Model Of Gene Flow Research Articles

Genetic Models of Adaptation and Gene Flow in Peripheral Populations

Genetic Models of Adaptation and Gene Flow in Peripheral Populations

Biochemical Genetic Variation Among Populations of the Greenshell Mussel, Perna canaliculus, from New Zealand: Preliminary Findings

Seven polymorphic and four monomorphic allozyme loci were assayed from nine wild populations and one cultured population of the endemic New Zealand greenshell mussel, Perna canaliculus. Genetic variation was examined to determine the extent of intra-population subdivision and inter-population variability. Five of seven polymorphic loci exhibited significant heterozygote deficiencies compared to Hardy-Weinberg equilibrium. Population F-statistics indicated that significant genetic heterogeneity exists among populations, indicating that there is insufficient gene flow between geographically isolated populations to create panmixia. The observed genetic heterogeneity among populations is best explained by an isolation-by-distance model of gene flow, which is modified by localized hydrographic conditions. These preliminary findings are discussed in the context of the one previous survey of population genetic variation in P. canaliculus and how this information relates to the gene flow of the greenshell mussel in New Zealand, which is often mediated by human transport from the main collection site of Kaitaia to aquaculture sites throughout the country.

Biochemical genetic variation among populations of the greenshell mussel, Perna canaliculus, from New Zealand: preliminary findings

Abstract Seven polymorphic and four monomorphic allozyme loci were assayed from nine wild populations and one cultured population of the endemic New Zealand greenshell mussel, Perna canaliculus . Genetic variation was examined to determine the extent of intra-population subdivision and inter-population variability. Five of seven polymorphic loci exhibited significant heterozygote deficiencies compared to Hardy-Weinberg equilibrium. Population F -statistics indicated that significant genetic heterogeneity exists among populations, indicating that there is insufficient gene flow between geographically isolated populations to create panmixia. The observed genetic heterogeneity among populations is best explained by an isolation-by-distance model of gene flow, which is modified by localized hydrographic conditions. These preliminary findings are discussed in the context of the one previous survey of population genetic variation in P. canaliculus and how this information relates to the gene flow of the greenshell mussel in New Zealand, which is often mediated by human transport from the main collection site of Kaitaia to aquaculture sites throughout the country.

Distinguishing between primary and secondary intergradation among morphologically differentiated populations of Anolis marmoratus

Distinguishing between primary and secondary intergradation among differentiated populations, and the relative importance of drift and selection, are persistent problems in evolutionary biology. An historical perspective on population interactions can provide insight into the nature of contacts, and thus help resolve these questions. Continuously distributed populations of Anolis marmoratus from the island of Basse Terre in the Guadeloupean archipelago of the Lesser Antilles show a striking degree of geographic variation in morphology. Initial surveys of mtDNA variation from throughout the Guadeloupean Archipelago revealed one case where levels of sequence difference and phylogenetic relationships of alleles from morphologically differentiated populations from the east coast of Basse Terre were consistent with primary intergradation. In this paper, I examine the genetic population structure of a series of populations spanning this north-south cline in morphological variation to test the hypothesis of primary intergradation. Sequences of the mitochondrial cytochrome-b gene from 50 individuals representing five populations spanning the cline were obtained and fourteen unique haplotypes (differing by 2% or less) were detected. Patterns of nucleotide substitution among haplotypes do not deviate from neutral expectation indicating no effect of selection at the level of mtDNA sequences. Estimates of population structure and gene flow were made using both summary statistics for nucleotide diversity (Nst) and cladistic methods. The results are sensitive to the choice of gene flow model, and this is discussed in detail. Mitochondrial variation in the northern populations may not be at equilibrium, and the phylogeny of alleles is consistent with a recent increase in effective population size. Estimates of nucleotide diversity, gene flow, and the phylogenetic relationships of haplotypes indicate that the southern-most population (representing the extreme of morphological variation along this cline) has been relatively isolated from populations to the north and has experienced a reduced effective population size. The apparent clinal variation between the southern population and the others may therefore reflect secondary contact and introgression rather than primary intergradation.

Population structure and gene flow in Stomion: a species swarm of flightless beetles of the Galápagos Islands

Stomion is a swarm of 13 flightless tenebrionid beetle species endemic to the Galapagos Islands. Their distribution is patchy and largely restricted to the littoral and arid zones of the archipelago. Each taxon is found on one or a few geographically close islands. Thirty-five populations representing nine of the taxa were examined at eight polymorphic enzyme loci using cellulose acetate electrophoresis to measure patterns of gene flow and investigate models of dispersal in a relatively young species group exhibiting spatially isolated populations. Genetic subdivision is high, particularly among populations of taxa which inhabit more than one island, revealing restricted gene flow and confirming the high potential for reproductive isolation among subpopulations. The mean FST across taxa was 0.30. The genetic differentiation occurring between spatially isolated populations may explain the exuberant speciation of the genus in the Galapagos. Tests of gene flow models give support to the stepping-stone model of dispersal. Because of their lack of flight wings, interisland dispersal of Stomion probably occurred by oceanic drift as pleuston or on floating debris, with individuals colonizing nearby islands more frequently than ones at a greater distance.

GENETIC CONSEQUENCES OF SEED DISPERSAL IN THREE SYMPATRIC FOREST HERBS. I. HIERARCHICAL POPULATION-GENETIC STRUCTURE.

To examine the effects of seed dispersal on spatial genetic structure, we compare three sympatric species of forest herbs in the family Apiaceae whose fruits differ widely in morphological adaptations for animal-attached dispersal. Cryptotaenia canadensis has smooth fruits that are gravity dispersed, whereas Osmorhiza claytonii and Sanicula odorata fruits have appendages that facilitate their attachment to animals. The relative seed-dispersal ability among species, measured as their ability to remain attached to mammal fur, is ranked Sanicula > Osmorhiza > Cryptotaenia. We use a nested hierarchical sampling design to analyze genetic structure at spatial scales ranging from a few meters to hundreds of kilometers. Genetic differentiation among population subdivisions, estimated by average genetic distance and hierarchical F-statistics, has an inverse relationship with dispersal ability such that Cryptotaenia > Osmorhiza > Sanicula. In each species, genetic differentiation increases with distance among population subdivisions. Stochastic variation in gene flow, arising from seed dispersal by attachment to animals, may partly explain the weak relationship between pairwise spatial and genetic distance among populations and heterogeneity in estimates of single locus F-statistics. A hierarchical island model of gene flow is invoked to describe the effects of seed dispersal on population genetic structure. Seed dispersal is the predominant factor affecting variation in gene flow among these ecologically similar, taxonomically related species.

Economic value of response from sire selection in dairy cattle—A linear programming model

Abstract A linear programming model that accounts for the economic consequences of response to selection in milk production, type, somatic cell score, days open (a reproductive trait), and liveweight to the producer enterprise over a given planning horizon is described. A procedure is given in detail for defining upper and lower bound constraints on variables that are correlated in the linear programming model. This procedure enabled the linear programming model to construct a feasible hyperspace that reflects the relationship between the variables in terms of the trend in the relationship and the variation around this trend. The linear programming model was used to compute optimal rates of response to selection for all traits in the model for four planning horizons. The optimal response to selection per year for the production traits was closest to the maximums achievable from a gene-flow model. Of all the nonproduction traits, days open had the greatest proportion of its maximum achievable from a gene-flow model. The linear programming model was used to compute relative economic weights (REVs) for the nine traits for each planning horizon. The REVs for milk, fat, and protein production were considerably larger than the REVs for the nonproduction traits for all planning horizons. Somatic cell score had the largest REVs of the nonproduction traits in all planning horizons.

Morphological variation in populations of Disa uniflora (Diseae: Orchidaceae) in the southwestern Cape, South Africa

Measurements of sixteen floral and vegetative morphological features of individuals from seven groups of populations of Disa uniflora Berg. (Orchidoideae: Orchidaceae) in the mountains of the southwestern Cape Province of South Africa were used to analyze patterns of intraspecific variation. Each population group had between one and three populations, and each population had between 2 and 30 (mean, 21) individuals. The data were analyzed using multivariate statistical clustering methods. The covariance biplot analysis indicated the patterns of variation, while the phenetic methods were used to test correlations between the patterns of variation and geographical isolation and altitudinal variation. The results are interpreted in the light of competing hypotheses on ecological versus gene-flow models driving interpopulational divergence. Variation was found to be largely between mountain blocks, whereas both geographical distance and altitudinal differences were found to be significantly correlated with the variation patterns. Conservation implications for D. uniflora as well as other Cape species are discussed. Key words: geographic variation analysis, Orchidaceae, intrapopulation variation.

Gene flow, spatial patterns and seed-collection zones

Abstract Models of gene flow, selection and demography were employed to determine the minimum limits and amplitudes of seed-collection and seed-transfer zones in forest trees, using loblolly pine ( Pinus taeda L.) and white spruce ( Picea glauca (Moench.) Voss.) as examples. Seed-collection zones were defined in terms of spatial patterns for polygenic (growth) characters. Fora given character, the boundaries of a seed-collection zone were found to be determined by interactions among gene flow, selection strength and genetic variation for that character at a specific age. For example, the width of a seed-collection zone for 5-year-old trees of loblolly pine was found to be larger if it was based on diameter rather than height growth. Conversely, the width of these zones differed between 5- and 15-year-old trees for either diameter or height growth. A similar trend was evident for white spruce. The average minimum width of seed-collection zones for loblolly pine and white spruce varied from 324 to 400 m. These results are consistent with many reports on gene flow, selection and differentiation in natural populations of forest trees. Seed-transfer zones should be based on the amount of genetic variation in the adaptive traits of each population representing a seed-collection zone. The adaptive amplitudes of populations from various seed-collection zones on a large geographical scale must be determined on the basis of the homeostatic properties specific to each population.

The genetic structure of a primate species: Rhesus macaques and other Cercopithecine monkeys

Among rhesus macaques (Macaca mulatta)and other cercopithecine monkeys, social groups occupying adjacent home ranges (i.e., members of the same local population) exchange individuals and genes and thus exhibit marked genetic similarities. To assess the degree to which this pattern extends beyond the local population, the genetic structure of M. mulattaand six other primate species was determined using Nei’s (1973) gene-diversity analysis. The genetic similarities seen among social groups in the Dunga Gali population of M. mulatta (Melnick et al.,1984a) can be seen over the entire species range. Comparison of these results with the structures of other similarly organized primate species indicates that (1) the average social group contains most of its local population’s genetic diversity, (2) the average local population contains the majority of the genetic diversity found in the region to which it belongs, and (3) the proportion of species gene diversity found in the average regional population varies substantially between species. Genetic homogeneity within local and regional populations is probably the product of gene flow. The application of a number of analytical models of selection and gene flow strongly suggests that gene flow, genetic drift, and zoogeography offer a more parsimonious and plausible explanation for interspecific variation in regional differentiation than does stabilizing selection.

Conservation Genetics in the Management of Desert Fishes

Abstract: The status and security of fishes in North American deserts have steadily declined in this century due to man's activities in this naturally fragile region. We address genetic aspects of the population structure of desert fishes as applicable to conservation and recovery programs by developing two zoogeographic models of isolation and gene flow. In the Death Valley model populations are isolated, with no chance of natural gene flow among them. Genetic diversity within populations tends to be low, but genetic divergence among populations within a species is high. In the Stream Hierarchy model, a complicated hierarchical genetic structure exists and is a function of geographic proximity and connectivity of habitats. Within‐habitat genetic diversity tends to be higher, and among‐habitat differentiation lower, than in the Death Valley model. These two systems must be recognized as distinct and managed differently. We also suggest three areas of experimentation needed to better understand and manage genetic stocks of desert fishes: relationships between heterozygosity and fitness, experimental mixing of similar stocks to examine effects of increased heterozygosity, and analysis of the relative roles of genetic adaptation and phenotypic plasticity in local differentiation.

Mitochondrial gene flow.

To account for the transmission of mitochondrial DNA between conspecific species Drosophila pseudoobscura and D. persimilis in sympatry reported by J.R. Powell [Powell, J.R. (1983) Proc. Natl. Acad. Sci. USA 80, 492-495], a simple model of gene flow and selection in infinite populations is analyzed. The model assumes two alleles at each of two loci, one of which is coded by an autosome and the other by mitochondrial DNA. Viability selection is presumed to be underdominant--i.e., heterozygous inferiority to the homozygotes--at an autosomal locus, and neutral or deleterious at a mitochondrial locus, with the combined action being multiplicative. Extremely strong selection against heterozygotes may prevent the transmission of mitochondrial DNA between two species, but otherwise the transmission can easily occur over species boundaries. The rate of approach to equilibrium is determined by the level of gene flow and is not affected much by selection against an autosomal locus. The divergence of the nuclear genomes of the two species is reexamined. Based on published data on enzyme loci, we conclude that there has been mitochondrial gene flow between these species for a long enough time that several nuclear loci examined could diverge because of accumulation of neutral mutations.

Geographic and Climatic Correlates of Morphological Variation in Eptesicus fuscus

Skull size in continental North American Eptesicus fuscus is inversely related to temperature as predicted by the classical form of Bergmann's rule, but it is positively related to moisture in contradiction to James' (1970) reformulation of the rule. Greater food availability in moister environments is suggested as the explanation of the latter relationship. Wing size is inversely related to temperature, but the relationship is not statistically significant. Wing size is significantly greater in moister environments, suggesting that evaporation from wings is an important selective factor. The geographic coordinates latitude, longitude, and elevation explain significant amounts of morphological variation, but differentiation among the insular subspecies of E. fuscus is not consistent with a gene flow model if a Central American ancestor is assumed. The relationship between body size and island area in the Antillean subspecies of E. fuscus supports Heaney's (1978) selective model of body size in insular mammals. Overall, the results suggest that both climate (natural selection) and geographic factors (barriers to gene flow) are important causes of differentiation in E. fuscus .

Dispersal Versus Gene Flow in Plants

Dispersal Versus Gene Flow in Plants

Population structure and anthropometric variation in rural western Ireland: migration and biological differentiation.

Models of population structure can be investigated using data on anthropometric variation among local populations. Anthropometric data collected by Dupertuis and Dawson during the 1930s were analysed from 347 males and 261 females in 12 towns in three counties of western Ireland. We hypothesized that recent migration would decrease the degree of among-group variation. To test this hypothesis, two additional samples were created by excluding known inter-county migrants from both male and female samples. Based on ethnographic data, a fifth sample was created using unmarried females only, in order to control partially for local migration upon marriage. Univariate and multivariate measures of relative differentiation were developed to compare different levels of migration and differences among the sexes. We found that the degree of among-group variation decreased as the amount of migration increased, in accordance with spatial models of population structure. Using non-parametric correlations of geographic and anthropometric distance, the observed patterns of differentiation were closely related to geography, suggesting a spatial model of gene flow to be appropriate in interpreting among-group variation. The female samples showed greater differentiation and higher correlations with geography than the males. It seems that this results from the sensitivity of males to developmental and local environmental influences, causing an increase in the relative amount of within-group variation.

THE RELATION OF GENETIC STRUCTURE TO ENVIRONMENTAL STRUCTURE: GAMMARUS MINUS IN A KARST AREA.

A species possesses an internal genetic structure that is compounded on the genetic architecture of constituent local populations. Each local population may be subtly unique in its coadapted gene ensemble, including the fine tuning of its genic regulatory system. More obviously and more approachably, populations may be differentiated in allele frequencies at single structural loci. This study describes the regional genetic patterns of allozyme loci in the amphipod crustacean Gammarus minus Say in a karst area of southeastern West Virginia. It is a part of an investigation of this species throughout the mid-Appalachian portion of its range, whose initial purpose is to map the genetic relationships among populations onto the highly structured environment of this region. Gammarus minus is highly differentiated in allele frequencies at several loci over short distances. There are at least three possible reasons for this fact. (1) As in other pericardians, females of G. minus brood their young in a ventral marsupium, and there is no specific migratory stage in the life history. (2) This species is principally found in freshwater lotic streams. Individual streams are essentially one-dimensional habitats from the viewpoint of gene flow, and drainage networks have a dimensionality between one and two (analogous to roads in Cavalli-Sforza and Bodmer, 1971). There is a greater decrease of genetic correlation with distance as dimensionality decreases in stepping-stone models of isotropic gene flow (Kimura and Weiss, 1964; Kimura and Maruyama, 1971). (3) G. minus in the Mid-Appalachians is largely confined to springs, spring runoff, and stream caves situated in carbonate strike valleys. Each local population is partially isolated from others in the local drainage system, and more profoundly isolated from populations in other streams and in other regions of carbonate outcrop. Holsinger (1969) discusses the Appalachian zoogeography of the Gammaridae in greater detail. In Virginia and West Virginia this species is widely distributed in the Ridge and Valley physiographic province and flanking areas, where its populations are mainly located in the headward reaches of feeders of trunk streams that trend northeast-southwest along linear strike valleys floored by Paleozoic limestones and dolomites. Some of these valleys are marked by karst development. A particularly extensive karst topography is displayed on the Greenbrier Limestone of Mississippian age in Pocahontas, Greenbrier, and Monroe Counties of southeastern West Virginia. This is a highly structured environment in which G. minus is found in three ecophenotypic forms. The primary aim of this study has been to map allozyme allele frequencies and levels of heterozygosity onto the structured topography to determine their concordance. The second goal has been to determine if the ecophenotypic

Environmental Heterogeneity: A Problem of Contradictory Selection Pressures, Gene Flow, and Local Polymorphism

The interaction of gene flow with contradictory selective pressures that are caused by environmental heterogeneity has been investigated by computer simulation. Generally there is a range of intermediate values of gene flow over which local polymorphism will occur. The length and location of the range depends, for the most part, upon the magnitude of the local selection pressures and upon the net effect of the overall average, or global, selection regime. For the cases studied, variables are introduced to characterize the magnitudes of the local and global selection pressures. The variable Z measures the net effect of the global regime near equilibrium relative to the square of the local selection pressures. In general, the range over which local polymorphism occurs will be long when the condition Z ≃ 1 is met. Within limits, asymmetry, in terms of the proportions of different local selection regimes within a mosaic environment, increases the range. Increased environmental complexity, in terms of the number of different kinds of local selection regimes, has a similar, but smaller, effect. Extension of these results to studies that used different analytic models of gene flow is discussed. Inferences are drawn concerning the evolutionary significance of gene flow.

A FURTHER STEP IN EVOLVING IDEAS ON POPULATION GENETICS

Endler makes a sterling effort to incorporate information from biogeography and field studies to support his models. For example, in Chapter 1 he examines biogeographic information on species distribution and morphology to find support for parapatric speciation. However, in Chapter 6 he rightly points out that it is generally impossible to distinguish between primary intergradation (the prerequisite for parapatric speciation) and secondary intergradation (the result of recontact of differentiated populations). Perhaps more successful is the effort to understand natural clines with gene flow and selection models. There is a useful summary of the information on natural clines and a good attempt to predict the width of specific clines with two basic models. Yet when Endler suggests that gene flow is underestimated here and overestimated there, it should remind the reader that there are probably many unknown factors involved and that biological reality is much more complicated than these models imply (see also the recent discussion of geographical variation in Cepaea by Jones et al., 1977). One of the major strengths of evolutionary biology is that evolutionary change is the result of genetic phenomena and that these genetic phenomena can then be incorporated into explanatory or predictive genetic models. However, demonstrating exactly which genetic phenomena are important in particular evolutionary events is a very difficult task indeed. It entails finding populations undergoing particular changes which can be monitored and tested: a circumstance which is often not possible because of the time involved or financial limitations. Otherwise, specific models can be tested in laboratory situations which are often suspect by many because of the artificial nature of the system. Furthermore, laboratory testing can be quite frustrating since evolutionary events such as speciation do not have to occur frequently to be important. Endler has done a fine service by documenting and clarifying some important aspects of evolutionary biology, although he could have given more suggestions for testing these models. Perhaps now others can further quantify the importance of parapatric speciation and how the interaction of selection and gene flow results in different patterns of genetic variation. This latest Princeton monograph is a well-written, thoroughly organized, and stimulating presentation of several difficult and controversial areas of evolutionary biology. Now if only someone had temporary ominiscience, they could give us the facts.

Gene flow and selection in a two-locus system.

A model of gene flow and selection in two linked loci is analyzed. The problems considered are the effects of linkage on the clines in frequencies at the two loci and the role of gene flow in producing linkage disequilibrium between the loci. Also, the possible significance of linkage as a mechanism for permitting a population of "track" spatial changes in the environment is considered. The results are that when the recombination fraction between the loci is of the same order of magnitude as the selection coefficients or smaller, then linkage is important in determining the gene frequencies and a substantial amount of linkage disequilibrium is present in the cline. Depending on the spatial pattern of selection on the two loci, linkage can either decrease or increase a population's response to local selection.