ESTIMATION OF PARAMETERS OF INBREEDING AND GENETIC DRIFT IN POPULATIONS WITH OVERLAPPING GENERATIONS

Many long‐lived plant and animal species have nondiscrete overlapping generations. Although numerous models have been developed to predict the effective sizes (N_e) of populations with overlapping generations, they are extremely difficult to apply to natural populations because of the large array of unknown and elusive life‐table parameters involved. Unfortunately, little work has been done to estimate the N_e of populations with overlapping generations from marker data, in sharp contrast to the situation of populations with discrete generations for which quite a few estimators are available. In this study, we propose an estimator (EPA, estimator by parentage assignments) of the current N_e of populations with overlapping generations, using the sex, age, and multilocus genotype information of a single sample of individuals taken at random from the population. Simulations show that EPA provides unbiased and accurate estimates of N_e under realistic sampling and genotyping effort. Additionally, it yields estimates of other interesting parameters such as generation interval, the variances and covariances of lifetime family size, effective number of breeders of each age class, and life‐table variables. Data from wild populations of baboons and hihi (stitchbird) were analyzed by EPA to demonstrate the use of the estimator in practical sampling and genotyping situations.     

EFFECTIVENESS OF SELECTION IN REDUCING THE GENETIC LOAD IN POPULATIONS OF PEROMYSCUS POLIONOTUS DURING GENERATIONS OF INBREEDING

It has been hypothesized that natural selection reduces the “genetic load” of deleterious alleles from populations that inbreed during bottlenecks, thereby ameliorating impacts of future inbreeding. We tested the efficiency with which natural selection purges deleterious alleles from three subspecies of Peromyscus polionotus during 10 generations of laboratory inbreeding by monitoring pairing success, litter size, viability, and growth in 3604 litters produced from 3058 pairs. In P. p. subgriseus, there was no reduction across generations in inbreeding depression in any of the fitness components. Strongly deleterious recessive alleles may have been removed previously during episodes of local inbreeding in the wild, and the residual genetic load in this population was not further reduced by selection in the lab. In P. p. rhoadsi, four of seven fitness components did show a reduction of the genetic load with continued inbreeding. The average reduction in the genetic load was as expected if inbreeding depression in this population is caused by highly deleterious recessive alleles that are efficiently removed by selection. For P. p. leucocephalus a population that experiences periodic bottlenecks in the wild, the effect of further inbreeding in the laboratory was to exacerbate rather than reduce the genetic load. Recessive deleterious alleles may have been removed from this population during repeated bottlenecks in the wild; the population may be close to a threshold level of heterozygosity below which fitness declines rapidly. Thus, the effects of selection on inbreeding depression varied substantially among populations, perhaps due to different histories of inbreeding and selection.     

放牧干扰对根茎冰草无性系种群年龄结构的影响

根茎冰草（Agropyron michnoi)是典型的无性系禾草。在松嫩平原,根茎冰草的分蘖节最老为４龄,可存活５个年度。不同季节的放牧干扰可导致根茎冰草无性系种群分蘖株的年龄结构发生变化,全年放牧样地和夏秋休牧样地均为稳定型年龄结构,两个冬季放牧样地均为增长型年龄结构。１龄分蘖株的生产力和营养繁殖力力均最大。４龄分蘖株均已丧失了营养繁殖力。营养繁殖芽的年龄结构可以更好地预测预报无性系种群和群落的未来。全年放牧可保护稳定型年龄结构,冬季放牧可保持增长型年龄结构,夏秋休牧可使种群发展为增长型年龄结构,两个冬季放牧样地将仍然保持单优势种群落,全年放牧地将仍为混生群落,夏季林牧样地具有向着单优势种群落演替的潜在趋势。     

中国人群遗传结构分析

本文根据红细胞血型基因频率,用Harpending和Jenkins(1973)方法计算了中国22个人群间的遗传距离,同时在国内首次运用主坐标分析及其排序方法展示了中华民族的遗传结构,反映出中国东西人群与南北人群间的基因流。     

ISOLATION BY DISTANCE IN EQUILIBRIUM AND NON-EQUILIBRIUM POPULATIONS

It is shown that for allele frequency data a useful measure of the extent of gene flow between a pair of populations is , which is the estimated level of gene flow in an island model at equilibrium. For DNA sequence data, the same formula can be used if F_ST is replaced by N_ST. In a population with restricted dispersal, analytic theory shows that there is a simple relationship between M? and geographic distance in both equilibrium and non-equilibrium populations and that this relationship is approximately independent of mutation rate when the mutation rate is small. Simulation results show that with reasonable sample sizes, isolation by distance can indeed be detected and that, at least in some cases, non-equilibrium patterns can be distinguished. This approach to analyzing isolation by distance is used for two allozyme data sets, one from gulls and one from pocket gophers.     

POPULATIONS OF MONARCH BUTTERFLIES WITH DIFFERENT MIGRATORY BEHAVIORS SHOW DIVERGENCE IN WING MORPHOLOGY

The demands of long‐distance flight represent an important evolutionary force operating on the traits of migratory species. Monarchs are widespread butterflies known for their annual migrations in North America. We examined divergence in wing morphology among migratory monarchs from eastern and western N. America, and nonmigratory monarchs in S. Florida, Puerto Rico, Costa Rica, and Hawaii. For the three N. American populations, we also examined monarchs reared in four common environment experiments. We used image analysis to measure multiple traits including forewing area and aspect ratio; for laboratory‐reared monarchs we also quantified body area and wing loading. Results showed wild monarchs from all nonmigratory populations were smaller than those from migratory populations. Wild and captive‐reared eastern monarchs had the largest and most elongated forewings, whereas monarchs from Puerto Rico and Costa Rica had the smallest and roundest forewings. Eastern monarchs also had the largest bodies and high measures of wing loading, whereas western and S. Florida monarchs had less elongated forewings and smaller bodies. Among captive‐reared butterflies, family‐level effects provided evidence that genetic factors contributed to variation in wing traits. Collectively, these results support evolutionary responses to long‐distance flight in monarchs, with implications for the conservation of phenotypically distinct wild populations.     

GENOTYPIC VARIATION AND CLONAL STRUCTURE IN CORAL POPULATIONS WITH DIFFERENT DISTURBANCE HISTORIES

Genotypic diversity in six populations of the endemic Hawaiian reef coral, Porites compressa, was directly related to habitat-disturbance history. The highest diversity (lowest amount of clonal proliferation) was found in populations that had been intensely or recently disturbed. In these populations, space was not limited and mean colony size was small (< 500 cm²), suggesting early stages of recolonization. In an undisturbed, protected habitat, lower genotypic diversity was a result of a significant degree of clonal replication of established genotypes. Unoccupied substratum was rare in this habitat, and average colony size was large (> 2500 cm²). Populations in intermediately disturbed habitats showed intermediate levels of diversity and clonal structure as a result of the combined contributions of sexual and asexual reproduction. Individual clones were distributed over small areas (< 4 m²) or distances (< 1 m) in young populations, and more broadly (> 256 m²) and over longer distances (> 90 m) in the older, undisturbed population. Interpretations of life-history parameters and estimates of total genetic variability in species that have the potential to reproduce asexually are dependent upon an assessment of the overall clonal structure of populations. The power of genotypic assays to reliably detect clonal versus unique colonies, as well as the spatial scales over which clonal populations are sampled, are critical to such assessments.     

THE OPPORTUNITY FOR BALANCING SELECTION IN EXPERIMENTAL POPULATIONS OF CAENORHABDITIS ELEGANS

The role of balancing selection in maintaining diversity during the evolution of sexual populations to novel environments is poorly understood. To address this issue, we studied the impact of two mating systems, androdioecy and dioecy, on genotype distributions during the experimental evolution of Caenorhabditis elegans. We analyzed the temporal trajectories of 334 single nucleotide polymorphisms, covering 1/3 of the genome, and found extensive allele frequency changes and little loss of heterozygosities after 100 generations. As modeled with numerical simulations, SNP differentiation was consistent with genetic drift and average fitness effects of 2%, assuming that selection acted independently at each locus. Remarkably, inbreeding by self‐fertilization was of little consequence to SNP differentiation. Modeling selection on deleterious recessive alleles suggests that the initial evolutionary dynamics can be explained by associative overdominance, but not the later stages because much lower heterozygosities would be maintained during experimental evolution. By contrast, models with selection on true overdominant loci can explain the heterozygote excess observed at all periods, particularly when negative epistasis or independent fitness effects were considered. Overall, these findings indicate that selection at single loci, including purging of recessive alleles, underlies most of the genetic differentiation accomplished during the experiment. Nonetheless, they also imply that maintenance of genetic diversity may in large part be due to balancing selection at multiple loci.     

MAINTENANCE OF POLYGENIC VARIATION IN SPATIALLY STRUCTURED POPULATIONS: ROLES FOR LOCAL MATING AND GENETIC REDUNDANCY

Although heritable genetic variation is critical to the evolutionary process, we know little about how it is maintained. Obviously, mutation-selection balance must play a role, but there is considerable doubt over whether it can account for heritabilities as high as 0.5, which are commonly found in natural populations. Most models of mutation-selection balance assume panmictic populations. In this paper we use Monte Carlo simulations to examine the effect of isolation by distance on the variation maintained by mutation in a polygenic trait subject to optimizing selection. We show that isolation by distance can substantially increase the total variation maintained in continuous populations over a wide range of dispersal patterns, but only if more than one genotype produces the optimal phenotype (genetic redundancy). Isolation by distance alone has only a slight effect on the variation maintained in the total population for neutral alleles. The combined effect of isolation by distance and genetic redundancy, however, allows the maintenance of substantial variation despite strong stabilizing selection. The mechanism is straightforward. Isolation by distance allows mutation and drift to operate independently in different parts of the population. Because of their independent evolutionary histories, different parts of the population independently draw from the available set of redundant genotypes. Because the genotypes are redundant, selection does not discriminate among them, and they will persist until eliminated by drift. The population as a whole maintains many distinct genotypes. We show that this process allows mutation to maintain high levels of variation, even under strong stabilizing selection, and that over a moderate range of dispersal patterns the amount of variation maintained in the entire population is independent of both the strength of selection and the variance of the dispersal distance. Furthermore, we show that individual heterozygosity is increased in locally mating populations when selection is strong. Finally, our simulations provide a rough picture of how selection and the dispersal pattern influence the spatial distribution of genetic and phenotypic variation.     

ARE POPULATIONS ISLANDS? ANALYSIS OF CHLOROPLAST DNA VARIATION IN AQUILEGIA

The degree to which conspecific populations are interconnected via ongoing gene flow remains an important focus of evolutionary biology. One major difficulty in distinguishing ongoing gene flow from historical subdivision is that either process can generate similar estimates of apparent gene flow. Thus, gene flow estimates themselves are insufficient to distinguish between these alternatives. However, genetic data coupled with additional information about demography and distribution do allow a distinction to be made. Here we address the specific question, does gene flow link populations of Aquilegia? In a survey of a 525 B.P. chloroplast DNA fragment sampled from 251 individual plants from 18 populations of three taxa, five haplotypes were identified. No significant relationship between geographic distance and apparent gene flow between population pairs existed. Further, the estimated level of gene flow was entirely compatible with a historical subdivision of Aquilegia populations during the late Pleistocene or early Holocene. Therefore, these patterns of variation are due not to ongoing gene flow, but rather to historical association among populations. Thus Aquilegia populations may be considered as distinct evolutionary entities with regard to seed-mediated processes. As a result, comparative analysis of ecological traits undergoing potentially rapid evolution (e.g., life histories, mating systems, inbreeding depression) should be possible in these taxa.     

DISCORDANT DISTRIBUTION OF POPULATIONS AND GENETIC VARIATION IN A SEA STAR WITH HIGH DISPERSAL POTENTIAL

Patiria miniata, a broadcast‐spawning sea star species with high dispersal potential, has a geographic range in the intertidal zone of the northeast Pacific Ocean from Alaska to California that is characterized by a large range gap in Washington and Oregon. We analyzed spatial genetic variation across the P. miniata range using multilocus sequence data (mtDNA, nuclear introns) and multilocus genotype data (microsatellites). We found a strong phylogeographic break at Queen Charlotte Sound in British Columbia that was not in the location predicted by the geographical distribution of the populations. However, this population genetic discontinuity does correspond to previously described phylogeographic breaks in other species. Northern populations from Alaska and Haida Gwaii were strongly differentiated from all southern populations from Vancouver Island and California. Populations from Vancouver Island and California were undifferentiated with evidence of high gene flow or very recent separation across the range disjunction between them. The surprising and discordant spatial distribution of populations and alleles suggests that historical vicariance (possibly caused by glaciations) and contemporary dispersal barriers (possibly caused by oceanographic conditions) both shape population genetic structure in this species.     

西双版纳不同斑块望天树种群的密度、结构和生物量

 根据5个不同斑块的野外调查数据,分析了残存分布在西双版纳的国家一级保护稀有树种望天树（Parashorea chinensis）的种群密度与数量、 年龄结构与生物量动态, 组建了不同生长发育阶段的望天树个体生长与年龄的回归模型、个体生物量模型及种群年龄结构模型, 编制了不同 斑块的望天树种群及整个种群的静态生命表、存活曲线和年龄结构图。结果表明,不同斑块的望天树种群因种群年龄及所受到的干扰方式的不 同,其种群密度和年龄结构差异很大,不同年龄阶段的死亡率也不同。面积最小的斑块缺乏成熟个体,并出现龄级结构缺省的现象。不同斑块 局部种群的生物量随林龄的变化近似于Logistic增长,但各斑块局部种群的最大生物量以及生物量随时间的动态变化有所不同。整个种群的年 龄结构为稳定增长型种群,1～60龄的种群个体的死亡率随林龄的增加而下降,60～150龄的个体死亡率随林龄的增加而上升,180 龄后种群呈 现生理衰退,个体出现死亡高峰。种群的生物量在180 龄前呈Logistic 增长,此后,生物量下降。部分斑块受到严重的人为干扰,已严重威胁 其局部种群的生存。     

山西云蒙山油松种群的年龄结构和动态特征

本文用样方法研究了山西云蒙山油松种群的年龄结构、存活曲线和数量动态规律,并且讨论了油松种群数量动态和群落演替的关系,结果表明:油松种群具有增长型和衰退型的动态特征,影响油松种群数量的内在机制主要是种内光资源的竞争。尽管大多数油松种群处于不断衰减状态,但其群落结构及性质在较长时期内将不会发生显著的变化。     

RECOMBINATION AND MIGRATION RATES IN NATURAL POPULATIONS OF BACILLUS SUBTILIS AND BACILLUS MOJAVENSIS

We have investigated the rates of recombination and migration in native populations of two closely related, naturally competent Bacillus species. Native soil isolates of Bacillus subtilis and Bacillus mojavensis were obtained from three continents and, within North America, from populations at a range of geographical distances from one another. The rate of recombination within populations of each species was estimated from restriction-site data for three genes. Recombination was shown to occur within each species at about the same rate as neutral mutation, whatever the geographical scale or phylogenetic scale over which strains were sampled. The rate of migration between populations was estimated by a cladistic analysis and was shown to be high (i.e., Nm > 1), even among populations on different continents. The level of migration within each species is sufficient to prevent neutral geographical divergence within species.     

ESTIMATING PHENOTYPIC SELECTION IN AGE‐STRUCTURED POPULATIONS BY REMOVING TRANSIENT FLUCTUATIONS

An extension of the selection differential in the Robertson–Price equation for the mean phenotype in an age‐structured population is provided. Temporal changes in the mean phenotype caused by transient fluctuations in the age‐distribution and variation in mean phenotype among age classes, which can mistakenly be interpreted as selection, will disappear if reproductive value weighting is applied. Changes in any weighted mean phenotype in an age‐structured population may be decomposed into between‐ and within‐age class components. Using reproductive value weighting the between‐age class component becomes pure noise, generated by previous genetic drift or fluctuating selection. This component, which we call transient quasi‐selection, can therefore be omitted when estimating age‐specific selection on fecundity or viability within age classes. The final response can be computed at the time of selection, but can not be observed until lifetime reproduction is realized unless the heritability is one. The generality of these results is illustrated further by our derivation of the selection differential for the continuous time age‐structured model with general age‐dependent weights. A simple simulation example as well as estimation of selection components in a house sparrow population illustrates the applicability of the theory to analyze selection on the mean phenotype in fluctuating age‐structured populations.     

SONG STRUCTURE AND SINGING BEHAVIOUR OF WILLOW WARBLER PHYLLOSCOPUS TROCHILUS ACREDULA IN POPULATIONS OF LOW AND HIGH DENSITY

ABSTRACT

Song structure and singing behaviour in two nesting populations of Willow Warbler Phylloscopus trochilus acredula of different density were studied in Moscow and Kostroma Regions, Russia, from 1994–1997. In total 520 songs of 9 males from the low-density population and 566 songs of 12 males from the high density population were analysed. Singing periods in the high density Willow Warbler population (11 males, 758 min.) were longer (58% of the time) than in the low density population (13 males, 866 min.) (58%). In the high density population, 51% of singing time consisted of “singing duels” (singing in gaps between the songs of a neighbouring male, and singing during the singing of a neighbouring male), while in the low density population only 7% did. The initial phrases of Willow Warbler songs were significantly longer in the high density population, with up to 9–13 elements, while there were no occurrences of this length in the low density population. Songs as a whole were longer and more variable in the high density population. Thus changes in structure, singing activity and behaviour are linked to male-male interaction and density of the nesting community.     

HOST-PATHOGEN INTERACTIONS IN NATURAL POPULATIONS OF LINUM MARGINALE AND MELAMPSORA LINI: II. LOCAL AND REGIONAL VARIATION IN PATTERNS OF RESISTANCE AND RACIAL STRUCTURE

Spatial variation in the resistance structure of Linum marginale (wild flax) populations to the rust fungus Melampsora lini, and in the racial structure of this pathogen, was investigated by sampling 10 populations distributed throughout the Kosciusko National Park, New South Wales, Australia. Considerable differences were found among populations in the structure of both host and pathogen. Host populations were divided into three broad categories: (1) populations susceptible to all testing races; (2) populations containing a strictly limited number of resistant phenotypes; and (3) populations with a considerable diversity of resistant phenotypes. The pathogen populations also showed a range of diversity. The major differences between these populations were determined by the occurrence and frequency of four common races of pathogen (races A, E, K, and N). These differences were apparent both at a regional spatial scale (over the 100 km separation of the most distant populations) and at a local scale where major differences were detected between two populations only 300 m apart. The distribution of the four common races of M. lini was consistent with the hypothesis that a fitness cost was associated with unnecessary virulence. In general, however, differences in the structure of pathogen populations from genetically very similar host populations implied that, in addition to host resistance genes, other evolutionary forces are also important in determining the genetic structure of individual pathogen populations.     

THE EVOLUTION OF SPITE: POPULATION STRUCTURE AND BACTERIOCIN‐MEDIATED ANTAGONISM IN TWO NATURAL POPULATIONS OF XENORHABDUS BACTERIA

Spite occurs when an individual harms itself in the act of harming other individuals. Such behaviors were once assumed to be of limited evolutionary importance, as the conditions for the evolution of spite were thought to be too restrictive. Recent theoretical work, however, suggests that spatial population structure, which allows local competition among genotypes, could favor the evolution of spite. One of the clearest examples of spite is the costly production and release by bacteria of toxins (called bacteriocins) that can kill unrelated strains of the same species. Here, we establish the existence of spatial structure in two natural populations of bacteriocin‐producing bacteria. Specifically, relatedness decreased with increasing spatial distance between the field isolates. In addition, toxin‐mediated inhibitions were found only between isolates that were collected more than 1 m apart and that were generally less than 80% similar in their genomic fingerprints. Taken together, the results suggest that the bacteria are spatially structured, with mixing of genotypes and spiteful interactions at the boundaries between demes.