Showing papers on "Genetic drift published in 1997"
TL;DR: Decreased genetic effective population size, which leads to both inbreeding and the loss of alleles by genetic drift, increased the probability of population extinction over that expected from demographic and environmental stochasticity alone.
Abstract: We established replicated experimental populations of the annual plant Clarkia pulchella to evaluate the existence of a causal relationship between loss of genetic variation and population survival probability. Two treatments differing in the relatedness of the founders, and thus in the genetic effective population size (Ne ), were maintained as isolated populations in a natural environment. After three generations, the low Ne treatment had significantly lower germination and survival rates than did the high Ne treatment. These lower germination and survival rates led to decreased mean fitness in the low Ne populations: estimated mean fitness in the low Ne populations was only 21% of the estimated mean fitness in the high Ne populations. This inbreeding depression led to a reduction in population survival: at the conclusion of the experiment, 75% of the high Ne populations were still extant, whereas only 31% of the low Ne populations had survived. Decreased genetic effective population size, which leads to both inbreeding and the loss of alleles by genetic drift, increased the probability of population extinction over that expected from demographic and environmental stochasticity alone. This demonstrates that the genetic effective population size can strongly affect the probability of population persistence.
609 citations
TL;DR: A general conclusion is that disruption of gene flow can cause evolutionary divergence, perhaps leading to speciation, in the absence of contributions from random genetic drift.
Abstract: We investigate the interplay between gene flow and adaptation in peripheral populations of a widespread species. Models are developed for the evolution of a quantitative trait under clinally varying selection in a species whose density decreases from the center of the range to its periphery. Two major results emerge. First, gene flow from populations at the range center can be a strong force that inhibits peripheral populations from evolving to their local ecological optima. As a result, peripheral populations experience persistent directional selection. Second, response to local selection pressures can cause rapid and substantial evolution when a peripheral population is isolated from gene flow. The amount of evolutionary change depends on gene flow, selection, the ecological gradient, and the trait's heritability. Rapid divergence can also occur between the two halves of a formerly continuous population that is divided by a vicariant event. A general conclusion is that disruption of gene flow can cause evolutionary divergence, perhaps leading to speciation, in the absence of contributions from random genetic drift.
556 citations
TL;DR: Small populations lose genetic variability because of genetic drift, and inbreeding within populations can further decrease individual variability, and genetic threats to population viability will be expressed through their effects on and interactions with demographic and ecological processes.
Abstract: Small populations lose genetic variability because of genetic drift, and inbreeding within populations can further decrease individual variability. Lower variation depresses individual fitness, resistance to disease and parasites, and flexibility in coping with environmental challenges. Lower variation decreases mean fitness of populations (population growth rates), resilience, and long-term adaptability. Genetic drift can threaten viability of populations not just by depleting variation, but also by replacing natural selection as the predominant force driving evolutionary change. Although most genetic studies use laboratory or domesticated populations, evidence is accumulating that the effects of inbreeding are at least as severe on wild animals in natural habitats. Natural selection is expected to reduce the frequency of deleterious alleles in populations that persist through bottlenecks, but as yet there is little evidence for such purging of the genetic load in mammalian populations. No species of mammal has been shown to be unaffected by inbreeding. Genetic problems are contributing to the decline and vulnerability of at least several mammalian taxa. Genetic threats to population viability will be expressed through their effects on and interactions with demographic and ecological processes. Theoretical analyses, experimental tests, field studies, and conservation actions should recognize the fundamental interdependency of genetic and non-genetic processes affecting viability of populations.
500 citations
TL;DR: Analysis of segregation of two different mtDNA genotypes in the tissues of heteroplasmic mice suggests that the coding sequence of mtDNA may represent a compromise between the competing demands of different tissues and point to the existence of unknown, tissue-specific nuclear genes important in the interaction between the nuclear and mitochondrial genomes.
Abstract: Mammalian mitochondrial DNA (mtDNA) is a highly polymorphic, high-copy-number genome that is maternally inherited. New mutations in mtDNA segregate rapidly in the female germline due to a genetic bottleneck in early oogenesis and as a result most individuals are homoplasmic for a single species of mtDNA. Sequence variants thus accumulate along maternal lineages without genetic recombination. Most of the extant variation in mtDNA in mammalian populations has been assumed to be neutral with respect to selection; however, comparisons of the ratio of replacement to silent nucleotide substitutions between species suggest that the evolution of mammalian mtDNA is not strictly neutral. To test directly whether polymorphic mtDNAs behave as neutral variants, we examined the segregation of two different mtDNA genotypes in the tissues of heteroplasmic mice. We find evidence for random genetic drift in some tissues, but in others strong, tissue-specific and age-related, directional selection for different mtDNA genotypes in the same animal. These surprising data suggest that the coding sequence of mtDNA may represent a compromise between the competing demands of different tissues and point to the existence of unknown, tissue-specific nuclear genes important in the interaction between the nuclear and mitochondrial genomes.
348 citations
TL;DR: The long-term survival of artificially restricted populations, particularly of relatively sedentary species, may require molecular monitoring, if genetic diversity is not to be lost by chance when facets of the species niche prove to be poorly understood.
Abstract: The ability to maintain small populations in quasi-natural settings is an issue of considerable importance in biodiversity conservation. The genetic structure of urban common frog (Rana temporaria) populations was determined by allozyme electrophoresis and used to evaluate the effects of restricted intersite migration. Despite the lack of any absolute barrier to movement between ponds, substantial genetic differentiation was found between sites separated by an average of only 2.3 km. Genetic distances between these town ponds correlated positively with geographical distances and were almost twice as great as those found between rural sites separated by an average of 41 km. Measures of genetic diversity and fitness were always lowest in the town, where the degree of subpopulation differentiation (FST = 0.388) was high. Population decline was not evident in the town, but molecular and fitness data indicated the presence of genetic drift and inbreeding depression. The long-term survival of artificially restricted populations, particularly of relatively sedentary species, may require molecular monitoring, if genetic diversity is not to be lost by chance when facets of the species niche prove to be poorly understood.
278 citations
TL;DR: Five research areas of particular importance to restoration biology that offer potentially unique opportunities to couple basic research with the practical needs of restorationists are discussed.
Abstract: A major goal of population biologists involved in restoration work is to restore populations to a level that will allow them to persist over the long term within a dynamic landscape and include the ability to undergo adaptive evolutionary change. We discuss five research areas of particular importance to restoration biology that offer potentially unique opportunities to couple basic research with the practical needs of restorationists. The five research areas are: (1) the influence of numbers of individuals and genetic variation in the initial population on population colonization, establishment, growth, and evolutionary potential; (2) the role of local adaptation and life history traits in the success of restored populations; (3) the influence of the spatial arrangement of landscape elements on metapopulation dynamics and population processes such as migration; (4) the effects of genetic drift, gene flow, and selection on population persistence within an often accelerated, successional time frame; and (5) the influence of interspecific interactions on population dynamics and community development. We also provide a sample of practical problems faced by practitioners, each of which encompasses one or more of the research areas discussed, and that may be solved by addressing fundamental research questions.
272 citations
01 Jan 1997
TL;DR: The chapter summarizes what is known about the control region of birds in terms of its organization, the location of markers within it, and presents exemplars from laboratory observations illustrating the potential and problems of fast-evolving sequences in elucidating the population structure and molecular systematics of closely related taxa.
Abstract: The chapter summarizes what is known about the control region of birds in terms of its organization, the location of markers within it, and presents exemplars from laboratory observations illustrating the potential and problems of fast-evolving sequences in elucidating the population structure and molecular systematics of closely related taxa. In addition to transition and transversion substitutions and numerous small indels, length differences accumulate through variation in a number of tandem repeats, and relatively large duplication or deletion of events. Both inter- and intraspecific variations are more common in the two flanking domains than in the conserved central blocks, with tandem repeats occurring primarily in domain III and larger duplications restricted to among-species comparisons. A thorough analysis of intraspecific sequence variation leads inevitably to the consideration of the population genetic processes responsible for major phylogenetic subdivisions in gene trees and to consideration of taxonomic recognition of these discrete clades as subspecies, phylogenetic species, or biological species. A major difficulty illustrated by the knot sequences is that populations are unlikely to be in equilibrium with respect to mutation and genetic drift.
253 citations
TL;DR: The existence of fitness classes of silent mutations is confirmed and maximum likelihood estimates suggest that selection intensity at silent sites is, on average, very weak in both D. pseudoobscura and D. simulans.
Abstract: In Escherichia coli, Saccharomyces cerevisiae, and Drosophila melanogaster, codon bias may be maintained by a balance among mutation pressure, genetic drift, and natural selection favoring translationally superior codons. Under such an evolutionary model, silent mutations fall into two fitness categories: preferred mutations that increase codon bias and unpreferred changes in the opposite direction. This prediction can be tested by comparing the frequency spectra of synonymous changes segregating within populations; natural selection will elevate the frequencies of advantageous mutations relative to that of deleterious changes. The frequency distributions of preferred and unpreferred mutations differ in the predicted direction among 99 alleles of two D. pseudoobscura genes and five alleles of eight D. simulans genes. This result confirms the existence of fitness classes of silent mutations. Maximum likelihood estimates suggest that selection intensity at silent sites is, on average, very weak in both D. pseudoobscura and D. simulans (|N(e)s| & 1). Inference of evolutionary processes from within-species sequence variation is often hindered by the assumption of a stationary frequency distribution. This assumption can be avoided when identifying the action of selection and tested when estimating selection intensity.
180 citations
Journal Article•
TL;DR: Investigation of a simple model of demographic and genetic change shows that, given the demographic changes in post-Famine Ireland, FST could in theory increase, decrease, or remain the same over short intervals of time.
Abstract: This study examines the genetic impact of the Great Famine (1846-1851) on the regional genetic structure of Ireland. The Great Famine resulted in a rapid decrease in population size throughout Ireland in a short period of time, increasing the possibility of genetic drift. Our study is based on migration and anthropometric data collected originally in the 1930s from 7211 adult Irish males. These data were subdivided into three time periods defined by year of birth: 1861-1880, 1881-1900, and 1901-1920. Within each time period the data were further subdivided into six geographic regions of Ireland. Estimates of Wright's FST were calculated from parent-offspring migration data and from 17 anthropometric variables (10 head measures, 7 body measures). Over time, the average population size decreased, but average rates of migration increased. The estimates of FST at equilibrium from migration matrix analysis suggest that the net effect of these opposite effects is a reduction in among-group variation. Closer examination shows that within each time period the rate of convergence to equilibrium is slow, meaning that the expected levels of genetic homogeneity revealed from migration matrix analysis are not likely to be seen over short intervals of time. Estimates of FST from anthropometric data show either relatively little change in microdifferentiation or some increase, depending on which variables are analyzed. Investigation of a simple model of demographic and genetic change shows that, given the demographic changes in post-Famine Ireland, FST could in theory increase, decrease, or remain the same over short intervals of time. Overall, the Great Famine appears to have had minimal impact on the genetic structure of Ireland on a regional level. Comparison with studies focusing on local genetic structure shows the opposite. It appears that the level of genetic impact depends strongly on the level of analysis; local populations are affected to a greater extent by demographic shifts than regional populations. We also provide formulas for the standard errors of FST from metric traits and related statistics.
149 citations
TL;DR: This chapter discusses contributions of population genetics to plant disease epidemiology and management, a field concerned with determining the extent and pattern of Genetic variation in populations with the goal of understanding the evolutionary processes affecting the origin and maintenance of genetic variation.
Abstract: Publisher Summary This chapter discusses contributions of population genetics to plant disease epidemiology and management. Population genetics and genetic variation in plant pathogens are subjects that have generated much interest since the late 1980s. Almost every recent issue of major plant pathological and mycological journals has at least one article on genetic variation of a plant pathogen species. Whether population genetics becomes an integral discipline within plant pathology depends, in part, on whether it can be integrated with epidemiology and disease management. Evolutionary biology and population genetics have the potential to deliver much basic information about plant pathogens. Population genetics is a field concerned with determining the extent and pattern of genetic variation in populations with the goal of understanding the evolutionary processes affecting the origin and maintenance of genetic variation. The conceptual framework is based on evolutionary biology and on the processes affecting the genetic composition of populations: selection, mutation, gene flow, genetic drift, and mating systems. The advances in technology also brought about a marked change in emphasis in population genetics of plant pathogens. The biology of pathogens at the population level and processes other than selection has been emphasized. Accurate population definition is essential so that sampling is done in a manner which will enable inferences to be made about the population of interest. From an operational perspective, recognition of population structure is very important when estimating population genetic parameters.
139 citations
TL;DR: Comparisons of wild and F1‐hatchery fish indicated significant changes in allele frequencies and losses of low‐frequency alleles but no reduction in heterozygosity, and estimates of variance and inbreeding population size indicated that susceptibility to genetic drift and in breeding in one population increased by twofold after only one generation of supplementation.
Abstract: Four tributaries of Lake St-Jean (Quebec, Canada) are used for spawning and juvenile habitat by land-locked Atlantic salmon. Spawning runs have drastically declined since the mid-1980s, and consequently, a supportive-breeding programme was undertaken in 1990. In this study, we analysed seven microsatellite loci and mtDNA, and empirically estimated effective population sizes to test the hypotheses that (i) fish spawning in different tributaries form genetically distinct populations and (ii) the supportive breeding programme causes genetic perturbations on wild populations. Allele frequency distribution, molecular variance and genetic distance estimates all supported the hypothesis of genetic differentiation among salmon from different tributaries. Gene flow among some populations was much more restricted than previously reported for anadromous populations despite the small geographical scale (40 km) involved. Both mtDNA and microsatellites revealed a more pronounced differentiation between populations from two tributaries of a single river compared with their differentiation with a population from a neighbouring river. The comparison of wild and F1-hatchery fish (produced from breeders originating from the same river) indicated significant changes in allele frequencies and losses of low-frequency alleles but no reduction in heterozygosity. Estimates of variance and inbreeding population size indicated that susceptibility to genetic drift and inbreeding in one population increased by twofold after only one generation of supplementation.
02 Sep 1997
TL;DR: While the paper covers many works on the theory and application of genetic algorithms, not much details are reported on genetic programming, parallel Genetic algorithms, in addition to more advanced techniques e.g. micro-genetic algorithms and multiobjective optimisation.
Abstract: This paper provides a review on current developments in genetic algorithms. The discussion includes theoretical aspects of genetic algorithms and genetic algorithm applications. Theoretical topics under review include genetic algorithm techniques, genetic operator technique, niching techniques, genetic drift, method of benchmarking genetic algorithm performances, measurement of difficulty level of a test-bed function, population genetics and developmental mechanism in genetic algorithms. Examples of genetic algorithm application in this review are pattern recognition, robotics, artificial life, expert system, electronic circuit design, cellular automata, and biological applications. While the paper covers many works on the theory and application of genetic algorithms, not much details are reported on genetic programming, parallel genetic algorithms, in addition to more advanced techniques e.g. micro-genetic algorithms and multiobjective optimisation.
TL;DR: An extension of current statistical methods is employed to reveal differences in the within and between species dynamics of preferred and unpreferred silent mutations in Drosophila simulans, where codon bias appears to be maintained under roughly equal magnitudes of natural selection and genetic drift.
TL;DR: The Mhc polymorphism is used to estimate the size of the founding population of Darwin's finches in the Galápagos Archipelago, and results indicate that the population could not have been smaller than 30 individuals.
Abstract: A key assumption of many allopatric speciation models is that evolution in peripheral or isolated populations is facilitated by drastic reductions in population size. Population bottlenecks are believed to lead to rapid changes in gene frequencies through genetic drift, to facilitate rapid emergence of novel phenotypes, and to enhance reproductive isolation via genetic revolutions. For such effects to occur, founding populations must be very small, and remain small for some time after founding. This assumption has, however, rarely been tested in nature. One approach is to exploit the polymorphism of the major histocompatibility complex ( Mhc ) to obtain information about the founding population. Here, we use the Mhc polymorphism to estimate the size of the founding population of Darwin9s finches in the Galapagos Archipelago. The results indicate that the population could not have been smaller than 30 individuals.
TL;DR: The probability of environmental changes sufficient to allow substantial morphological evolution or reproductive isolation is large relative to the probability that similar changes could occur by processes requiring genetic drift, even with very small population sizes.
Abstract: Two similar evolutionary theories, the shifting balance theory and founder-flush models, invoke random genetic drift to allow evolution on complex adaptive landscapes. These models, in their usual incarnations, deal with fitness as a static entity, and the probability of transition from one form to another is predicted to be quite small by analysis of these models. Fitness itself can change, however, and the amount of change in the parameters of the fitness functions required to allow deterministic evolution to new adaptive peaks is very small. The probability of environ- mental changes sufficient to allow substantial morphological evolution or reproductive isolation is large relative to the probability that similar changes could occur by processes requiring genetic drift, even with very small population sizes. The rapid evolution or speciation following a population founding event is more closely linked with environmental changes than genetic drift.
TL;DR: Analysis of genetic differentiation among eight Daphnia magnapopulations, separated by less than 100 m to more than500 km from each other, suggests that in populations in which only a fewabundant clones are present after a period of strongclonal selection, among-populational geneticdifferentiation as revealed by allozyme markers is inflated as a result of stochasticity involving chance associations of alleles with specific abundantgenotypes.
Abstract: Using allozyme data based on four polymorphic enzyme loci, we present an analysis of genetic differentiation among eight Daphnia magna populations, separated by less than 100 m to more than 500 km from each other. In spite of the large range of geographic distances, there was only a slight tendency for an increase in genetic differentiation with increasing geographic distance between populations, and the relation was not significant. This was mainly due to the fact that neighbouring populations were already highly genetically differentiated. Our results suggest that in populations in which only a few abundant clones are present after a period of strong clonal selection, among-populational genetic differentiation as revealed by allozyme markers is inflated as a result of stochasticity involving chance associations of alleles with specific abundant genotypes. Indices quantifying genetic differentiation were much higher among populations with a low clonal diversity than among populations with a high clonal diversity.
TL;DR: Estimating an effective population size as the mean number of selected S1 lines over 12 cycles, the observed loss of variation was consistent with theoretical expectations resulting from genetic drift of neutral alleles.
Abstract: Iowa Stiff Stalk Synthetic (BSSS) and Iowa Corn Borer Synthetic #1 (BSCB1) are undergoing reciprocal recurrent selection as part of Iowa's Federal-State maize (Zea mays L.) breeding program. This study focused on molecular genetic variation in BSSS(R) and BSCB1(R) cycle 0 (C0) and cycle 12 (C12) populations, as well as the inbred progenitor lines (P) used to synthesize BSSS and BSCBI. The objectives were to quantify amounts of genetic variation within populations, to estimate what proportion remained after selection, and to compare genetic diversities between BSSS and BSCBI populations. Genotypic data for 82 restriction fragment length polymorphism (RFLP) loci were collected from 100 randomly sampled individuals from each C0 and C12 population, 16 BSSS(R) progenitors, and 12 BSCB1(R) progenitors. Progenitor lines were highly homozygous as expected. No single progenitor made excessive genetic contributions to C0 or C12. The BSSS and BSCB1 progenitor populations were initially genetically similar (Nei's genetic distance 0.07). After 12 cycles of selection, they substantially diverged (Nei's distance = 0.66). Gene diversity (expected heterozygosity under random mating) across progenitor populations was very broad (mean gene diversity = 0.6) and remained at that level to C12. Within both populations, the polymorphism level decreased from about 99 to 75%, and gene diversity decreased from about 0.6 to 0.3 between P and C12. The mean number of alleles per locus dropped from about four to less than three. Assuming an effective population size as the mean number of selected S1 lines over 12 cycles, the observed loss of variation was consistent with theoretical expectations resulting from genetic drift of neutral alleles.
TL;DR: Surprisingly, an inverse correlation between genetic variation within certain CTL epitopes and the frequency distribution of HLA alleles that present the CSL epitopes was seen, suggesting that the evolutionary pressures on the C TL epitopes of the virus may be toward their conservation rather than their inactivation.
Abstract: The question whether immune pressure exerted by cytotoxic T lymphocytes (CTLs) can influence the long-term evolution of genetically stable viruses such as Epstein-Barr virus (EBV) has generated considerable scientific interest, primarily due to its important implications for the overall biology of the virus. While arguing for a role of CTLs in the evolution of viruses, it is important to differentiate between genetic variation in virus and immune recognition of these variant virus by CTLs. To assess the role of genetic selection in the long-term evolution of EBV, we have analyzed a large panel of type 1 EBV isolates from African, Southeast Asian, Papua-New Guinean (PNG), and Australian Caucasian individuals. Seven different regions of the EBV genome, which include nine CTL epitopes restricted through a range of HLA class I alleles, were sequenced and compared. Although numerous nucleotide changes were identified within these isolates, comparison of synonymous and nonsynonymous substitutions in the CTL epitope indicated that the genetic variation was generated mostly independently of immune selection pressure. Surprisingly, an inverse correlation between genetic variation within certain CTL epitopes and the frequency distribution of HLA alleles that present the CTL epitopes was seen, suggesting that the evolutionary pressures on the CTL epitopes of the virus may be toward their conservation rather than their inactivation. Furthermore, molecular evolutionary genetic analysis of nucleotide sequences revealed that viral isolates from PNG are evolving as a lineage distinct from isolates from African, Southeast Asian, and Australian Caucasian individuals.
TL;DR: That some populations exhibit much more variability than others suggests that historical demographic processes may play a significant role, and a hierarchical analysis of the distribution of the variability across populations indicates a strong pattern of isolation by distance, whatever the geographical scale considered.
Abstract: The distribution of neutral genetic variability within and among sets of populations results from the com- bined actions of genetic drift, migration, extinction and recolonization processes, mutation, and the mating system. We here analyzed these factors in 38 populations of the hermaphroditic snail Bulinus truncatus. The sampling area covered a large part of the species range. The variability was analyzed using four polymorphic microsatellite loci. A very large number of alleles (up to 55) was found at the level of the whole study. Observed heterozygote deficiencies within populations are consistent with very high selfing rates, generally above 0.80, in all populations. These should depress the variability within populations, because of low effective size, genetic hitchhiking, and background selection, whatever the model of mutation assumed. However, that some populations exhibit much more variability than others suggests that historical demographic processes (e.g., population size variation, bottlenecks, or founding events) may play a significant role. A hierarchical analysis of the distribution of the variability across populations indicates a strong pattern of isolation by distance, whatever the geographical scale considered. Our analysis also illustrates how the mutation rate may affect population differentiation, as different mutation rates result in different levels of homoplasy at microsatellite loci. The effects of both genetic drift and gene flow vary with the temporal and spatial scales considered in B. truncatus populations.
TL;DR: The island subspecies population has evolved sufficient genetic independence to justify ongoing conservation efforts to counter demographic collapse and genetic erosion; the course of genetic erosion can now be monitored non–invasively, as demonstrated by this study, based on DNA amplified from feathers.
Abstract: Polymorphic nuclear microsatellite loci were used to characterize genetic variation in contemporary and historic populations of the San Clemente Island loggerhead shrike ( Lanius ludovicianus mearnsi ), an endangered bird with a current population of 30 individuals that is endemic to to one of the California Channel Islands. We also compared the population of the shrike with two contemporary populations of the still abundant subspecies, L. l. gambeli , which live 120 km away on the adjacent mainland. The current population of L. l. mearnsi has 60 per cent of the genetic variation of the mainland shrike populations and is strongly differentiated from them. Comparison of living birds with 19 birds collected in 1915 shows that most of the variation within the island population was lost before the recent 90 per cent decline in population size, and the 20 per cent decrease in variation this century is probably attributable to genetic drift. Mitochondrial DNA control region sequence data from 80 year old specimens show that there may have been limited introgression to L. l. mearnsi , this century, from another island subspecies, L. l. anthonyi , found in the northern Channel Islands. Today, gene flow between L. l. mearnsi and mainland L. l. gambel is very low, even though a few mainland birds visit the island annually. The island subspecies population has evolved sufficient genetic independence to justify ongoing conservation efforts to counter demographic collapse and genetic erosion; the course of genetic erosion can now be monitored non–invasively, as demonstrated by this study, based on DNA amplified from feathers.
TL;DR: The genetic data suggest that the founding North American population(s) was substantial, and there was a measure of genetic differentiation caused by drift, but this differentiation was small with respect to the large geographical distances among the sampled populations.
TL;DR: Populations tend to diverge genetically because of genetic drift, but their differences are reduced by the exchange of individuals or gametes—in brief, by gene flow.
Abstract: Populations tend to diverge genetically because of genetic drift, but their differences are reduced by the exchange of individuals or gametes—in brief, by gene flow. The evolutionary weight of drift depends on a property of each single population—its effective size. Conversely, the rate—and, therefore, the impact—of gene flow depends on the relationships between populations. Geographers say that everything is related to everything else but that close objects are more closely related than distant objects.
TL;DR: It is shown that both the selection scheme and the mating scheme proposed in this paper could increase the effective size and decrease inbreeding in any generation compared with the classical selection and mating schemes.
Abstract: A selection scheme and a mating scheme are proposed to control the inbreeding and genetic drift in conserved or control animal populations with different numbers of males and females. Recurrence equations for the inbreeding coefficient and formulae for effective size are derived for autosomal loci, sex-linked loci with males being heterogametic and sex-linked loci with females being heterogametic under each of four breeding systems. It is shown that both the selection scheme and the mating scheme proposed in this paper could increase the effective size and decrease inbreeding in any generation compared with the classical selection and mating schemes. Among the four breeding systems considered, the most efficient one could increase the effective size by as much as 19 per cent for autosomal loci and 50 per cent for sex-linked loci in comparison with the classical breeding system usually utilized in conserved or control populations.
TL;DR: The results support a demographic model of population expansions and contractions under which available habitats, after the rainy season, are colonized by individuals originating from a smaller number of refuges (areas that never dry out in the deepest parts of the ponds) and selfing appeared to be an important force affecting the genetic structure in permanent ponds.
Abstract: We analyzed short-term forces acting on the genetics of subdivided populations based on a temporal survey of the microsatellite variability in the hermaphrodite freshwater snail Bulinus truncatus . This species inhabits temporary habitats, has a short generation time and exhibits variable rates of selfing. We studied the variability over three sampling dates in 12 Sahelian populations (1161 individuals). Classical genetic parameters (estimators of H o , H e , f selfing rate and Fst ) showed limited change over time whereas important temporal changes of allelic frequencies were detected for 10 of the ponds studied. These variations are not easily explained by selection, sampling drift and genetic drift alone and may be due to periodic migration. Indeed the habitats occupied by the populations studied are subject to large temporal fluctuations owing to annual cycles of drought and flood. In such ponds our results support a demographic model of population expansions and contractions under which available habitats, after the rainy season, are colonized by individuals originating from a smaller number of refuges (areas that never dry out in the deepest parts of the ponds). In contrast, selfing appeared to be an important force affecting the genetic structure in permanent ponds.
TL;DR: This study suggests that other reasons for low fruit set in Jones cycladenia, such as resource or pollinator limitation, or genetic load, should be explored in future research.
Abstract: The rare endemic Cycladenia humilis var. jonesii (Jones cycladenia) has low levels of sexual reproduction. Enzyme electrophoresis was used to explore possible causes of low seed set and high fruit abortion by assessing the clonal structure and genetic diversity in populations. The seven populations studied were composed of multiple, highly interdigitated clones; thus low fruit set is not likely to be due to a scarcity of mates. Genotype frequencies did not differ significantly from Hardy-Weinberg proportions, suggesting that populations are not highly inbred. Jones cycladenia exhibited high levels of genetic diversity at both the population level (A = 1.7; P = 37; He = 0.14) and the taxon level (A = 2.7; P = 60) in comparison to other plants. These data suggest that genetic drift is unlikely to have left this taxon genetically depauperate. Furthermore, we detected little divergence among geographically disjunct populations of Jones cycladenia (FST = 0.10). In comparison, Jones cycladenia populations were highly differentiated from a population of the taxon's close relative, C. h. var. humilis (mean genetic identity = 0.76). Our study suggests that other reasons for low fruit set in Jones cycladenia, such as resource or pollinator limitation, or genetic load, should be explored in future research.
TL;DR: The results indicate the operation of founder effect and subsequent genetic drift within the small reintroduced herds, but also may be influenced by small sample sizes from herds that were difficult to sample, and suggest management practices that should minimize the loss of genetic variation from reintroduced populations of bighorn sheep.
Abstract: We compared allozyme variability in 4 reintroduced populations of bighorn sheep (Ovis canadensis) with their common source population in Wyoming to understand how reintroduction affects genetic variability. Founder size was low (8-69) and effective population size (N e ) remained low 10-20 years after release. Allele frequencies at 2 of 4 polymorphic loci differed between the reintroduced herds and the common source (P < 0.01), and the number of alleles per locus was reduced in reintroduced herds (P = 0.04). Multi-locus heterozygosity (at 29 loci) was lower (P = 0.03) in 3 of the reintroduced herds than in the common source population. Simulations of genetic drift in reintroduced herds indicated that allele frequencies were within expectations of model predictions, but that heterozygosity sometimes varied from predictions. Our results indicate the operation of founder effect and subsequent genetic drift within the small reintroduced herds, but also may be influenced hy small sample sizes from herds that were difficult to sample. We suggest management practices that should minimize the loss of genetic variation from reintroduced populations of bighorn sheep.
TL;DR: Using newly developed theory and results from computer simulations, the genetic aspects of age-structured populations with overlapping generations are discussed, especially the detection of contamination and the genetic dynamics following hybridization.
Abstract: Although an increasing number of natural fish populations are being contaminated by exogenous immigrants, knowledge is poor regarding the genetic changes expected to occur in a wild stock once an introgression has taken place. One reason for this lack of knowledge appears to be that the theory for the genetic dynamics is poorly developed and complicated for age-structured populations with overlapping generations. Using newly developed theory and results from computer simulations, the genetic aspects of age-structured populations with overlapping generations are discussed, especially the detection of contamination and the genetic dynamics following hybridization. When generations overlap, the amount of temporal allele frequency shift is generally larger than for a population of equal genetically effective size with discrete generations. This is even more pronounced for the separate cohorts than for the population as a whole. Therefore, when testing for temporal genetic heterogeneity, a higher frequency of statistically significant results may be expected than can be explained by genetic drift caused by a restricted effective population size. During introgression, a sudden infusion of new genes initiates marked allele frequency fluctuations, and in salmonids this “genetic instability” may persist for several decades. In spite of these potentially dramatic fluctuations, even a massive influx of exogenous genes may be very difficult to detect, particularly in the absence of genetic and demographic monitoring data from the natural population prior to immigration. A conservative attitude is recommended when interpreting allele frequency differences within populations where the history and the demographic characteristics are poorly known. The risk of incorrect interpretations is particularly apparent for many salmonid species where only a subset of the existing age classes may be available for sampling.
TL;DR: This review discusses the results of laboratory studies examining the possibility of an advantage of sex through Muller's ratchet in RNA viruses and a model is presented for how an observed pattern of molecular evolution in wild populations may be explained by Muller's Ratchet and the addition of compensatory mutations to Ohta's model of evolution by slightly deleterious mutations.
TL;DR: Divergence among Italian populations was found to be about as large as that between Italian and Greek populations, and large randomly generated disequilibrium between alleles at neutral, unlinked loci was expected.
Abstract: In this study, 23 previously identified Mendelian RAPD markers and 16 polymorphic allozymic markers were used to assess divergence among two Greek populations and five Italian populations of Pinus leucodermis. Confidence intervals of observed genetic divergence were obtained using bootstrap analysis. Divergence among Italian populations was found to be about as large as that between Italian and Greek populations. Since it is likely that the split of two nuclei took place more than 10,000 years ago, a larger differentiation between, rather than within, the above nuclei was expected. If genetic drift was responsible for the larger divergence of Italian populations, large randomly generated disequilibrium between alleles at neutral, unlinked loci was expected. Indeed, the proportion of pairs of loci showing a non-random association of alleles within each of the Italian populations was larger than what was expected by pure chance (7.95–10.88%). Effective population size based on randomly generated disequilibrium was quite small for three out of the five populations considered (N
e
=17.31±1.88, 16.57± 1.73, and 31.41±7.26, respectively). The implications of the results with respect to the conservation of endangered species of trees are discussed.
TL;DR: Recent studies of the genetic factors that may modify susceptibility to infection in humans are focused on and some recent developments in microbial genetics that may play an important role in this interaction are touched on.
Abstract: The notion that selection during epidemics or longer periods of exposure to infectious diseases may have had a major effect in modifying the constitution of the human genome is not new. It was proposed, at least in outline, by A.E. Garrod in 1931. In his remarkable book, The Inborn Factors in Disease, he suggested that infectious diseases may have been a major selective force in human evolution and in shaping our biochemical individuality. In 1948, J.B.S. Haldane, unimpressed with the idea that the extremely high frequency of thalassemia in certain racial groups from the Mediterranean region might reflect an unusually high mutation rate in these populations, proposed that these diseases might have come under intense selection because of heterozygote advantage against malaria. It was, in effect, Haldane’s remarkable insight that opened up the field of the investigation of genetic susceptibility to infection. Although considerable progress was made in relating the frequency and distribution of different protein polymorphisms to past or present infection, until recently the field was bedeviled by difficulties relating to population homogeneity, founder effects, and gene drift. However, with an increasing ability to analyze human variability at the DNA level, progress has been much more rapid. Comparing the sequences of genes common to rodents and humans, Murphy (1993), found that host defense genes are much more diverse than those for other families of proteins, an observation suggesting that selection in many species has resulted from exposure to different infectious agents. The way in which DNA analysis is transforming our understanding of the reasons for the distribution and high frequency of the human hemoglobin variants, the problem first posed by Haldane, is reviewed by Flint et al. (1993). Clearly, an analysis of the human genome with respect to variable susceptibility to infection is already beginning to provide important new insights into the mechanisms of human diversity. In considering genetic susceptibility to infectious disease it is important not only to consider the genetic makeup of the host but also that of the infectious agent. Here, we focus mainly on recent studies of the genetic factors that may modify susceptibility to infection in humans and touch only briefly on some recent developments in microbial genetics that may play an important role in this interaction. Mouse models for studying genetic susceptibility to infection have been reviewed recently elsewhere (McLeod et al. 1995).