▪ Abstract  Inbreeding depression critically influences both mating system evolution and the persistence of small populations prone to accumulate mutations. Under some circumstances, however, inbreeding will tend to purge populations of enough deleterious recessive mutations to reduce inbreeding depression (ID). The extent of purging depends on many population and genetic factors, making it impossible to make universal predictions. We review 52 studies that compare levels of ID among species, populations, and lineages inferred to differ in inbreeding history. Fourteen of 34 studies comparing ID among populations and species found significant evidence for purging. Within populations, many studies report among-family variation in ID, and 6 of 18 studies found evidence for purging among lineages. Regression analyses suggest that purging is most likely to ameliorate ID for early traits (6 studies), but these declines are typically modest (5–10%). Meta-analyses of results from 45 populations in 11 studies reveal no significant overall evidence for purging, but rather the opposite tendency, for more selfing populations to experience higher ID for early traits. The likelihood of finding purging does not vary systematically with experimental design or whether early or late traits are considered. Perennials are somewhat less likely to show purging than annuals (2 of 10 vs. 7 of 14). We conclude that although these results doubtless reflect variation in population and genetic parameters, they also suggest that purging is an inconsistent force within populations. Such results also imply that attempts to deliberately reduce the load via inbreeding in captive rearing programs may be misguided. Future studies should examine male and female fitness traits over the entire life cycle, estimate mating histories at all levels (i.e. population and families within populations), report data necessary for meta-analysis, and statistically test for purging of genetic loads.

The population structure of an organism reflects its evolutionary history and influences its evolutionary trajectory. It constrains the combination of genetic diversity and reveals patterns of past gene flow. Understanding it is a prerequisite for detecting genomic regions under selection, predicting the effect of population disturbances, or modeling gene flow. This paper examines the detailed global population structure of Arabidopsis thaliana. Using a set of 5,707 plants collected from around the globe and genotyped at 149 SNPs, we show that while A. thaliana as a species self-fertilizes 97% of the time, there is considerable variation among local groups. This level of outcrossing greatly limits observed heterozygosity but is sufficient to generate considerable local haplotypic diversity. We also find that in its native Eurasian range A. thaliana exhibits continuous isolation by distance at every geographic scale without natural breaks corresponding to classical notions of populations. By contrast, in North America, where it exists as an exotic species, A. thaliana exhibits little or no population structure at a continental scale but local isolation by distance that extends hundreds of km. This suggests a pattern for the development of isolation by distance that can establish itself shortly after an organism fills a new habitat range. It also raises questions about the general applicability of many standard population genetics models. Any model based on discrete clusters of interchangeable individuals will be an uneasy fit to organisms like A. thaliana which exhibit continuous isolation by distance on many scales.

Plants of a self-incompatible species, which occur in small populations, may have reduced fitness due to the limited availability of compatible mates. Self-incompatibility decreases inbreeding by allowing successful mating to occur only with individuals which differ by at least one-allele at the S-locus. A computer simulation model was developed to test the effect of small population size upon the diversity and the relative frequency of the S-alleles which determine the number of available mates. In a large population at equilibrium, the greater the number of S-alleles the greater the frequency of available mates for all individuals in the population. In small populations (less than 50 individuals), they are unable to maintain a high diversity of S-alleles and therefore there is a decrease in the frequency of available mates. In addition, in small populations there is an increase in the variance of available mates. The number of mates in these populations depends on the genotype of a particular individual. Two patterns would be expected in a small population of incompatible species: (1) a lower seed set per individual due to limited mates, and (2) an increase in variation of seed set among individuals due to the variance in available mates. Lower seed set would lead to a decrease in fitness of particular genotypes and could increase the possibility of local extinction of the species.

As plant populations decrease in size, reduced seed set may contribute to their ultimate extirpation. In this study, effects of pollen quantity and compatibility relationships (quality) on seed set were investigated in a rare species (Eupatorium resinosurn) and a closely related common species (E. perfoliatumi. The impact of pollen quantity was studied through pollen supplementation experiments in two populations of each species. Addition of pollen increased seed set only in the smaller population of E. resinosurn. Compatibility relationships (pollen quality) were investigated in a diallel crossing experiment using ten genotypes from the same populations. Plants from the smaller population of E. resinosurn were found to be 40% cross-incompatible, which was higher than the larger population of E. resinosurn and the two populations of E. perfoliatum, the latter showing signs of self-compatibility in some individuals. In addition the variance in number of compatible matings per individual was higher in the smaller population of E. resinosurn. These results are consistent with a computer simulation model that investigated the effect of small population size on S-allele diversity. Sufficient pollination accompanied by a partial breakdown of the incompatibility system may account, in part, for the relative success of E. perfoliatum.