statistical inference and biological interpretation. Evolution 41:1149-1161. Orr, R. T. 1945. A study of captive Galapagos finches of the genus Geospiza. Condor 47:177-201. Partridge, L. 1976a. Individual differences in feeding efficiencies and feeding preferences of captive great tits. Anim. Behav. 24:230-240. 1976b. Field and laboratory observations on the foraging and feeding techniques of blue tits (Parus caeruleus) and coal tits (P. ater) in relation to their habitats. Anim. Behav. 24:534544. Price, T. D. 1984. Sexual selection and body size, plumage and territory variables in a population of Darwin's finches. Evolution 38:327-341. 1987. Diet variation in a population of Darwin's finches. Ecology 68:1015-1028. Price, T. D., and P. R. Grant. 1984. Life history traits and natural selection for small body size in a population of Darwin's finches. Evolution 38:483-494. Price, T. D., P. R. Grant, and P. T. Boag. 1984. Genetic change in the morphological differentiation of Darwin's ground finches. Pages 49-66 in K. Wohrmann and V. Loeschcke, eds. Population biology and evolution. Springer-Verlag, Berlin. Ratcliffe, L. M., and P. R. Grant. 1983. Species recognition in Darwin's finches (Geospiza, Gould). I. Discrimination by morphological cues. Anim. Behav. 31:1139-1153. Schluter, D. 1982. Seed and patch selection by Galapagos ground finches: relation to foraging efficiency and food supply. Ecology 63:1106-1120. Schluter, D., and P. R. Grant. 1982. The distribution of Geospiza difficilis in relation to G. fiuliginosa the Galapagos Islands: tests of three hypotheses. Evolution 36:1213-1226. . 1984a. Ecological correlates of morphological evolution in a Darwin's finch, Geospiza difficilis. Evolution 38:856-869. . 1984b. Determinants of morphological patterns in communities of Darwin's finches. Am. Nat. 123:175-196. Schluter, D., and J. N. M. Smith. 1986a. Natural selection on beak and body size in the song sparrow. Evolution 40:221-232. 1986b. Genetic and phenotypic correlations in a natural population of song sparrows. Biol. J. Linn. Soc. 29:23-36. Schluter, D., T. D. Price, and P. R. Grant. 1985. Ecological character displacement in Darwin's finches. Science (Wash., D.C.) 227:1056-1059. Simpson, G. G. 1944. Tempo and mode in evolution. Columbia University Press, New York. Smith, J. N. M., and H. P. A. Sweatman. 1976. Feeding habits and morphological variation in Cocos finches. Condor 78:244-248. Smith, J. N. M., P. R. Grant, B. R. Grant, I. Abbott, and L. K. Abbott. 1978. Seasonal variation in feeding habits of Darwin's ground finches. Ecology 59:1137-1150. Snodgrass, R. E. 1902. The relation of the food to the size and shape of the bill in the Galapagos genus Geospiza. Auk 19:367-381. Werner, T. K., and T. W. Sherry. 1987. Behavioral feeding specialization in Pinaroloxias inornata, the "Darwin's finch" of Cocos Island, Costa Rica. Proc. Natl. Acad. Sci. USA 84:5506-5510. Wright, S. 1988. Surfaces of selective value revisited. Am. Nat. 131:115-123. This content downloaded from 134.124.189.112 on Mon, 05 Feb 2024 18:58:39 +00:00 All use subject to https://about.jstor.org/terms
