Purging of inbreeding depression and fitness ... - Wiley Online Library

Purging of inbreeding depression and ®tness decline in bottlenecked populations of Drosophila melanogaster P. S. MILLER* & P. W. HEDRICK *Conservation Breeding Specialist Group, 12101 Johnny Cake Ridge Road, Apple Valley, MN 55124±8151, USA Department of Biology, Arizona State University, Tempe, AZ 85287±1501, USA

Keywords:

Abstract

bottleneck; conservation; Drosophila; founder-¯ush; inbreeding depression; purging.

Drastic reductions in population size, or bottlenecks, are thought to signi®cantly erode genetic variability and reduce ®tness. However, it has been suggested that a population can be purged of the genetic load responsible for reduced ®tness when subjected to bottlenecks. To investigate this phenomenon, we put a number of Drosophila melanogaster isofemale lines known to differ in inbreeding depression through four `founder-¯ush' bottleneck cycles with ¯ush sizes of 5 or 100 pairs and assayed for relative ®tness (single-pair productivity) after each cycle. Following the founder-¯ush phase, the isofemale lines, with a large ¯ush size and a history of inbreeding depression, recovered most of the ®tness lost from early inbreeding, consistent with purging. The same isofemale lines, with a small ¯ush size, did not regain ®tness, consistent with the greater effect of genetic drift in these isofemale lines. On the other hand, the isofemale lines that did not show initial inbreeding depression declined in ®tness after repeated bottlenecks, independent of the ¯ush size. These results suggest that the nature of genetic variation in ®tness may greatly in¯uence the way in which populations respond to bottlenecks and that stochastic processes play an important role. Consequently, an attempt intentionally to purge a population of detrimental variation through inbreeding appears to be a risky strategy, particularly in the genetic management of endangered species.

Introduction When a population passes through a generation with a small number of individuals, called a bottleneck, traditionally it has been thought that genetic drift results in a reduction in the overall heterozygosity, the number of different alleles, and the amount of additive genetic variation (James, 1971; Nei et al., 1975). For example, the low level of polymorphism observed for some genetic markers in cheetahs (O'Brien et al., 1983, 1985; Hedrick, 1996) and northern elephant seals (Bonnell & Selander, 1974; Hoelzel et al., 1993; Hedrick, 1995; Weber et al., 2000) has been attributed to bottlenecks in these species. The loss of genetic variation is also thought to have Correspondence: Dr P. W. Hedrick, Department of Biology, Arizona State University, Tempe, AZ 85287±1501, USA. Tel.: +1 480 965 0799; fax: +1 480 965 2519; e-mail: [email protected]

J. EVOL. BIOL. 14 (2001) 595±601 ã 2001 BLACKWELL SCIENCE LTD

negative consequences for population ®tness, both by ®xing detrimental variants and reducing the genetic variation available for future adaptation. On the other hand, bottlenecks have been suggested as a potential mechanism for the reorganization of genetic variation that could result in speciation (Carson & Templeton, 1984; however, see Barton & Charlesworth, 1984; Galiana et al., 1993). In addition, some experimental (Bryant et al., 1986; Carson & Wisotzkey, 1989; LoÂpezFanjul & Villaverde, 1989; Fernandez et al., 1995) and theoretical (Robertson, 1952; Goodnight, 1987; Cockerham & Tachida, 1988; Willis & Orr, 1993) studies have suggested that bottlenecks may, in some instances, lead to increased additive genetic variance. However, generally in such cases the bottlenecks also result in reduction of mean ®tness from the level found before the bottleneck and the conditions for an increase in additive genetic variance are quite restrictive (LoÂpez-Fanjul et al., 1999).

595

596

P. S. MILLER AND P. W. HEDRICK

Fitness is reduced in bottlenecked populations through random changes in frequencies for alleles in¯uencing ®tness. It has been suggested (Lande & Schemske, 1985) that populations may experience a smaller reduction in ®tness over time. In other words, the set of detrimental alleles primarily responsible for lowered ®tness from inbreeding or bottlenecks may be effectively purged by the combination of genetic drift and/or inbreeding and selection. For example, Templeton & Read (1983, 1984) suggested that the initial inbreeding depression for survival in a captive population of Speke's gazelle (Gazella spekei) was reduced in animals whose parents were inbred (however, see Kalinowski et al., 2000), Bryant et al. (1990) reported that early inbreeding depression within serially bottlenecked isofemale lines of the house¯y (Musca domestica) was largely ameliorated after later bottlenecks, and Saccheri et al. (1996) observed that a low hatching rate in a butter¯y (Bicyclus anynana) caused by inbreeding was generally eliminated in later generations. However, Willis (1999) has recently suggested that it is dif®cult in laboratory studies to distinguish between purging of detrimental variants and adaptation unless the ®tness of inbred individuals at the end of the experiment is evaluated. The increase in subsequent ®tness as the result of past inbreeding is not always apparent. For example, Ballou (1997) examined 25 captive species and found little effect of past inbreeding on ®tness. A review of the plant literature by Byers & Waller (1999) found little evidence for purging among 52 studies. Moreover, the ability to purge a population of its detrimental variation appears highly dependent on the nature of that variation (Hedrick, 1994; Charlesworth & Charlesworth, 1999; Wang et al., 1999; Kirkpatrick & Jarne, 2000). Detrimental variation consisting primarily of recessive lethals is much more easily eliminated than that due to detrimentals with higher levels of dominance and lower effects. In this paper, we report the results of experiments designed to investigate the effect of multiple bottlenecks on ®tness in populations of Drosophila melanogaster. The general experimental protocol is similar to founder-¯ush studies of speciation (reviewed by Galiana et al., 1989; see also Galiana et al., 1993; LoÂpez Bueno et al., 1993) in which bottlenecks (founder events) are alternated over time with generations exhibiting larger population numbers (¯ush periods). To gain insight into evolutionary dynamics and potential of the process, we examined populations that previously had either shown inbreeding depression or those that had not.

Materials and methods The isofemale lines used in this study were collected in Miami, Florida, by M. Tracey and were maintained in captivity for a minimal period of time before the initiation of the experiment. A population from south Florida was chosen because D. melanogaster populations

in this area remain large throughout the year, with an associated high amount of detrimental genetic variation (Lewontin, 1974). A given isofemale line is established from the progeny of a single wild-caught female and includes at least four wild genomes (two from the female, two from a male) but may include more variation if the female was mated more than once. Once established, these isofemale lines were maintained at high numbers so little further inbreeding occurred. Examining different isofemale lines allows documentation of variation in a natural population for speci®c genetic characteristics (Hoffman & Parsons, 1988) without the problems associated with using long-term inbred lines. These isofemale lines and all experimental cultures were maintained in controlled conditions in environmental chambers with a 12-h photoperiod at 25 1 °C under constant humidity (65 5% RH) with the media consisting of measured amounts of yeasted Carolina Instant Medium. Founder-¯ush protocol Nineteen isofemale lines were initially subjected to three generations of single-pair full-sib mating (generations FS-1 to FS-3) in 8-dram vials, resulting in an inbreeding coef®cient in the ®nal set of progeny equal to f 0.50. Outbred controls were constructed by crossing virgin adults between the isofemale lines. More speci®cally, virgin adults of both sexes were randomly chosen and mated between the 19 isofemale lines for each generation of the three inbred generations, f 0.25, 0.375 and 0.5. For each of these generations, the control productivity (see below) was measured as the mean productivity of these crosses. Following the ®nal generation of inbreeding, 10 inbred isofemale lines were chosen for the founder-¯ush phase of the experiment: ®ve isofemale lines that displayed considerable inbreeding depression relative to the control, and ®ve isofemale lines showing comparatively little inbreeding depression relative to the control. This strategy allowed us to investigate the genetics of founder-¯ush dynamics in isofemale lines that differed substantially in their initial inbreeding depression. In particular, this protocol allowed us to examine both whether there was any improvement in ®tness in those isofemale lines initially exhibiting inbreeding depression and whether isofemale lines initially having high ®tness maintained their ®tness. For each of these 10 isofemale lines, two replicate halfpint culture bottles were set up for each of two ¯ush treatments (40 total bottles). For both ¯ush treatments, ®ve pairs of adults were placed in each bottle on day 0, with adults removed three days later. On day 14, for the ®rst ¯ush treatment, called ¯ush-5, ®ve pairs were transferred to bottles containing new medium. For the second ¯ush treatment, called ¯ush-100, the ®rst ¯ush generation was the same as ¯ush-5 while for the second


Purging of inbreeding depression

597

Fig. 1 Actual population sizes throughout the course of the experiment for the ¯ush-5 and ¯ush-100 isofemale lines. Fitness assays were conducted during the single-pair bottlenecks.

generation, newly emerged adults up to a maximum of 100 pairs were similarly transferred to new bottles in each of the ¯ush-100 lines. Each set of treatment bottles was maintained in this way for 7±10 generations, when a single-pair bottleneck was imposed on all lines (see Fig. 1). Following each bottleneck, the isofemale lines were again allowed to ¯ush to their appropriate population sizes. All isofemale lines were taken through a total of four founder-¯ush cycles, designated FF-1 to FF-4, with the entire experiment encompassing 38 generations (76 weeks). The two ¯ush treatments were intended to investigate the difference between a large ¯ush size in which there is little potential for ®nite population size effects but opportunity for recombination and selection, and a low ¯ush size in which genetic drift would be expected to dominate genetic change. Fitness assessment During the three initial generations of full-sib mating and each of the single-pair bottlenecks of the founder-¯ush cycles, ®tness was measured as the total productivity of single pairs of adults in 8-dram vials. Productivity is a composite ®tness measure, which includes female fecundity, male-mating success, and egg-to-adult survival. One of the two replicate bottles for each founder-¯ush line was chosen at random for the ®tness measurement. Four replicate productivity vials were set up for each line during the continuous full-sib mating portion of the experiment, while ®ve replicate vials were set up for each line in the two founder-¯ush treatments. Progeny were counted until day 17 for all vials, with the ®ve pairs used to initiate the next set of ¯ush bottles chosen at random from all available progeny.


The productivity control used in the fourth founder¯ush cycle was generated by randomly choosing and mating ¯ies from the 10 different ¯ush-100 isofemale lines. We had planned to randomly choose and mate ¯ies from stocks of the 10 selected isofemale lines for a control but stocks of several of these isofemales lines were lost over the 76-week experimental period. Because all 10 selected isofemale lines were represented in the ¯ush-100 lines, we used crosses between them as a control because they provided the most complete genetic representation of the 10 selected isofemale lines available.

Results Following the ®nal generation of continuous full-sib mating (f 0.5), inbreeding depression among the 19 inbred isofemale lines was shown by analysis of variance to be statistically signi®cant (P 0.005). From these isofemale lines, the 10 most divergent were chosen to begin the founder-¯ush stage of the experiment: isofemale lines 3, 4, 12, 17 and 22 were designated as low®tness isofemale lines, and isofemale lines 7, 15, 25, 30 and 31 as high-®tness isofemale lines. Figure 2 shows the productivity data for the 10 selected isofemale lines during the continuous full-sib mating phase of the experiment. For the third generation of full-sib mating (f 0.5), the means ( standard error) for the control, low-®tness and high-®tness isofemale lines were 90.6 9.9, 42.8 12.5 and 83.3 8.1, respectively. Analysis of variance of these data showed a signi®cant effect of ®tness group (P 0.017), with the low-®tness group showing signi®cant inbreeding depression relative to the control (P < 0.05, Tukey's a posteriori

598


Fig. 2 Productivity (mean SE) for the full-sib mating portion of the study. Data for the two ®tness groups include only those nine isofemale lines used in the founder-¯ush protocol. Controls used during the experiment are also indicated.

comparison) while the high-®tness isofemale lines did not (P > 0.50). Analysis of variance also showed that the mean productivity of the high-®tness group and the control during the continuous full-sib mating phase were not signi®cantly different. Further, the control did not show signi®cant variation over time, suggesting that there were no signi®cant environmental trends or variation during this period. The productivities of the ¯ush-5 and ¯ush-100 founder-¯ush isofemale lines and the controls at the beginning and end of the experiment are shown for the low-®tness (Fig. 3A) and high-®tness (Fig. 3B) isofemale lines. For the low-®tness isofemale lines, productivity declined from the initial value of 42.8 12.5 in the ¯ush-5 treatment to 29.5 8.0 and increased in the ¯ush-100 treatment to 48.1 6.2 (line 4 was lost after the ®rst founder-¯ush cycle). For the ®nal founder-¯ush cycle, analysis of variance demonstrated that the ¯ush-5, low-®tness isofemale lines exhibited signi®cantly lower productivity compared to the control, while the ¯ush100 isofemale lines did not differ signi®cantly from the control (Table 1). Among the high-®tness isofemale lines (Fig. 3B), a fairly consistent decline in productivity was seen from the initial value of 83.8 8.1 to 31.9 3.7 and 29.3 3.7 for the ¯ush-5 and ¯ush-100 treatments, respectively. The two treatments did not differ from each other in productivity for the ®nal founder-¯ush cycle and both showed signi®cantly lower productivity compared to the simultaneous control (Table 1).

Fig. 3 Productivity (mean SE) for the (A) low-®tness and (B) high-®tness groups over the four founder-¯ush cycles. Controls at the beginning and end of the experiment are also indicated.

Table 2 presents the productivity of each line, grouped by ®tness and expressed as a proportion of their respective control values. For example, the FS-3 productivity for line 3 was 0.268 that of the control value and, using the Least Signi®cant Difference (LSD) method for multiple planned comparisons (Sokal & Rohlf, 1995), it had signi®cantly lower productivity compared to the simultaneous control. Isofemale lines 17 and 22 showed similarly low productivity in FS-3, but the differences were not quite statistically signi®cant at the P 0.05 level. None of the ®ve high-®tness isofemale lines shows signi®cantly lower productivity relative to the simultaneous control in the FS-3 generation. For the low-®tness isofemale lines in the FF-4 generation, line 3 showed signi®cantly lower productivity in



Table 1 Productivity ( SE) for the fourth and ®nal founder-¯ush cycle where N is the number of test vials in each category (®ve replicates times the number of remaining lines for the experimental categories) and P is the signi®cance level of each A N O V A comparing treatment means to the simultaneous outbred control. Low ®tness Control

Flush-5

High ®tness Flush-100

Flush-5

Flush-100

N 25 15 20 25 25 Productivity 60.1 7.2 29.5 8.0 48.1 6.2 31.9 3.7 29.3 3.7 P 0.009 0.228 0.001 0.001

Table 2 Productivity for each line expressed as a proportion of related control productivity for FS-3, the last generation of full-sib mating, and FF-4, after the last founder-¯ush cycle. Values indicated with *, ** and *** were signi®cantly lower than the associated control values at the P < 0.05, 0.01 and 0.001 levels, respectively. ± indicates loss of treatment line before the ®nal productivity assay. FF-4

(A) Low ®tness

(B) High ®tness

Line

FS-3

Flush-5

Flush-100

3 12 17 22

0.268* 0.665 0.489 0.469

0.183* 0.672 ± 0.619

0.553* 1.385 0.599* 0.666

Mean

0.473**

0.492**

0.801

7 15 25 30 31

1.084 0.966 0.927 0.883 0.739

0.503* 0.616 0.336* 0.410* 0.789

0.523* 0.739 0.599* 0.196* 0.386*

Mean

0.920

0.531***

0.510***

both the ¯ush treatments, while line 17 had signi®cantly lower productivity in the ¯ush-100 treatment. However, average proportion over all low-®tness isofemale lines was unchanged for the ¯ush-5 treatment while it was increased for all isofemale lines in the ¯ush-100 treatment with a mean increase of 69%. In marked contrast to these results, the high-®tness isofemale lines displayed much lower productivity following the FF-4 generation for both ¯ush treatments. Isofemale lines 7, 25, and 30 showed signi®cant reduction in productivity in both ¯ush treatments and line 31 showed a similar reduction in the ¯ush-100 treatment. As a result, the high-®tness isofemale lines as a group exhibit a signi®cant reduction in ®tness following the founder¯ush protocol; average productivity in these isofemale lines was 92% of the control value after FS-3 and only 52% of the control at the end of the experiment.

Discussion Inbreeding, and its deleterious consequences for ®tness, has been studied in considerable depth and is a topic of


599

extensive current research (e.g. Charlesworth & Charlesworth, 1999; Bijlsma et al., 2000; Hedrick & Kalinowski, 2000). Nevertheless, the generality of the process by which the detrimental genetic variation of a population can be purged through inbreeding or bottlenecks remains a controversial area (Bijlsma et al., 1999; Byers & Waller, 1999; Willis, 1999). Here we have observed that in the protocol design most likely to result in purging, such an outcome appeared in fact to occur. That is, in our examination of low-®tness isofemale lines (isofemale lines that had initially shown inbreeding depression) in the ¯ush-100 regime (isofemale lines that were kept at 100 pairs between one-pair bottlenecks), the ®tness relative to the simultaneous control increased from 0.473 to 0.801, an increase of 69%. For the low-®tness isofemale lines in the ¯ush-5 regime (isofemale lines that were kept at 5 pairs between bottlenecks), ®tness did not recover and was statistically indistinguishable from the initial level. We conclude that the higher ¯ush numbers between bottlenecks in the ¯ush-100 regime allowed selection, recombination, or perhaps mutation to regenerate the ®tness initially lost from inbreeding. The absence of a ®tness rebound for the ¯ush-5 regime can be explained by the continued importance of genetic drift, even in the ¯ush generations. However, because the ®tness did not decrease in these isofemale lines further from the initial loss, even in the face of four one-pair bottlenecks, this suggests that the constraints caused by genetic drift appeared to be balanced by some effects of selection and recombination, or perhaps mutation, increasing ®tness. On the other hand, the high-®tness isofemale lines (isofemale lines that had not initially shown inbreeding depression) declined in ®tness over time relative to the simultaneous control from 0.920 to 0.520, a decline of 57%. Potentially these isofemale lines could have increased in ®tness if they had become more adapted to the laboratory environment (Willis, 1999). The decline was similar for both the ¯ush-5 and ¯ush-100 regimes, suggesting that the relative lack of genetic drift in the ¯ush period for ¯ush-100 regime did not overcome the effects of the four one-pair bottlenecks. We would have initially predicted that the high-®tness isofemale lines would have shown little further decline in ®tness because the three generations of full-sib mating may have purged detrimental variants. On the contrary, these isofemale lines showed the greatest loss in ®tness over the course of the experiment, independent of the ¯ush size. A possible explanation is that the high-®tness isofemale lines were still segregating for detrimental genetic variants that were not expressed in the initial three generations of full-sib mating and that some of these variants became ®xed in later bottlenecks. Our results demonstrate the variance associated with the nature of ®xation of the deleterious genetic variation in a population. Purely by chance, a population may become ®xed or purged for detrimentals after just one

600


bottleneck, or many bottlenecks may occur before the population experiences signi®cant ®xation or elimination of its deleterious genetic variation. One has no a priori knowledge of a particular population's genetic response to either one or a series of bottlenecks. While there will always be unexplained variation in experimental studies of inbreeding depression that requires very large sample sizes to overcome (Lynch, 1988), the variation seen in our results may be explained by the stochastic nature of the change induced by population bottlenecks. The results reported here might have important implications for application of evolutionary genetics principles in conservation biology. Templeton & Read (1983, 1984) concluded from their study in Speke's gazelle that it may be possible to remove a large portion of the deleterious genetic variation in small captive populations of endangered species through deliberate selective inbreeding (see Kalinowski et al., 2000, for an alternative explanation). The rebounds in ®tness reported by Bryant et al. (1990) and Saccheri et al. (1996) are also suggestive that inbreeding depression can be eliminated. However, from our study, and the reviews of Ballou (1997) and Byers & Waller (1999) it appears that a population's response to bottlenecks is dependent not only on the population's initial genetic constitution and the traits involved, but also on the chance genetic processes associated with bottlenecks. Application of the Speke's gazelle breeding program to other animals may have quite different, and perhaps severe, consequences. Also, surveys have found that species differ widely in the severity of inbreeding depression (Ralls et al., 1988; for a review, see Hedrick & Kalinowski, 2000) and, in addition, theory predicts that a population of historically small size should have a lower equilibrium of lethal variants (Nei, 1968; Lande & Barrowclough, 1987; Bataillon & Kirkpatrick, 2000). As a result, one must expect the response of different species to population bottlenecks will vary considerably.

Acknowledgments We thank Marty Tracey for collecting D. melanogaster samples for this research, Elizabeth King and Barb Webb for their technical assistance in the laboratory, and Ed Bryant for his comments on an earlier draft. This research was supported by NSF.

References Ballou, J.D. 1997. Ancestral inbreeding only minimally affects inbreeding depression in mammalian populations. J. Hered. 88: 169±178. Barton, N.H. & Charlesworth, B. 1984. Genetic revolutions, founder effects, and speciation. Ann. Rev. Ecol. Syst. 15: 133±164. Bataillon, T. & Kirkpatrick, M. 2000. Inbreeding depression due to mildly deleterious mutation in ®nite populations: size does matter. Genet. Res. 75: 75±81.

Bijlsma, R., Bundgaard, J. & Boerema, A.C. 2000. Does inbreeding affect the extinction risk of small populations?: predictions from Drosophila. J. Evol. Biol. 13: 502±514. Bijlsma, R., Bundgaard, J. & Van Putten, W.F. 1999. Environmental dependence of inbreeding depression and purging in Drosophila melanogaster. J. Evol. Biol. 12: 1125±1137. Bonnell, M.L. & Selander, R.K. 1974. Elephant seals: genetic variation and near extinction. Science 184: 908±909. Bryant, E.H., McCommas, S.A. & Combs, L.M. 1986. The effect of an experimental bottleneck upon quantitative genetic variation in the house¯y. Genetics 114: 1191±1211. Bryant, E.H., Meffert, L.M. & McCommas, S.A. 1990. Fitness rebound in serially bottlenecked populations of the house¯y. Amer. Natur. 136: 542±549. Byers, D.L. & Waller, D.M. 1999. Do plant populations purge their genetic load? Effects of population size and mating history on inbreeding depression. Ann. Rev. Syst. Ecol. 30: 479±513. Carson, H.L. & Templeton, A.R. 1984. Genetic revolution in relation to speciation phenomena: the founding of new populations. Ann. Rev. Ecol. Syst. 15: 97±131. Carson, H.L. & Wisotzkey, R.G. 1989. Increase in genetic variance following a population bottleneck. Amer. Natur. 134: 668±673. Charlesworth, B. & Charlesworth, D. 1999. The genetic basis of inbreeding depression. Genet. Res. 74: 329±340. Cockerham, C.C. & Tachida, H. 1988. Permanency of response to selection for quantitative characters in ®nite populations. Proc. Natl. Acad. Sci. USA. 85: 1563±1565. Fernandez, A., Toro, M.A. & Lopez-Fanjul, C. 1995. The effect of inbreeding on the redistribution of genetic variance of fecundity and viability in Tribolium castaneum. Heredity 75: 376±381. Galiana, A., Ayala, F.J. & Moya, A. 1989. Flush-crash experiments in Drosophila. In: Evolutionary Biology of Transient Unstable Populations. (A. Fontevila, ed.), pp. 58±73. SpringerVerlag, Munich. Galiana, A., Moya, A. & Ayala, F.J. 1993. Founder-¯ush speciation in Drosophila pseudoobscura: a large-scale experiment. Evolution 47: 432±444. Goodnight, C.J. 1987. On the effect of founder events on epistatic genetic variance. Evolution 41: 80±91. Hedrick, P.W. 1994. Purging inbreeding depression and the probability of extinction: full-sib mating. Heredity 73: 363±372. Hedrick, P.W. 1995. Elephant seals and the estimation of a population bottleneck. J. Hered. 86: 232±235. Hedrick, P.W. 1996. Bottleneck (s) or metapopulation in cheetahs. Cons. Biol. 10: 897±899. Hedrick, P.W. & Kalinowski, S.T. 2000. Inbreeding depression in conservation biology. Ann. Rev. Ecol. Syst. 31: 139±162. Hoelzel, A.R., Halley, J., O'Brien, S.J., Campagna, C., Arnbom, T., Le Boeuf, B., Ralls, K. & Dover, G.A. 1993. Elephant seal genetic variation and the use of simulation models to investigate historical population bottlenecks. J. Hered. 84: 443±449. Hoffmann, A.A. & Parsons, P.A. 1988. The analysis of quantitative variation in natural populations with isofemale lines. Genet. Select. Evol. 20: 87±98. James, J.W. 1971. The founder effect and response to arti®cial selection. Genet. Res. 16: 241±250. Kalinowski, S.T., Hedrick, P.W. & Miller, P.S. 2000. Inbreeding depression in the Speke's gazelle captive breeding program. Cons. Biol. 14: 1375±1384.



Kirkpatrick, M. & Jarne, P. 2000. The effects of bottleneck on inbreeding depression and the genetic load. Amer. Natur. 155: 154±167. Lande, R. & Barrowclough, G.F. 1987. Effective population size, genetic variation, and their use in population management. In: Viable Populations for Conservation (M. E. SouleÂ, ed.), pp. 87±123. Cambridge University Press, Cambridge. Lande, R. & Schemske, D.W. 1985. The evolution of selffertilization and inbreeding depression in plants. I. Genetic models. Evolution 39: 24±40. Lewontin, R.C. 1974. The Genetic Basis of Evolutionary Change. Columbia University Press, New York. LoÂpez Bueno, J., Moya, A.A. & GonzaÂlez-Candelas, F. 1993. The effect of periodic bottlenecks on the competitive ability of Drosophila pseudoobscura isofemale lines. J. Hered. 70: 60±66. LoÂpez-Fanjul, C., Fernandez, A. & Toro, M.A. 1999. The role of epistasis in the increase in the additive genetic variance after populations bottlenecks. Genet. Res. 73: 45±59. LoÂpez-Fanjul, C. & Villaverde, A. 1989. Inbreeding increases genetic variance for viability in Drosophila melanogaster. Evolution 43: 1800±1804. Lynch, M. 1988. Design and analysis of experiments on random drift and inbreeding depression. Genetics 120: 791±807. Nei, M. 1968. The frequency distribution of lethal chromosomes in ®nite populations. Proc. Natl. Acad. Sci. USA 60: 517±523. Nei, M., Maruyama, T. & Chakraborty, R. 1975. The bottleneck effect and genetic variability in populations. Evolution 29: 1±10. O'Brien, S.J., Roelke, M.E., Marker, L., Newman, A., Winkler, C.A., Meltzer, D., Colly, L., Everman, J.F., Bush, M. & Wildt, D.E. 1985. Genetic basis for species vulnerability in the cheetah. Science 227: 1428±1434. O'Brien, S.J., Wildt, D.E., Goldman, D., Merril, C.R. & Bush, M. 1983. The cheetah is depauperate in genetic variation. Science 221: 459±462.


601

Ralls, K., Ballou, J.D. & Templeton, A.R. 1988. Estimates of lethal equivalents and the cost of inbreeding in mammals. Cons. Biol. 2: 185±193. Robertson, A. 1952. The effect of inbreeding on variation due to recessive genes. Genetics 37: 189±207. Saccheri, I.J., Brake®eld, P.M. & Nichols, R.A. 1996. Severe inbreeding depression and rapid ®tness rebound in the butter¯y Bicyclus anynana (Satyridae). Evolution 50: 2000±2013. Sokal, R.R. & Rohlf, F.J. 1995. Biometry, 4th edn. W. H. Freeman. New York. Templeton, A.R. & Read, B. 1983. Elimination of inbreeding depression in a captive herd of Speke's gazelle. In: Genetics and Conservation: a Reference for Managing Wild Animal and Plant Populations (C. M. Schonewald-Cox, S. M. Chambers, B. MacBryde & L. Thomas, eds), pp. 241±261. Benjamin/ Cummings, Menlo Park, California. Templeton, A.R. & Read, B. 1984. Factors eliminating inbreeding depression in a captive herd of Speke's gazelle. Zoo Biol. 3: 177±199. Wang, J., Hill, W.G., Charlesworth, D. & Charlesworth, B. 1999. Dynamics of inbreeding depression due to deleterious mutations in small populations: mutation parameters and inbreeding rate. Genet. Res. 74: 165±178. Weber, D.S., Stewart, B.S., Garza, J.C. & Lehman, N. 2000. An empirical genetic assessment of the severity of the northern elephant seal population bottleneck. Curr. Biol. 10: 1287±1290. Willis, J.H. 1999. The role of genes of large effect on inbreeding depression in Mimulus guttatus. Evolution 53: 1678±1691. Willis, J.H. & Orr, H.A. 1993. Increased heritable variation following population bottlenecks: the role of dominance. Evolution 47: 949±957. Received 2 March 2001; accepted 30 April 2001