Evohaion, 43(6),1989, pp. 1273-1284 INBREEDING IN DARWIN'S MEDIUM GROUND FINCHES (GEOSPIZA FORTIS) H, LISLE GIBBS'" Museum ofZoology and Department ofBiology, The University ofMichigan, Ann Arbor, MI48109-1079 AND PETER R. GRANT Department ofBiology, Princeton University, Princeton, NJ 08544-1008 Abstract. - We studied the frequency and causes of inbreeding and its effect on reproductive success in a population of Darwin's Medium Ground Finches iGeospiza fortis) on Isla Daphne Major, Galapagos, during four breeding seasons (1981, 1983, 1984, and 1987). Pedigree analysis showed that levels of inbreeding were low but comparable with those observed in other passerine birds. For pairs with at least half of their grandparents known, approximately 20% of all pairings were between detectably related birds. The frequency ofpairings between closely related birds (coefficient of kinship [</>] ~ 0.250) among all pairs was 0.6%. We detected no effect of inbreeding on repro­ ductive success, although sample sizes were small. The observed reproductive output of related pairs was not significantly different from the output ofunrelated pairs, and there was no correlation between a pair's kinship coefficient and an estimate of the potential magnitude of inbreeding depression. Comparisons with a study ofGreat Tits (Parus major) by van Noordwijk and Scharloo (1981) suggest that, even if present, the fitness costs of inbreeding in this population of G. fortis would be low. Observed levels of inbreeding in each breeding episode were accurately predicted by simulations of random mating in which relatedness had no influence on pairing between in­ dividuals. This result suggests that levels ofinbreeding in this population are determined more by demographic factors than by behavioral avoidance of mating with kin. Received June I, 1988. Accepted March 24,1989 It has often been argued that inbreeding direct estimates of inbreeding and the has important evolutionary consequences mechanisms responsible can only be ob­ in vertebrate populations (Mayr, 1963; tained from long-term studies of marked Wright, 1969). For example, levels of in­ individuals (e.g., studies on the Great Tit breeding can influence the overall amount [Parus major]: van Noordwijk and Scharloo of genetic variation present within popu­ [1981]; van Noordwijk et at [1985]; van lations and the degree ofkinship among in­ Tienderen and van Noordwijk [1988]). dividuals, while the negative effects of in­ However, the following key questions about breeding on fitness may play an important inbreeding remain largely unanswered. 1) role in the evolution of a wide variety of How frequently does inbreeding occur? 2) behaviors, including patterns ofmate choice What is the effect, if any, of inbreeding on (Bateson, 1978) and sex-specific patterns of reproductive success? 3) What is the relative dispersal within species (Greenwood and importance of factors such as dispersal, Harvey, 1982). population structure, and behavioral avoid­ Inbreeding is generally defined as mating ance ofmating with kin in determining levels between related individuals, with kinship of inbreeding? judged from some arbitrarily defined ref­ Population-level estimates ofinbreeding, erence population or pedigree (see Shields inferred from pedigrees, have been calcu­ [1982] and Ralls et at [1986] for recent dis­ lated for several bird species (Bulmer, 1973; cussions). Despite its potential importance, Greenwood et at, 1978; van Noordwijk and inbreeding has been rarely studied in nat­ Scharloo, 1981; Payne et at, 1985; Ralls et ural populations of birds, mainly because at, 1986; Rowley et at, 1986; Craig and Jamieson, 1988). Most studies, however, have only estimated levels of inbreeding I Present address: Department of Biology, Queen's University, Kingston, ON K7L 3N6, Canada. from matings between close relatives (par­ 2 To whom all correspondence should be addressed. ents and offspring or siblings breeding with 1273 1274 H. L. GIBBS AND P. R. GRANT each other). As Templeton (1987) suggests, ulation size, degree of population subdivi­ and van Noordwijk and Scharloo's (1981) sion, and variance in reproductive success study ofinbreeding in Great Tits (Parus ma­ among individuals. jor) demonstrates, matings between more The purpose ofour study was to examine distant relatives can make an important the level, causes, and fitness consequences contribution to observed levels of inbreed­ of inbreeding in a population of Darwin's ing and should be incorporated into esti­ Medium Ground Finches iGeospiza fortis) mates of the overall frequency of inbreed­ on Isla Daphne Major, Galapagos. This ing. population has been intensively studied Studies that examine the fitness conse­ since 1976 (see Boag and Grant, 1984; Price, quences ofinbreeding in wild birds are even 1985; Gibbs and Grant, 1987a), with the rarer. Inbreeding in domesticated or captive result that practically all adults in the pop­ birds generally has a negative effect on com­ ulation have been individually marked since ponents of fitness such as egg hatchability 1978. Thus, detailed pedigrees and life-his­ and production (Shoffner, 1948; Lerner, tory information are available for many in­ 1954; Sittmann et aI., 1966; but see Daniell dividuals. The specific goals of the study and Murray [1986]). However, the effects of were to I) measure the level of inbreeding, inbreeding on reproductive success in wild using a pedigree analysis, 2) determine birds vary. Hatching success is negatively whether there was an effect of pair-relat­ correlated with pair relatedness in Great Tits edness on reproductive success, and 3) iden­ (van Noordwijk and Scharloo, 1981; see also tify the factors responsible for the observed Greenwood et al. [1978]). In contrast, highly level of inbreeding in the population. inbred pairs ofSplendid Fairy Wrens (Mal­ urus splendens) show no decrease in success compared with less inbred birds (Payne et MATERIALS AND METHODS aI., 1985; Rowley et aI., 1986). Clearly, more Finch Populations studies are needed to determine whether in­ The population of Darwin's Medium breeding usually is associated with a fitness Ground Finches (Geospizafortis) has been cost. studied intensively on Isla Daphne Major, Moreover, the relative importance ofbe­ Galapagos, for the first 6-9 months ofeach havioral avoidance and population struc­ year from 1976 until the present (Boag and ture in generating observed levels of in­ Grant, 1984; Price, 1985; Gibbs and Grant, breeding in wild bird populations is largely 1987a). Isla Daphne Major (henceforth, unknown. There is indirect evidence, based "Daphne") is a small island «40 ha) sit­ on dispersal patterns, that individuals in uated in the middle of the Galapagos ar­ some species avoid mating with close kin chipelago. A description ofits geological and (Koenig and Pitelka, 1979). Ralls et ai. floristic characteristics is given in Boag and (1986) suggest that the low percentage of Grant (1984). Over 95% of all G. fortis in­ matings observed between close kin in a dividuals in the population have been band­ number ofpopulations implies that behav­ ed with a metal band and a unique com­ ioral avoidance of mating with close rela­ bination ofthree colored plastic bands since tives is common among birds. These stud­ 1978. Due to the demography of the pop­ ies, however, did not attempt to compare ulation, the 1981 breeding season was the observed levels ofinbreeding with those ex­ first in which matings between related birds pected ifbirds were mating at random with could be detected. Owing to annual varia­ respect to relatedness. This comparison has tion in rainfall, birds do not breed in every only been attempted for a single population year. Here, we present data on inbreeding of Great Tits (van Noordwijk et aI., 1985; in four years (1981, 1983, 1984, and 1987) van Tienderen and van Noordwijk, 1988), when enough reproduction occurred for us and the results demonstrated that pairing to identify breeding pairs reliably and to was random with respect to relatedness. This obtain information on their reproductive finding suggests that the level ofinbreeding success. in this population is determined mainly by Finches breed in response to rain, which aspects ofpopulation structure such as pop- can fall any time during December-July. INBREEDING IN FINCHES 1275 During each of the four breeding seasons are based on calculations involving effective reported here, all nests were marked, and population size (see Crow and Kimura, the parents were identified. Nest contents 1970): 1) the demographic data needed to were regularly checked to determine clutch estimate N; are extensive and difficult to size, hatching, and fledging success. We are obtain for natural populations; 2) changes confident that more than 95% of all nests in our knowledge of ancestry in the popu­ were found, because repeated systematic lation, which strongly influence detected searches ofall potential nest sites were con­ levels of inbreeding, can be incorporated ducted and because fledglings without bands into our estimates of expected levels of in­ were rarely seen. Young birds that were alive breeding; and 3) with simulations, we were in January ofthe year following the season able to generate a distribution of expected of their birth were recorded and identified values ofkinship under random mating and, during repeated and standardized visual hence, to obtain a plausible estimate of the censuses and other observations (Boag and variance of the distribution. Grant, 1984; Gibbs and Grant,