A simulation study on detecting purging of inbreeding depression in captive populations

Inbreeding depression threatens the survival of small populations of both captive and wild outbreeding species. In order to fully understand this threat, it is necessary to investigate what role purging plays in reducing inbreeding depression. Ballou (1997) undertook such an investigation on 25 mammalian populations, using an ancestral inbreeding regression model to detect purging. He concluded that there was a small but highly significant trend of purging on neonatal survival across the populations. We tested the performance of the regression model that Ballou used to detect purging on independently simulated data. We found that the model has low statistical power when inbreeding depression is caused by the build-up of mildly deleterious alleles. It is therefore possible that Ballou's study may have underestimated the effects of ancestral inbreeding on the purging of inbreeding depression in captive populations if their inbreeding depression was caused mainly by mildly deleterious mutations. We also developed an alternative regression model to Ballou's, which showed an improvement in the detection of purging of mildly deleterious alleles but performed less well if deleterious alleles were of a large effect.     

Levels of inbreeding depression over seven generations of selfing in the androdioecious clam shrimp, Eulimnadia texana

Androdioecy (mixtures of males and hermaphrodites) is a rare mating system in both plants and animals. Theory suggests that high levels of inbreeding depression can maintain males in androdioecious populations if hermaphrodites commonly self-fertilize. However, if inbreeding depression (delta) can be 'purged' from selfing populations, maintaining males is more difficult. In the androdioecious clam shrimp, Eulimnadia texana, delta is estimated to be as high as 0.7. Previous work suggests that this high level is maintained in the face of high levels of inbreeding due to an associative overdominance of fitness-related loci with the sex-determining locus. Such associative overdominance would make purging of inbreeding depression difficult to impossible. The current experiment was designed to determine if delta can be purged in these shrimp by tracking fitness across seven generations in selfing and outcrossing treatments. Evidence of purging was found in one of four populations, but the remaining populations demonstrated a consistent pattern of delta across generations. Although the experimental design allowed ample opportunity for purging, the majority of populations were unable to purge their genetic load. Therefore, delta in this species is likely due to associative overdominance caused by deleterious recessive alleles linked to the sex determining locus.     

Temporal change in inbreeding depression in life‐history traits in captive populations of guppy (Poecilia reticulata): evidence for purging?

Inbreeding depression, which generally affects the fitness of small populations, may be diminished by purging recessive deleterious alleles when inbreeding persists over several generations. Evidence of purging remains rare, especially because of the difficulties of separating the effects of various factors affecting fitness in small populations. We compared the expression of life-history traits in inbred populations of guppy (Poecilia reticulata) with contemporary control populations over 10 generations in captivity. We estimated inbreeding depression as the difference between the two types of populations at each generation. After 10 generations, the inbreeding coefficient reached a maximum value of 0.56 and 0.16 in the inbred and control populations, respectively. Analysing changes in the life-history traits across generations showed that inbreeding depression in clutch size and offspring survival increased during the first four to six generations in the populations from the inbred treatment and subsequently decreased as expected if purging occurred. Inbreeding depression in two other traits was weaker but showed similar changes across generations. The loss of six populations in the inbred treatment indicates that removal of deleterious alleles also occurred by extinction of populations that presumably harboured high genetic load.     

No correlation between inbreeding depression and delayed selfing in the freshwater snail Physa acuta

Inbreeding depression, one of the main factors driving mating system evolution, can itself evolve as a function of the mating system (the genetic purging hypothesis). Classical models of coevolution between mating system and inbreeding depression predict negative associations between inbreeding depression and selfing rate, but more recent approaches suggest that negative correlations should usually be too weak or transient to be detected within populations. Empirical results remain unclear and restricted to plants. Here, we evaluate, for the first time, the within-population genetic correlation between inbreeding depression and a trait that controls the amount of self-fertilization (the waiting time) in a self-fertile hermaphroditic animal, the freshwater snail Physa acuta. Using a large quantitative-genetic design (36 grand-families and 348 families), we observe abundant within-population family-level genetic variation for both inbreeding depression (estimated for survival, fecundity, and size) and the degree of behavioral selfing avoidance. However, we detected no correlation between waiting time and inbreeding depression across families. In agreement with recent models, this result shows that mutational variance rather than differential purging accounts for most of the genetic variance in inbreeding depression within a population.     

SEXUAL SELECTION COUNTERACTS EXTINCTION OF SMALL POPULATIONS OF THE BULB MITES

Genetic drift in small populations can increase frequency of deleterious recessives and consequently lead to inbreeding depression and population extinction. On the other hand, as homozygosity at deleterious recessives increases, they should be purged from populations more effectively by selection. Sexual selection has been postulated to strengthen selection against deleterious mutations, and should thus decrease extinction rate and intensify purging of inbreeding depression. We tested these predictions in the bulb mite Rhizoglyphus robini. We created 100 replicate lines of small populations (five males and five females) and in half of them experimentally removed sexual selection by enforcing monogamy. The lines were propagated for eight generations and then assayed for purging of inbreeding depression. We found that proportion of lines which went extinct was lower with sexual selection than without. We also found evidence for purging of inbreeding depression in the lines with sexual selection, but not in lines without sexual selection. Our results suggest that purging of inbreeding depression was more effective against mutations with relatively large deleterious effects. Thus, although our data clearly indicate a positive impact of sexual selection on short‐term survival of bottlenecked populations, long‐term consequences are less clear as they may be negatively impacted by accumulation of deleterious mutations of small effect.     

Evaluation of ancestral inbreeding coefficients: Ballou’s formula versus gene dropping

Estimation of the purging of detrimental effects through inbreeding and selection is an important issue in conservation genetics opening new perspectives for the management of small populations. In 1997 Ballou proposed the ancestral inbreeding coefficient, which is calculated recursively via pedigree inbreeding coefficients, as a tool for evaluating the purging of deleterious alleles in zoo populations. The formula of Ballou assumes independence of inbreeding and ancestral inbreeding coefficients at any stage of the recursion. This study investigates the consequences of this inaccuracy on the estimation of true ancestral inbreeding, i.e. the proportion of alleles within a genome that has undergone inbreeding in the past. As an alternative we propose the estimation of ancestral inbreeding by the method of gene dropping. The methods are compared by stochastic simulation for various models with respect to mode of inheritance (neutral, detrimental and lethal alleles) and different settings for population size and initial allele frequencies. In all scenarios the proportion of alleles within a genome that has undergone inbreeding in the past was overestimated by Ballou’s formula. The overestimation was more pronounced in smaller populations but was not affected by genetic model or initial allele frequency. In contrast, the ancestral inbreeding coefficient calculated by gene dropping provided a robust estimate of ancestral inbreeding in most models and settings. A marginal overestimation was observed only in models with lethal alleles. Therefore, we recommend applying the gene dropping approach to estimate ancestral inbreeding coefficients.     

Effects of population size on performance and inbreeding depression in <Emphasis Type="Italic">Lupinus perennis</Emphasis>

We investigated the relationships among population size, offspring performance, and inbreeding depression (δ) in Lupinus perennis by examining the effect of population size category (large vs. small) on seed production and offspring performance for three pollination treatments (open pollination, hand crossing and hand selfing). In each of our four pairs of populations, one member of the pair was substantially larger than the other. We then grew seeds from this factorial design (2 sizes × 4 pairs × 3 pollination treatments) in the greenhouse to investigate whether population size affects offspring performance in a common environment, and how small size affects purging of the inbreeding load. Multiplicative performance across four early life-stage components (seed production, seedling emergence, seedling survival and seedling growth) of smaller populations was not significantly lower, although biomass of seedlings declined in smaller populations. Self-pollination reduced seed production, seedling emergence and seedling growth, reflecting substantial inbreeding depression (δ = 0.404 ± 0.043). Population size categories did not consistently differ in levels of inbreeding depression, suggesting that purging of genetic load in smaller populations has been limited, and that all populations still harbor inbreeding load. We also found a significant decrease in log performance with increases in the population inbreeding coefficient. These results indicate that even in seemingly large populations, lupines are susceptible to considerable fitness declines through both inbreeding load within populations, and drift load via genetic erosion and fixation of deleterious alleles between populations.     

Has the inbreeding load for a condition‐dependent sexual signalling trait been purged in insular lizard populations?

Sexually selected traits are often condition‐dependent and are expected to be affected by genome‐wide distributed deleterious mutations and inbreeding. However, sexual selection is a powerful selective force that can counteract inbreeding through purging of deleterious mutations. Inbreeding and purging of the inbreeding load for sexually selected traits has rarely been studied across natural populations with different degrees of inbreeding. Here we investigate inbreeding effects (measured as marker‐based heterozygosity) on condition‐dependent sexually selected signalling trait and other morphological traits across islet‐ and mainland populations (n = 15) of an endemic lizard species (Podarcis gaigeae). Our data suggest inbreeding depression on a condition‐dependent sexually selected signalling character among mainland subpopulations with low or intermediate levels of inbreeding, but no sign of inbreeding depression among small and isolated islet populations despite their higher overall inbreeding levels. In contrast, there was no such pattern among ten other morphological traits which are primarily naturally selected and presumably not involved in sexual signalling. These results are in line with purging of recessive deleterious alleles, or purging in combination with stochastic fixation of alleles by genetic drift, for a sexual signalling character in the islet environment, which is characterized by low population sizes and strong sexual selection. Higher clutch sizes in islet populations also raise interesting questions regarding the possibility of antagonistic pleiotropy. Purging and other non‐exclusive explanations of our results are discussed.     

Reduced inbreeding depression due to historical inbreeding in Drosophila melanogaster: evidence for purging

An important issue in conservation biology and the study of evolution is the extent to which inbreeding depression can be reduced or reversed by natural selection. If the deleterious recessive alleles causing inbreeding depression can be 'purged' by natural selection, outbred populations that have a history of inbreeding are expected to be less susceptible to inbreeding depression. This expectation, however, has not been realized in previous laboratory experiments. In the present study, we used Drosophila melanogaster as a model system to test for an association between inbreeding history and inbreeding depression. We created six 'purged' populations from experimental lineages that had been maintained at a population size of 10 male-female pairs for 19 generations. We then measured the inbreeding depression that resulted from one generation of full-sib mating in the purged populations and in the original base population. The magnitude of inbreeding depression in the purged populations was approximately one-third of that observed in the original base population. In contrast to previous laboratory experiments, therefore, we found that inbreeding depression was reduced in populations that have a history of inbreeding. The large purging effects observed in this study may be attributable to the rate of historical inbreeding examined, which was slower than that considered in previous experiments.     

Epistasis,inbreeding depression,and the evolution of self-fertilization

Inbreeding depression resulting from partially recessive deleterious alleles is thought to be the main genetic factor preventing self-fertilizing mutants from spreading in outcrossing hermaphroditic populations. However, deleterious alleles may also generate an advantage to selfers in terms of more efficient purging, while the effects of epistasis among those alleles on inbreeding depression and mating system evolution remain little explored. In this article, we use a general model of selection to disentangle the effects of different forms of epistasis (additive-by-additive, additive-by-dominance, and dominance-by-dominance) on inbreeding depression and on the strength of selection for selfing. Models with fixed epistasis across loci, and models of stabilizing selection acting on quantitative traits (generating distributions of epistasis) are considered as special cases. Besides its effects on inbreeding depression, epistasis may increase the purging advantage associated with selfing (when it is negative on average), while the variance in epistasis favors selfing through the generation of linkage disequilibria that increase mean fitness. Approximations for the strengths of these effects are derived, and compared with individual-based simulation results.     

Population type can influence the magnitude of inbreeding depression in Clarkia concinna (Onagraceae)

Fragmented populations may face high risk of extinction due to the deleterious consequences of increased inbreeding or of genetic drift in small and isolated populations. Theories on the mechanisms of inbreeding depression predict that the severity of inbreeding depression can eventually decrease in populations that persistently inbreed, and hence populations that are isolated through habitat fragmentation might experience a decrease in inbreeding depression over time. In this study, we tested this hypothesis using the patchily distributed, outcrossing annual plant, Clarkia concinna concinna (Onagraceae), which naturally experiences many fragmentation effects. We collected seeds from isolated and central subpopulations and created artificially inbred and outcrossed lines. Progeny from these crosses were planted into the field and greenhouse and assayed for fitness traits over the course of a growing season. Overall, inbreeding depression was substantial, ranging as high as 0.76 (for cumulative fitness in the field), and significant for plant height, fecundity, and above-ground biomass in all experiments. No inbreeding depression was detected for germination or survival rates in the greenhouse experiments, but in the field, survival was significantly depressed for inbred progeny. There was no evidence to support our hypothesis that increased inbreeding in isolated populations would lead to the purging of deleterious alleles and a decrease in the severity inbreeding depression. The most likely hypothesis to explain our results is that purging is not occurring more strongly in the isolated populations due to details of a number of genetic factors (e.g., selection against deleterious alleles is inconsistent or insufficient, or drift has caused fixation of deleterious alleles in these populations). This study supports the view that even when inbreeding depression is predicted to be less problematic, it may still be an important force influencing the fitness of populations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Detecting purging of inbreeding depression by a slow rate of inbreeding for various traits: the impact of environmental and experimental conditions

Inbreeding depression (ID) has since long been recognized as a significant factor in evolutionary biology. It is mainly the consequence of (partially) recessive deleterious mutations maintained by mutation-selection balance in large random mating populations. When population size is reduced, recessive alleles are increasingly found in homozygous condition due to drift and inbreeding and become more prone to selection. Particularly at slow rates of drift and inbreeding, selection will be more effective in purging such alleles, thereby reducing the amount of ID. Here we test assumptions of the efficiency of purging in relation to the inbreeding rate and the experimental conditions for four traits in D. melanogaster. We investigated the magnitude of ID for lines that were inbred to a similar level, F ≈ 0.50, reached either by three generations of full-sib mating (fast inbreeding), or by 12 consecutive generations with a small population size (slow inbreeding). This was done on two different food media. We observed significant ID for egg-to-adult viability and heat shock mortality, but only for egg-to-adult viability a significant part of the expressed inbreeding depression was effectively purged under slow inbreeding. For other traits like developmental time and starvation resistance, however, adaptation to the experimental and environmental conditions during inbreeding might affect the likelihood of purging to occur or being detected. We discuss factors that can affect the efficiency of purging and why empirical evidence for purging may be ambiguous.Subject terms: Evolutionary genetics, Inbreeding     

Effects of Interference Between Selected Loci on the Mutation Load,Inbreeding Depression,and Heterosis

A classical prediction from single-locus models is that inbreeding increases the efficiency of selection against partially recessive deleterious alleles (purging), thereby decreasing the mutation load and level of inbreeding depression. However, previous multilocus simulation studies found that increasing the rate of self-fertilization of individuals may not lead to purging and argued that selective interference among loci causes this effect. In this article, I derive simple analytical approximations for the mutation load and inbreeding depression, taking into account the effects of interference between pairs of loci. I consider two classical scenarios of nonrandomly mating populations: a single population undergoing partial selfing and a subdivided population with limited dispersal. In the first case, correlations in homozygosity between loci tend to reduce mean fitness and increase inbreeding depression. These effects are stronger when deleterious alleles are more recessive, but only weakly depend on the strength of selection against deleterious alleles and on recombination rates. In subdivided populations, interference increases inbreeding depression within demes, but decreases heterosis between demes. Comparisons with multilocus, individual-based simulations show that these analytical approximations are accurate as long as the effects of interference stay moderate, but fail for high deleterious mutation rates and low dominance coefficients of deleterious alleles.     

Royal dynasties as human inbreeding laboratories: the Habsburgs

The European royal dynasties of the Early Modern Age provide a useful framework for human inbreeding research. In this article, consanguineous marriage, inbreeding depression and the purging of deleterious alleles within a consanguineous population are investigated in the Habsburgs, a royal dynasty with a long history of consanguinity over generations. Genealogical information from a number of historical sources was used to compute kinship and inbreeding coefficients for the Habsburgs. The marriages contracted by the Habsburgs from 1450 to 1750 presented an extremely high mean kinship (0.0628±0.009), which was the result of the matrimonial policy conducted by the dynasty to establish political alliances through marriage. A strong inbreeding depression for both infant and child survival was detected in the progeny of 71 Habsburg marriages in the period 1450–1800. The inbreeding load for child survival experienced a pronounced decrease from 3.98±0.87 in the period 1450–1600 to 0.93±0.62 in the period 1600–1800, but temporal changes in the inbreeding depression for infant survival were not detected. Such a reduction of inbreeding depression for child survival in a relatively small number of generations could be caused by elimination of deleterious alleles of a large effect according with predictions from purging models. The differential purging of the infant and child inbreeding loads suggest that the genetic basis of inbreeding depression was probably very different for infant and child survival in the Habsburg lineage. Our findings provide empirical support that human inbreeding depression for some fitness components might be purged by selection within consanguineous populations.     

Inbreeding depression and mixed mating in Leptosiphon jepsonii: a comparison of three populations

BACKGROUND AND AIMS: Inbreeding depression is thought to play a central role in the evolution and maintenance of cross-fertilization. Theory indicates that inbreeding depression can be purged with self-fertilization, resulting in positive feedback for the selection of selfing. Variation among populations of Leptosiphon jepsonii in the timing and rate of self-fertilization provides an opportunity to study the evolution of inbreeding depression and mating systems. In addition, the hypothesis that differences in inbreeding depression for male and female fitness can stabilize mixed mating in L. jepsonii is tested. METHODS: In a growth room experiment, inbreeding depression was measured in three populations with mean outcrossing rates ranging from 0.06 to 0.69. The performance of selfed and outcrossed progeny is compared at five life history stages. To distinguish between self-incompatibility and early inbreeding depression, aborted seeds and unfertilized ovules were counted in selfed and outcrossed fruits. In one population, pollen and ovule production was quantified to estimate inbreeding depression for male and female fitness. KEY RESULTS: Both prezygotic barriers and inbreeding depression limited self seed set in the most outcrossing population. Cumulative inbreeding depression ranged from 0.297 to 0.501, with the lowest value found in the most selfing population. Significant inbreeding depression for early life stages was found only in the more outcrossing populations. Inbreeding depression was not significant for pollen or ovule production. CONCLUSIONS: The results provide modest support for the hypothesized relationship between inbreeding depression and mating systems. The absence of early inbreeding depression in the more selfing populations is consistent with theory on purging. Differences in male and female expression of inbreeding depression do not appear to stabilize mixed mating in L. jepsonii. The current estimates of inbreeding depression for L. jepsonii differ from those of previous studies, underscoring the effects of environmental variation on its expression.     

Role of inbreeding depression and purging in captive breeding and restoration programmes

Inbreeding depression is a major force affecting the evolution and viability of small populations in captive breeding and restoration programmes. Populations that experience small sizes may be less susceptible to future inbreeding depression because they have been purged of deleterious recessive alleles. We review issues related to purging, as they apply to the management of small populations, and discuss an experiment we conducted examining purging in populations of mosquitofish (Gambusia affinis). Purging is an important process in many small populations, but the literature contains a diversity of responses to purging both within and among studies. With the exception that slow inbreeding results in more purging and less threat to population viability, there seem to be few consistent trends that aid in prediction of how a purging event will affect a population. In our examination of purging on population viability in mosquitofish, single or multiple bottlenecks do not appear to have resulted in any purging of the influence of genetic load on population growth. Rather, serial bottlenecks resulted in a marked decline in population growth and an increase in extinction. Our results, taken together with those of reviewed studies, suggest that in small populations there is great uncertainty regarding the success of any single purging event in eliminating inbreeding depression, together with the high likelihood that purging will depress population viability through the fixation of deleterious alleles. In management of captive breeding and restoration programmes, the common practice of avoiding inbreeding and small population sizes should be followed whenever possible.     

Reduced population size can induce quick evolution of inbreeding depression in the invasive ladybird <Emphasis Type="Italic">Harmonia axyridis</Emphasis>

Understanding biological invasion is currently one of the main scientific challenges for ecologists. The introduction process is crucial for the success of an invasion, especially when it involves a demographic bottleneck. A small introduced population is expected to face a higher risk of extinction before the first stage of invasion is complete if inbreeding depression, caused by the expression of deleterious alleles, is important. Changes in mating regimes or in population size can induce the evolution of deleterious allele frequencies, either by selection or by drift, possibly resulting in the purging or the fixation of such alleles within the population. The harlequin ladybird Harmonia axyridis became invasive on several continents following a scenario including at least one event of demographic bottleneck. Although native populations suffered from severe inbreeding depression, it was greatly reduced in invasive ones suggesting that deleterious alleles were purged during the invasion process. In this study, we performed an experiment designed to manipulate the effective population size of H. axyridis across successive generations to mimic contrasting introduction events. We used the measurement of two fitness-related phenotypic traits in order to test (1) if inbreeding depression can evolve at the time-scale of an invasion; and (2) if the changes in inbreeding depression following a bottleneck in laboratory conditions are compatible with the purging of deleterious alleles observed in this species. We found that two generations of very low population size are enough to induce a substantial change in inbreeding depression. Although the genetic changes mostly consisted in fixation of deleterious alleles, purging did also occur, sometimes simultaneously with fixation.     

Inbreeding depression and drift load in small populations at demographic disequilibrium

Inbreeding depression is a major driver of mating system evolution and has critical implications for population viability. Theoretical and empirical attention has been paid to predicting how inbreeding depression varies with population size. Lower inbreeding depression is predicted in small populations at equilibrium, primarily due to higher inbreeding rates facilitating purging and/or fixation of deleterious alleles (drift load), but predictions at demographic and genetic disequilibrium are less clear. In this study, we experimentally evaluate how lifetime inbreeding depression and drift load, estimated by heterosis, vary with census (N_c) and effective (estimated as genetic diversity, H_e) population size across six populations of the biennial Sabatia angularis as well as present novel models of inbreeding depression and heterosis under varying demographic scenarios at disequilibrium (fragmentation, bottlenecks, disturbances). Our experimental study reveals high average inbreeding depression and heterosis across populations. Across our small sample, heterosis declined with H_e, as predicted, whereas inbreeding depression did not vary with H_e and actually decreased with N_c. Our theoretical results demonstrate that inbreeding depression and heterosis levels can vary widely across populations at disequilibrium despite similar H_e and highlight that joint demographic and genetic dynamics are key to predicting patterns of genetic load in nonequilibrium systems.     

Perspective: purging the genetic load: a review of the experimental evidence

Inbreeding depression, the reduction in fitness that accompanies inbreeding, is one of the most important topics of research in evolutionary and conservation genetics. In the recent literature, much attention has been paid to the possibility of purging the genetic load. If inbreeding depression is due to deleterious alleles, whose effect on fitness are negative when in a homozygous state, then successive generations of inbreeding may result in a rebound in fitness due to the selective decrease in frequency of deleterious alleles. Here we examine the experimental evidence for purging of the genetic load by collating empirical tests of rebounds in fitness-related traits with inbreeding in animals and plants. We gathered data from 28 studies including five mammal, three insect, one mollusc, and 13 plant species. We tested for purging by examining three measures of fitness-component variation with serial generations of inbreeding: (1) changes in inbreeding depression, (2) changes in fitness components of inbred lines relative to the original outbred line, and (3) purged population (outcrossed inbred lines) trait means as a function of ancestral outbred trait means. Frequent and substantial purging was found using all three measures, but was particularly pronounced when tracking changes in inbreeding depression. Despite this, we found little correspondence between the three measures of purging within individual studies, indicating that the manner in which a researcher chooses to estimate purging will affect interpretation of the results obtained. The discrepancy suggests an alternative hypothesis: rebounds in fitness with inbreeding may have resulted from adaptation to laboratory conditions and not to purging when using outcrossed inbred lines. However, the pronounced reduction in inbreeding depression for a number of studies provides evidence for purging, as the measure is likely less affected by selection for laboratory conditions. Unlike other taxon-specific reviews on this topic, our results provide support for the purging hypothesis, but firm predictions about the situations in which purging is likely or the magnitude of fitness rebound possible when populations are inbred remain difficult. Further research is required to resolve the discrepancy between the results obtained using different experimental approaches.     

Inbreeding depression and mating-distance dependent offspring fitness in large and small populations of Lychnis flos-cuculi (Caryophyllaceae)

The spatial structure of four Lychnis flos-cuculi populations, varying in size and degree of isolation, was studied by comparing the fitness of offspring resulting from self-pollination and pollinations by neighbouring plants, plants within the same population, and plants from other populations. Selfed offspring had the lowest fitness of the four offspring groups. No significant difference was found between the performance of offspring from pollinations by neighbouring plants and offspring pollinated by plants further apart but within the same population. A lower fitness of offspring from pollinations between neighbours would be expected if these matings, on average, yielded inbred offspring which suffered from inbreeding depression. These results imply that either a tight neighbourhood structuring is not present, or that the inbreeding depression for offspring by neighbours is too low to detect, although these are inbred. Crossings between populations produced offspring with a significantly higher fitness than offspring sired within populations. There were no significant differences in response to inbreeding among the populations, and differences in mean fitness among populations had no clear relation to the population size or degree of isolation. A reduced fitness of small populations due to inbreeding depression or a less severe response to experimental inbreeding due to purging of deleterious alleles is therefore not supported by our results.