Showing papers on "Genetic drift published in 1990"

Mutational Order: A Major Stochastic Process in Evolution

Abstract: Computer simulations in which selection acts on a quantitative character show that the randomness of mutations can contribute significantly to evolutionary divergence between populations. In different populations, different advantageous mutations occur, and are selected to fixation, so that the populations diverge even when they are initially identical, and are subject to identical selection. This stochastic process is distinct from random genetic drift. In some circumstances (large populations or strong selection, or both) mutational order can be greatly more important than random drift in bringing about divergence. It can generate a 'disconnection' between evolution at the phenotypic and genotypic levels, and can give rise to a rough 'molecular clock', albeit episodic, that is driven by selection. In the absence of selection, mutational order has little or no effect.

The Seed Bank as a Source of Genetic Novelty in Plants

Abstract: Populations of many annual species have seed banks that reduce the between-year variation in population size, thereby reducing the risk of population extinction. Seed banks may buffer populations against change in genetic composition resulting from genetic drift or selection. In this paper, I propose that seed banks may be an important source of genetic variation, if mutation rates in seeds aged in the field are as high as those of seeds aged in the laboratory. Laboratory-aged seeds often have genic and chromosomal mutation rates two to three orders of magnitude greater than those of one-year-old seeds. Unusually high mutation rates in the seed bank would increase the evolutionary flexibility of populations and increase the likelihood of fixing a novel chromosomal arrangement.

Theoretical study of near neutrality. I. Heterozygosity and rate of mutant substitution.

Abstract: In order to clarify the nature of "near neutrality" in molecular evolution and polymorphism, extensive simulation studies were performed. Selection coefficients of new mutations are assumed to be small so that both random genetic drift and selection contribute to determining the behavior of mutants. The model also incorporates normally distributed spatial fluctuation of selection coefficients. If the system starts from "average neutrality," it will move to a better adapted state, and most new mutations will become "slightly deleterious." Monte Carlo simulations have indicated that such adaptation is attained, but that the rate of such "progress" is very low for weak selection. In general, the larger the population size, the more effective the selection becomes. Also, as selection becomes weaker, the behavior of the mutants approaches that of completely neutral genes. Thus, the weaker the selection, the smaller is the effect of population size on mutant dynamics. Increase of heterozygosity with population size is very pronounced for subdivided populations. The significance of these results is discussed in relation to various observed facts on molecular evolution and polymorphism, such as generation-time dependency and overdispersion of the molecular clock, or contrasting patterns of DNA and protein polymorphism among some closely related species.

Mitochondrial DNA evolution in experimental populations of Drosophila subobscura.

Abstract: When two mitochondrial DNA (mtDNA) haplotypes of Drosophila subobscura compete in experimental populations with discrete generations, one or the other approaches fixation, depending on the nuclear background with which they are associated. The approach to fixation, however, is strongly dependent on the effective number of females in the population, Nf. Whether or not the ultimate fate of a given mtDNA haplotype is determined by random genetic drift depends on Nf as well as on the relative fitnesses. Our experimental results show that the mtDNA polymorphisms observed in natural populations are affected by interactions among nuclear polymorphisms, random genetic drift, and direct selection on the mtDNA haplotypes.

Dispersal, gene flow, and allelic diversity between local populations of thomomys bottae pocket gophers in the coastal ranges of california.

Abstract: We studied Thomomys bottae pocket gophers from 1979 to 1982 to determine if the amount of gene flow between local populations was sufficient to reduce allele frequency differences between them. Dispersal was quantified using three different trap techniques, and genetic changes in the population were monitored using protein variants. Additional allele frequency data were available for 1976 and 1977. We observed dispersal to be common in pre-reproductive juvenile females throughout the breeding season of their birth. Males on the other hand tended to disperse only from the end of the breeding season. Although dispersal was common, 63% of adults appeared to be recruited within 40 m of where they were born. Gene flow occurred into both established populations and into vacant habitat, but it was too low to reduce the differences in gene frequencies between the fields over seven years. We conclude that allelic diversity in T. bottae populations is a balance between random drift due to small effective population size and gene flow.

Genotype-environment interaction for climate and competition in a natural population of flour beetles, tribolium castaneum

Abstract: The norm of reaction, the set of average phenotypes produced by a genotype in different environments, can be affected by spatial variation in natural selection especially when there exists genotype-environment interaction. In subdivided populations, the greater the genotype-environment interaction variance and the lower the migration rate, the more independent are the possible evolutionary trajectories for local adaptation. I examined genotype-environment interaction in the rate of population increase for lineages randomly derived from a wild population of Tribolium castaneum across a series of ecologically important environments. The lineages were derived from an outbred, wild-caught population by 14 generations of random genetic drift, during which the effective size of each lineage was approximately 22 breeding adults. The environments studied were the classic temperate-wet and cold-dry climates of Park (1954) in factorial combination with two genetic strains of a congeneric competitor, T. confusum. Much among-lineage genetic variation for rate of population increase was found for each of these ecologically important environments of climate and competition. Genotype-environment interaction accounted for 40.5% of the total among-lineage variance in rate of population increase signifying that the performance of a lineage in one environment is not necessarily a good predictor of its performance in another. Changing the genetic identity of the competitor changed the rate of increase of some lineages as much or more than changing the climatic conditions of temperature and humidity. This is the first empirical study to characterize the genotype-environment interaction variance associated with genetic variation in a competing congeneric species. This competitor-specific genetic variation in competitive ability may play an important role in coevolution in subdivided populations.

The Breeding System, Genetic Diversity and Pollen Sterility in Eucalyptus pulverulenta, a Rare Species with Small Disjunct Populations

Abstract: There are eight known populations of E. pulverulenta, which has a disjunct distribution in south-eastern Australia. Levels of genetic variation were studied in four populations of about 5000 plants in all. Of a total of 16 allozyme loci examined eight were polymorphic, but the level of polymorphism was generally low. Within populations there was a mean 1.42 alleles per locus. Both the total species and mean population genetic diversities were low for a tree species (0.10 and 0.07 respectively), while the between-population genetic diversity was high at 30%. These data are consistent with the effects expected of genetic drift. Population structure may thus have been critical in determining levels of genetic diversity throughout the species' range. Analysis of half-sib arrays in three populations showed that the mean outcrossing rate (t) was 70%, comparable to values from other Eucalyptus species. The mean level of inbreeding (Wright's F) was 0.19, although both F and t varied considerably between populations. Significant levels of pollen sterility have been reported in this species, and data presented here show that this trait extends through much of the species' range. Overall, these data suggest that neither its disjunct population structure nor its tendency to male sterility caused the species' detectable level of outcrossing to differ markedly from levels reported in other eucalypt species. However, it remains possible that pollen sterility may have had some effect in at least one population. Strategies for conservation of this rare species are also considered.

Population genetics and systematics of the Morchella esculenta complex.

Abstract: Allelic frequencies for strains of an early occurring gray form of Morchella and the tan M. esculenta were determined for collections from west central Illinois and southwestern Wisconsin. Horizontal starch gel electrophoresis was used to determine electromorph (allele) frequencies from fourteen enzyme systems encoded by twenty presumptive structural loci. A total of 122 monoascosporous isolates from 72 ascocarps were studied. Electrophoretic polymorphisms were found in strains of both forms demonstrating that they exist as Mendelian populations. In general, the gray form and M. esculenta had a high genetic similarity when they occurred at the same locality, indicating that they were likely derived from a common ancestral population. The Plymouth strains were genetically more similar to the Eagle Township strains than the strains from the Richland Center area, which had a larger separation than the other collections. Geographic distance between Illinois and Wisconsin strains resulted in substantial genetic differentiation attributable to genetic drift (Fst = 0.165). Strains ofthe two different forms, however, do not cluster separately in a phenetic analysis indicating that the two phenotypes are not different taxa.

Genetic-Variation Among Insular Populations of the Sleepy Lizard, Trachydosaurus-Rugosus Gray (Squamata, Scincidae)

Abstract: Seven island populations of the sleepy lizard, Trachydosaurus rugosus, in South Australia were studied to establish the genetic effects of isolation. These effects were assessed by comparing genetic characteristics (using allozyme electrophoresis) of the island populations with those of three adjacent mainland populations. Heterozygosity levels did not vary significantly among the populations although the island populations exhibited reduced allelic diversity. Alleles that were rare on the mainland were not present in the island populations. Genetic divergence among the island populations was much greater than among populations on the mainland, reinforcing the notion that evolutionary forces, probably genetic drift, were greatest among the insular populations. This study demonstrates that the intra-specific component of variation can be significant, and that the importance of this component will increase with the fragmentation and isolation of populations. This finding serves to emphasise the importance of considering the population as the unit of conservation.

Genetic variation and population structure of the Florida Wood Stork

Abstract: The Wood Stork (Mycteria americana) is a colonial wading bird of the tropical and lower subtropical zones of the Americas. In Florida, Wood Storks are nonvisual, tactile feeders and require high densities of small aquatic organisms in shallow water for foraging (Kahl 1964). These conditions occur when prey are concentrated as water levels drop during the annual dry season (Kahl 1964, Kushlan et al. 1975). Colony formation in Wood Storks is correlated with the onset of these hydrologic conditions, and successful nesting requires the continuation of these conditions for foraging (Kahl 1964, Kushlan et al. 1975). Yearly fluctuations and long-term alterations of the hydrologic regime result in abandonment of some colony sites. Nesting records indicate that although storks exhibit colony-site tenacity, most-if not allcolonies are transitory in nature (Ogden and Patty 1981, Kushlan and Frohring 1986). Censuses also have documented gradual site shifts among regions (Kushlan and Frohring 1986, Ogden et al. 1987), which implies that following colony abandonment, adult storks disperse widely to renest in more favorable areas. Although large-scale shifts of storks among areas have been documented (Kushlan and Frohring 1986, Ogden et al. 1987), relatively little is known about the dispersal and fate of adult storks following colony abandonment. These individuals move to areas where favorable foraging conditions exist, but it is unclear where or how far they move, whether or not they reattempt nesting, and if pair bonds remain intact. Without this information, it is impossible to assess the effects of colony-site shifts on stork population structure. Shifts of large numbers of individuals from one site to another could have a potentially dramatic effect on population structure. If breeding individuals are interchanged among colonies, then population shifts would represent gene flow on a massive scale. Large-scale movement of individuals would likely swamp evolutionary processes creating population structure (e.g. selection, random genetic drift) and reduce differentiation among colonies. We collected allele frequency data from Florida colonies to estimate levels of heterozygosity, to examine patterns of population structure, and to estimate levels of &ene flow. We sampled from 15 colonies in 1985 and 1986 (Table 1, Fig. 1). Two growing, centrally located primary feathers (one from each wing) were plucked, placed in plastic vials, and frozen in liquid nitrogen within one hour of collection. Samples were stored in an ultra-cold freezer (-76?C). One individual from each nest was sampled, and sample sizes reflect the number of nests sampled within colonies. Colony sizes at the time of sampling ranged from 76-592 nests. Our samples represented 5-24% of all nests present (x= 12%). Pulp was squeezed from the feather shaft and homogenized with 5-6 ml of 0.01 M Tris-0.001 M EDTA pH 7.0 buffer solution. Gels of 11-12% starch were run overnight. General staining followed Selander et al. (1971) and Harris and Hopkinson (1976). Buffer conditions for 20 presumptive gene loci are provided in Table 2. Presumptive loci were numbered from anode to cathode. Allozymes were designated alphabetically in order of relative mobility from anode to cathode, with the letter "C" chosen to represent the most common allele. We used the statistical package BIOSYS-1 (Swofford and Selander 1981) to analyze allele frequencies, genetic variability measures (heterozygosity, mean number of alleles per locus, percent polymorphic loci), Chi-square deviation from Hardy-Weinberg proportions, Nei's (1978) and Rogers' (1972) genetic distance, and F-statistics (Nei 1977, Wright 1978). Heterozygosity data were arcsine square-root transformed before analysis of variance (ANOVA) to test for differences among populations. We used a Chi-square test with Levene's (1949) correction for small sample size to detect departure from Hardy-Weinberg proportions. For comparative purposes we present two measures of genetic distance (Rogers 1972, Nei 1978) that differ in their assumptions and methodologies. A dendrogram of the genetic relationships among colonies was generated from the matrix of Rogers' distance values. A permutational Mantel test (1967, Sokal 1979) was used to compare matrices of straightline geographic and genetic distance for congruence of pattern. To examine population structure, we used Wright's F-statistics (Wright 1978). The among-population component of genetic variance, FST, measures the extent to which species are organized into subpopulations or demes. FsT-values ranged from 0 (indicating lack of differentiation) to 1 (suggesting fixation of alternate alleles and complete differentiation). FlS and FIT measure heterozygote deficiency or excess within subpopulations and the total population, respectively, and are commonly used as inbreeding indices. Both values range from -1 to + 1, with positive values indicating heterozygote deficiency (e.g. as may occur from inbreeding). Precise interpretation of F-statistics required detailed knowledge about the breeding structure of the species examined. Because this information is usually lacking, inferences about pop-

Genetic aspects of quality control in tsetse colonies

Abstract: Tsetse colonized either for laboratory studies or for release in S. I. T. programmes are assumed to be healthy and genetically similar to flies in natural populations. However, insect colonies are subjected to many of the same evolutionary forces that influence genetic changes in natural populations, i.e. drift, selection, hitch-hiking, mutations, assortative mating and immigration. The influence of these on genetic structure of tsetse fly colonies is outlined, and examples are presented from several species. There is little or no evidence for adaptation during the early phases of laboratory colonization of five species of tsetse. A model is presented indicating that with as little as a 5 % fitness difference between males, some colonies have existed long enough to have undergone significant changes in the relative numbers of males having “standard” and “enhanced” fitness. Slight changes in heterozygosity of colonized flies is documented by comparing colonies and field-collected flies and by comparisons within colonies over periods of several generations or years. An example of hitch-hiking is illustrated with the closely linked genes Sr (sex ratio) and Est-X in Glossina morsitans submorsitans. A possible interaction between alleles at these loci is discussed. A summary is presented of Polyacrylamide gel electrophoretic methods for monitoring 16 polymorphic loci distributed among the X chromosome and autosomes of tsetse.

Genetic monitoring of Pacific salmon hatcheries.

Abstract: In the last few decades, and in response to substantial reductions in the abundance of wild ~o~ulations of Pacific salmon, an enormous amount of resources in both Asia and North America has been devoted to artificial propagation programs. Several factors increase the possibility of rapid (often detrimental) genetic change in cultured populations, but genetic considerations are often overlooked in the effort to increase short-term productivity. Here, we discuss recent studies using electrophoretic data for chinook salmon, Oncorhynchus tshawytscha, that address three important concerns for hatchery populations: levels of genetic variability, stability of allele frequencies, and genetic interactions (due to straying or overplanfing) between hatchery and wild populations. Results indicate that although there is no evidence for a general reduction in levels of genetic variability in hatchery stocks relative to wild populations from the same geographic area, allele frequencies over a period of one generation changed much more in samples from hatchery populations in Oregon than in nearby wild populations. The genetic changes in the hatchery stocks appear to be due fo a combination of two factors: genetic drift due to reduced effective population size, and (in some cases) the infusion of genes from other populations through straying or transfer of broodstock between hatcheries.

Genetic variability in domesticated and wild sheep based on blood protein characters

Abstract: 1. 1. The study examined genetic variation, as indicated by eight blood protein of 539 animals from 11 genotypes (14 flocks) of domesticated sheep and three genotypes of wild sheep. 2. 2. The polymorphic distribution among the animal populations presented a random pattern with wide variation. 3. 3. Genetic variability both between and within population was larger in domesticated sheep than in their wild ralatives. 4. 4. This increased genetic variability observed in domesticated sheep may have resulted from the process of domestication through accelerated genetic drift and promoted gene flow.

Maintaining genetic variation in a one-way, two island model

Abstract: I used a stochastic simulation model to examine the loss of heterozygosity and allelic diversity in a bottlenecked population and a sink population subject to various migration rates from source populations of different sizes. In the bottlenecked population, the initial founder size influenced the level of allelic diversity more than heterozygosity, although the subsequent patterns of loss of the 2 measures were similar. The "rescue effect," whereby migration from a source population offsets genetic drift in a sink population, was shown to have a detrimental counterpart termed the "imperil effect," which may lead to the erosion of genetic variation. This effect is manifested when the source population has less genetic variation than the sink population. When genetic data about a source population are lacking or scant, to avoid the "imperil effect," migration rates should be <1 migrant per generation. J. WILDL. MANAGE. 54(4):676-682 A suggested goal of captive breeding and genetic management is to maintain 90% of the heterozygosity of the source (wild) population over a period of 200 years (Soule et al. 1986). This goal becomes complicated when economic and spatial constraints dictate that population sizes of managed species remain small, making the populations susceptible to genetic drift. In addition to heterozygosity, which has been correlated with individual fitness (Beardmore 1983, Allendorf and Leary 1986), another populationlevel measure of genetic variance, allelic diversity, is crucial to the long-term adaptability of a population (Frankel and Soule 1981) and should also be considered for genetic management. When a large population is reduced in size to N individuals, the average heterozygosity per locus is expected to decrease by 1/(2N) (Nei et al. 1975), and the number of polymorphic alleles at a particular locus is expected to decrease by 2 (1 pj)2N (Denniston 1978), where p, is the -1ee frequency, and k is the number of alleles allele frequency, and k is the number of alleles at a locus. Both measures will continue to decline from generation to generation until a drift-mutation equilibrium is attained (Lacy 1987). If the population rebounds from the bottleneck, heterozygosity may show little or no reduction, but rare alleles have a high probability of being lost (Allendorf 1986, Fuerst and Maruyama 1986). Management techniques designed to maintain allelic diversity and heterozygosity are conflicting. Allelic diversity is promoted through the subdivision of populations (Chesser 1983), whereas heterozygosity is maintained through a large effective population size (N,) (Simberloff 1988). Limited migration among subpopulations or 1-way migration from an infinitely large population decreases loss of heterozygosity caused by genetic drift (Wright 1931, Lacy 1987). This "rescue effect" (Brown and KodricBrown 1977) provided by an impulse of new genetic material could reduce the extinction probability of an insular population by preventing the loss of genetic variation. Migration This content downloaded from 157.55.39.243 on Wed, 05 Oct 2016 04:47:51 UTC All use subject to http://about.jstor.org/terms J. Wildl. Manage. 54(4):1990 RESCUE EFFECT * Weishampel 677 rates of <1 (Foose et al. 1986, Off. Technol. Assessment 1987), 1 (Avery 1978, Allendorf 1983), 1-2 (Franklin 1980), and 1-5 migrants per generation (Frankel and Soul6 1981) have been proposed. In my study, I used a stochastic simulation model to examine how bottleneck severity, immigration rate, and source and sink population sizes affect heterozygosity and allelic diversity. This work was inspired by E. F. Connor, J. J. Murray, and H. H. Shugart. I wish to thank them, J. A. Yeakley, and 3 anonymous reviewers for their input. This research was supported in part by an NSF grant presented under Interagency Agreement No. BSR-8718168 with the Department of Energy and by the National Aeronautics and Space Administration Earth Sciences Division (UPN 677-80-06-05) to W. E.

Mitochondrial DNA variation in British house mice (Mus domesticus, Rutty).

Abstract: Morphometric, karyological and historical evidence indicates that Caithness and Orkney House mice (Mus domesticus Rutty) are genetically distinct from other British mice, suggesting they are descended from introductions. Mitochondrial DNA is a small, rapidly evolving, maternally inherited molecule; hence each mtDNA molecule carries in its sequence the history of its lineage uncomplicated by recombination. Thus, mtDNA RFLPs can be used for analysing possible patterns of colonisation and gene flow in these populations. Highly purified mtDNA was isolated from each mouse and mapped, using the high resolution restriction method, with respect to the published sequence of mouse mtDNA. This allowed the types and incidence of mutational change by which mtDNA evolves in the House mouse to be evaluated. A total of 23 mtDNA composite genotypes, assayed using 14 restriction enzymes, were recognised among the British mice examined and a genetic "break" observed between individuals from the north of Britain (Orkney, Ireland and N.E. Scotland; N.W lineage) and those from the south (British mainland, south of Caithness and Sutherland; S.E lineage). The approximate location of this "break" corresponds with the Great Glen fault, which marks a boundary between inhospitable moorland, occupied by Apodemus. Geographic orientation of mtDNA variability is concordant with data from other sources, including the paternal Y-chromosome DNA. The House mouse is unlikely to have survived the last glaciation, dating tlie earliest possible British colonisation to about 10,000 B.P. An integrated approach, using evidence from anthropological, palaeontological, genetical and historical sources, permits the progression of the house mouse to be followed through Europe. These data indicate that Mas domesticus probably reached North-West Europe and Britain in the Iron Age. Hence, divergences of such magnitude between the N.W and S.E lineages are inconsistent with substitutions accumlating in situ since their arrival; consistent with the N.W and S.E forms originating from separate introduction events from different ancestral sources. Such a distinct "break" could have been maintained by a number of either extrinsic (geographical barriers) and/or intrinsic factors including, maintenance of territories, and specific mate preferences. A popular view is that house mice live in behaviourally isolated tribes or demes of between 4-6 individuals, with very little gene flow between them. It is believed that as a consequence of this rigid structure, immigrants into an establised population are unlikely to be reproductively successful, and genetic drift will become important in shaping their population structure. This concept may be too inflexible, as virtually every longitudinal study of feral mice has shown some population mixing. The Isle of May introduction experiment investigated the relative importance of these intrinsic factors. House mice from Eday (Orkney) released into an established population on the Isle of May (Firth of Forth) in April 1982, subsequently bred with the endemic mice. The relative maternal and paternal contributions to the success of this introduction were studied using mtDNA and Y-chromsome markers. Differential introgression was observed: Eday Y-chromosome apparently spread at a similar rate to the autosomal genes, while Eday mtDNA increased in incidence and distribution at only one-third the rate. The temporal and spatial distribution of Eday derived DNA showed that males disperse and introgress more rapidly than Eday females, and form a significantly higher proportion of the mating population than May males. Clearly, there seems to be no social barriers to gene flow in this feral population. The Isle of May introduction has allowed evaluation of mtDNA as a genetic marker, describing population structure and matrilineal kinship on a microgeographical scale.

Dispersal, effective population size, and the genetic structure of the contemporary United States.

Abstract: I estimate effective population size (Ne ) and the inbreeding coefficient (FST ) for contemporary United States using Wright's isolation by distance model (Wright: Genetics 28:114-138, 1943) and parent-offspring dispersal distances obtained from individuals surveyed as part of a study of modern dispersal patterns. Ne is estimated to be minimally 3.61 × 107 and more likely closer to 8.05 × 107 ; based on these values, FST is between 1.59 × 10-7 and 9.28 × 10-9 , depending on whether it is measured relative to the United States population or the world at large. Not all the assumptions of the isolation by distance model are met by modern populations, and thus the results must be interpreted with caution. They suggest, however, that both mobility within and immigration into contemporary United States are great enough to make the probability of inbreeding and random genetic drift negligible factors in producing future evolutionary change. In contrast, gene flow, acting as both a constraint against geographic differentiation within the United States and by introducing new genes via international immigration, is likely to be a dominant evolutionary force in this population.

Properties of Single-Locus Models under Several Microevolutionary Pressures

Abstract: The narrow-sense genetic drift model considered above is a kind of starting or reference point in a study of the effect a microevolutionary pressure has on the fate of a population. This role is similar to that of the random mating (panmixia) model among deterministic models of population genetics.

Random Genetic Drift in the Narrow Sense

Abstract: By the genetic drift, or more strictly, by the random genetic drift in the narrow sense one usually means a random process of variation in gamete concentrations due to the effects of sampling alone at the change of the generations. Initially, the biologist who had paid attention to this phenomenon stressed the important role the drift played in reaching the genetic homogeneity under considerable variations in the population size (causing cancellation of some genetic variants when the size decreases), or under the presence of rare genes (which may happen to be absent in the next generation by pure chance). The situation is however the same when the population size is constant and the gene concentrations are arbitrary in the population.