Morphological variation of major human populations based on nonmetric dental traits

The patterns of inter- and intra-regional variation among 12 major geographical groups from around the world were investigated based on 15 nonmetric dental traits. The R-matrix method was applied using a pooled within-group variance-covariance matrix estimated with the maximum likelihood method (tetrachoric correlation matrix) and the threshold value for each trait estimated by univariate probit analysis. Using average heritability rates that range from 0.40 to 1.00, the inter-regional variation represented by Fst falls between 7.19% and 16.23% of the total variance. This range of variation is compatible with those obtained by genetic, craniometric, and odontometric data. Subsaharan Africans show the largest intra-regional diversity among the groups compared. The degree of intra-regional variation shows, moreover, rough clinalities from subsaharan Africa to peripheral regions. The relationship between regional variation and geographic distance from subsaharan Africa supports serial bottlenecks and the founder effect of ancient populations originating in Africa. The variation of East/Northeast Asians is relatively large, suggesting a complex population history such as possible earlier divergence and multiple migrations from outside sources. The present findings are in agreement with both the recent African model for the origin of anatomically modern humans and the current scenario for human migration history suggested by genetic analyses.     

TESTING MODELS OF MIGRATION AND ISOLATION AMONG POPULATIONS OF CHINOOK SALMON (ONCORHYNCHUS TSCHAWYTSCHA)

The chinook salmon (Oncorhynchus tschawytscha) is a behaviorally, morphologically, and ecologically variable species distributed over a large geographic range. Although previous genetic surveys have revealed considerable genetic differences among populations with different life history types and from different major river drainages, it is not clear to what degree these genetically distinct populations are connected by low levels of gene flow. The work described in this paper addresses this question by surveying DNA restriction site variation at six nuclear genes from nine populations encompassing most of the species's North American range, and then attempting to fit the patterns of variation observed at these genes to five different evolutionary models using computer simulations of the coalescent process. Two commonly used constant population size models, one hypothesizing no migration among populations and one hypothesizing equal rates of migration among populations, were found to be statistically inconsistent with the observed patterns of variation. The other three models, which involved either recent divergence among populations coupled with large changes in populations size, unequal migration rates among populations, or selection, were all found to be consistent with the observed patterns of variation. Assuming selective neutrality, these results suggest that either the populations have all descended from a common ancestral population within the last ~50,000 years and have all suffered large declines in effective population size since that time, or that they have a more ancient divergence time but are connected by low levels of gene flow. These conclusions rest on the assumption of selective neutrality. With the methods employed, it was not possible to simultaneously test hypotheses of both selective neutrality and population structure.     

Age, phylogeography and population structure of the microendemic banded spring snail, Mexipyrgus churinceanus

Recent theoretical and empirical studies of phylogeography and population structure indicate that many processes influence intraspecific evolutionary history. The present study represents the first examination of various forces influencing the spatial and temporal patterns of sequence variation in the freshwater Mexican banded spring snail, Mexipyrgus churinceanus. This snail occurs in one of the most critically endangered centres of freshwater endemism, the desert ecosystem of Cuatro Ciénegas. From cytochrome b mtDNA sequence variation, there is strong evidence of long-term isolation of three regions, suggesting that these regions represent evolutionarily distinct lineages. Molecular clock estimates of clade age indicate a time to most recent common ancestor of approximately 2.5 million years ago (Ma). The three regions differ considerably in the historical and demographic forces affecting population structure. The western populations have extremely low mtDNA diversity consistent with a severe bottleneck dating to 50,000 years before present (bp). The nearby Rio Mesquites drainage is characterized by fragmentation events, restricted gene flow with isolation by distance, and higher levels of mtDNA polymorphism. These patterns are consistent with the long-term stability of this drainage along with habitat heterogeneity and brooding contributing to population isolation and restricted gene flow. Southeastern populations show evidence of range expansion and a strong influence of genetic drift. Migration rates between drainages indicate very little gene flow between drainages except for asymmetric migration from the Rio Mesquites into both western and southeastern drainages.     

Origin and phylogeography of African savannah elephants (<Emphasis Type="Italic">Loxodonta africana</Emphasis>) in Kruger and nearby parks in southern Africa

African savannah elephants (Loxodonta africana) occur in fragmented and isolated populations across southern Africa. Transfrontier conservation efforts aim at preventing the negative effects of population fragmentation by maintaining and restoring linkages between protected areas. We sought to identify genetic linkages by comparing the elephants in Kruger National Park (South Africa) to populations in nearby countries (Botswana, Mozambique, Zambia and Zimbabwe). We used a 446 base pair mitochondrial DNA (mtDNA) control region fragment (141 individuals) and 9 nuclear DNA (nDNA) microsatellite markers (69 individuals) to investigate phylogenetic relationships and gene flow among elephant populations. The mtDNA and nDNA phylogeographic patterns were incongruent, with mtDNA patterns likely reflecting the effects of ancient female migrations, with patterns persisting due to female philopatry, and nDNA patterns likely reflecting male-mediated dispersal. Kruger elephant heterozygosity and differentiation were examined, and were not consistent with genetic isolation, a depleted gene pool or a strong founder effect. Mitochondrial DNA geographic patterns suggested that the Kruger population was founded by elephants from areas both north and south of Kruger, or has been augmented through migration from more than one geographic source. We discuss our findings in light of the need for conservation initiatives that aim at maintaining or restoring connectivity among populations. Such initiatives may provide a sustainable, self-regulating management approach for elephants in southern Africa while maintaining genetic diversity within and gene flow between Kruger and nearby regions.     

SPECIATION AND POPULATION GENETIC STRUCTURE IN TROPICAL PACIFIC SEA URCHINS

Unlike populations of many terrestrial species, marine populations often are not separated by obvious, permanent barriers to gene flow. When species have high dispersal potential and few barriers to gene flow, allopatric divergence is slow. Nevertheless, many marine species are of recent origin, even in taxa with high dispersal potential. To understand the relationship between genetic structure and recent species formation in high dispersal taxa, we examined population genetic structure among four species of sea urchins in the tropical Indo-West Pacific that have speciated within the past one to three million years. Despite high potential for gene flow, mtDNA sequence variation among 200 individuals of four species in the urchin genus Echinometra shows a signal of strong geographic effects. These effects include (1) substantial population heterogeneity; (2) lower genetic variation in peripheral populations; and (3) isolation by distance. These geographic patterns are especially strong across scales of 5000-10,000 km, and are weaker over scales of 2500-5000 km. As a result, strong geographic patterns would not have been readily visible except over the wide expanse of the tropical Pacific. Surface currents in the Pacific do not explain patterns of gene flow any better than do patterns of simple spatial proximity. Finally, populations of each species tend to group into large mtDNA regions with similar mtDNA haplotypes, but these regional boundaries are not concordant in different species. These results show that all four species have accumulated mtDNA differences over similar spatial and temporal scales but that the precise geographic pattern of genetic differentiation varies for each species. These geographic patterns appear much less deterministic than in other well-known coastal marine systems and may be driven by chance and historical accident.     

Outbreaks, gene flow and effective population size in the migratory locust, Locusta migratoria: a regional-scale comparative survey

The potential effect of population outbreaks on within and between genetic variation of populations in pest species has rarely been assessed. In this study, we compare patterns of genetic variation in different sets of historically frequently outbreaking and rarely outbreaking populations of an agricultural pest of major importance, the migratory locust, Locusta migratoria . We analyse genetic variation within and between 24 populations at 14 microsatellites in Western Europe, where only ancient and low-intensity outbreaks have been reported (non-outbreaking populations), and in Madagascar and Northern China, where frequent and intense outbreak events have been recorded over the last century (outbreaking populations). Our comparative survey shows that (i) the long-term effective population size is similar in outbreaking and non-outbreaking populations, as evidenced by similar estimates of genetic diversity, and (ii) gene flow is substantially larger among outbreaking populations than among non-outbreaking populations, as evidenced by a fourfold to 30-fold difference in F _ST values. We discuss the implications for population dynamics and the consequences for management strategies of the observed patterns of genetic variation in L. migratoria populations with contrasting historical outbreak frequency and extent.     

Geographic mitochondrial DNA variation in the rock hyrax, Procavia capensis.

Restriction-fragment-length polymorphisms in mitochondrial DNA (mtDNA) were used to evaluate geographic population genetic structure in the rock hyrax, Procavia capensis, a species which occurs widely, though restricted to rocky habitat, throughout South Africa. Ten restriction endonucleases were employed to assay mtDNAs from 55 specimens representing 10 localities. Haplotypes showed strong geographic patterning, and estimates of nucleotide sequence divergence indicate two major clades thought to be dispersing along separate routes. The divergence time of approximately 2 Myr between clades is relatively high for intraspecific variation. We speculate that the marked genetic break distinguishing the northwestern populations from those constituting the south/central clade may be indicative of two species in what has conventionally been regarded as P. capensis.     

Relative influences of historical and contemporary forces shaping the distribution of genetic variation in the Atlantic killifish, Fundulus heteroclitus

A major goal of population genetics research is to identify the relative influences of historical and contemporary processes that serve to structure genetic variation. Most population genetic models assume that populations exist in a state of migration-drift equilibrium. However, in the past this assumption has rarely been verified, and is likely rarely achieved in natural populations. We assessed the equilibrium status at both local and regional scales of the Atlantic killifish, Fundulus heteroclitus . This species is a model organism for the study of adaptive clinal variation, but has also experienced a complicated history of range expansion and secondary contact following allopatric divergence, potentially obscuring the influence of contemporary evolutionary processes. Presumptively neutral genetic markers (microsatellites) demonstrated zones of secondary intergradation among coastal populations centred around northern New Jersey and the Chesapeake Bay region. Analysis of genetic variation indicated isolation by distance among some populations and provided supporting evidence that the Delaware Bay, but not the Chesapeake Bay, has acted as a barrier to dispersal among coastal populations. Bayesian estimates indicated large effective population sizes and low migration rates, and were in good agreement with empirically derived estimates of population and neighbourhood size from mark–recapture studies. These data indicate that populations are not in migration-drift equilibrium at a regional scale, and suggest that contributing factors include large population size combined with relatively low migration rates. These conditions should be considered when interpreting the evolutionary significance of the distribution of genetic variation among F. heteroclitus populations.     

EFFECTS OF INTRINSIC AND EXTRINSIC FACTORS ON POPULATION FRAGMENTATION IN THREE SPECIES OF NORTH AMERICAN MINNOWS (TELEOSTEI: CYPRINIDAE)

Geographic patterns of genetic variation (mitochondrial DNA [mtDNA] and allozymes) were used to examine effects of intrinsic characteristics (e.g., vagility, habitat specificity, and reproductive behaviors) and extrinsic factors (e.g., climatic and geological history) on population fragmentation. The three species of cyprinid fishes examined (Tiaroga cobitis, Meda fulgida, and Agosia chrysogaster) occupied similar historical ranges within the lower Colorado River drainage, but differ in intrinsic characteristics conducive to population fragmentation. Relationships among populations were similar across species, reflecting common historical influences, but results indicate the distribution of variation among species is strongly affected by intrinsic characteristics. Variation within two species (T. cobitis and M. fulgida) is subdivided among populations, suggesting little gene flow among rivers. In contrast, similarity of A. chrysogaster populations throughout the Gila River drainage supports the hypothesis that levels of gene flow are high for this species. Levels of mtDNA divergence were much higher than expected for both T. cobitis and A. chrysogaster suggesting long-term isolation of geographic regions. These results indicate that both long-term and short-term extrinsic factors have shaped basic patterns of variation within these fishes; however, the intrinsic characteristics of each species have strongly affected the population genetic structure of these fishes.     

Phylogeography and population history of the endangered golden sun moth (Synemon plana) revealed by allozymes and mitochondrial DNA analysis

A combination of allozyme and mitochondrial DNA markers were used to determine the contribution of recent and ancient causes of patterns of genetic variation within and among 46 populations of the endangered golden sun moth, Synemon plana. Allozyme analysis grouped the 46 populations into 5 major genetic clusters that corresponded closely with geographic location following a classic isolation-by-distance model. Phylogenetic analysis of 14 mtDNA haplotypes revealed two reciprocally monophyletic groups. One of these groups (containing 4 geographically distant populations) was clearly identified by allozyme analysis and represents a distinct evolutionary unit. The remaining 4 allozyme groups were not distinguishable by mtDNA analysis. The evidence suggests that the populations within these groups derived from a small founding population that underwent rapid demographic expansion in ancient times. This was followed by more recent population bottlenecks resulting from habitat fragmentation associated with the widespread introduction of agriculture into the region. The generally low levels of allozyme and nucleotide diversity within these populations support this hypothesis. This revised version was published online in July 2006 with corrections to the Cover Date.     

The influence of history and contemporary stream hydrology on the evolution of genetic diversity within species: an examination of microsatellite DNA variation in bull trout,Salvelinus confluentus (Pisces: Salmonidae)

An understanding of the relative roles of historical and contemporary factors in structuring genetic variation is a fundamental, but understudied aspect of geographic variation. We examined geographic variation in microsatellite DNA allele frequencies in bull trout (Salvelinus confluentus, Salmonidae) to test hypotheses concerning the relative roles of postglacial dispersal (historical) and current landscape features (contemporary) in structuring genetic variability and population differentiation. Bull trout exhibit relatively low intrapopulation microsatellite variation (average of 1.9 alleles per locus, average He = 0.24), but high levels of interpopulation divergence (F(ST) = 0.39). We found evidence of historical influences on microsatellite variation in the form of a decrease in the number of alleles and heterozygosities in populations on the periphery of the range relative to populations closer to putative glacial refugia. In addition, one region of British Columbia that was colonized later during deglaciation and by more indirect watershed connections showed less developed and more variable patterns of isolation by distance than a similar region colonized earlier and more directly from refugia. Current spatial and drainage interconnectedness among sites and the presence of migration barriers (falls and cascades) within individual streams were found to be important contemporary factors influencing historical patterns of genetic variability and interpopulation divergence. Our work illustrates the limited utility of equilibrium models to delineate population structure and patterns of genetic diversity in recently founded populations or those inhabiting highly heterogeneous environments, and it highlights the need for approaches incorporating a landscape context for population divergence. Substantial microsatellite DNA divergence among bull trout populations may also signal divergence in traits important to population persistence in specific environments.     

Inferring population history and demography using microsatellites, mitochondrial DNA, and major histocompatibility complex (MHC) genes

Microsatellites and mitochondrial DNA (mtDNA) have traditionally been used in population genetics because of their variability and presumed neutrality, whereas genes of the major histocompatibility complex (MHC) are increasingly of interest because strong selective pressures shape their standing variation. Despite the potential for MHC genes, microsatellites, and mtDNA sequences to complement one another in deciphering population history and demography, the three are rarely used in tandem. Here we report on MHC, microsatellite, and mtDNA variability in a single large population of the eastern tiger salamander (Ambystoma tigrinum tigrinum). We use the mtDNA mismatch distribution and, on microsatellite data, the imbalance index and bottleneck tests to infer aspects of population history and demography. Haplotype and allelic variation was high at all loci surveyed, and heterozygosity was high at the nuclear loci. We find concordance among neutral molecular markers that suggests our study population originated from post-Pleistocene expansions of multiple, fragmented sources that shared few migrants. Differences in N(e) estimates derived from haploid and diploid genetic markers are potentially attributable to secondary contact among source populations that experienced rapid mtDNA divergence and comparatively low levels of nuclear DNA divergence. We find strong evidence of natural selection acting on MHC genes and estimate long-term effective population sizes (N(e)) that are very large, making small selection intensities significant evolutionary forces in this population.     

Melanesian mtDNA complexity

Melanesian populations are known for their diversity, but it has been hard to grasp the pattern of the variation or its underlying dynamic. Using 1,223 mitochondrial DNA (mtDNA) sequences from hypervariable regions 1 and 2 (HVR1 and HVR2) from 32 populations, we found the among-group variation is structured by island, island size, and also by language affiliation. The more isolated inland Papuan-speaking groups on the largest islands have the greatest distinctions, while shore dwelling populations are considerably less diverse (at the same time, within-group haplotype diversity is less in the most isolated groups). Persistent differences between shore and inland groups in effective population sizes and marital migration rates probably cause these differences. We also add 16 whole sequences to the Melanesian mtDNA phylogenies. We identify the likely origins of a number of the haplogroups and ancient branches in specific islands, point to some ancient mtDNA connections between Near Oceania and Australia, and show additional Holocene connections between Island Southeast Asia/Taiwan and Island Melanesia with branches of haplogroup E. Coalescence estimates based on synonymous transitions in the coding region suggest an initial settlement and expansion in the region at approximately 30-50,000 years before present (YBP), and a second important expansion from Island Southeast Asia/Taiwan during the interval approximately 3,500-8,000 YBP. However, there are some important variance components in molecular dating that have been overlooked, and the specific nature of ancestral (maternal) Austronesian influence in this region remains unresolved.     

Environmental Heterogeneity Explains the Genetic Structure of Continental and Mediterranean Populations of Fraxinus angustifolia Vahl

Tree species with wide distributions often exhibit different levels of genetic structuring correlated to their environment. However, understanding how environmental heterogeneity influences genetic variation is difficult because the effects of gene flow, drift and selection are confounded. We investigated the genetic variation and its ecological correlates in a wind-pollinated Mediterranean tree species, Fraxinus angustifolia Vahl, within a recognised glacial refugium in Croatia. We sampled 11 populations from environmentally divergent habitats within the Continental and Mediterranean biogeographical regions. We combined genetic data analyses based on nuclear microsatellite loci, multivariate statistics on environmental data and ecological niche modelling (ENM). We identified a geographic structure with a high genetic diversity and low differentiation in the Continental region, which contrasted with the significantly lower genetic diversity and higher population divergence in the Mediterranean region. The positive and significant correlation between environmental and genetic distances after controlling for geographic distance suggests an important influence of ecological divergence of the sites in shaping genetic variation. The ENM provided support for niche differentiation between the populations from the Continental and Mediterranean regions, suggesting that contemporary populations may represent two divergent ecotypes. Ecotype differentiation was also supported by multivariate environmental and genetic distance analyses. Our results suggest that despite extensive gene flow in continental areas, long-term stability of heterogeneous environments have likely promoted genetic divergence of ashes in this region and can explain the present-day genetic variation patterns of these ancient populations.     

Features of evolution and expansion of modern humans,inferred from genomewide microsatellite markers

We study data on variation in 52 worldwide populations at 377 autosomal short tandem repeat loci, to infer a demographic history of human populations. Variation at di-, tri-, and tetranucleotide repeat loci is distributed differently, although each class of markers exhibits a decrease of within-population genetic variation in the following order: sub-Saharan Africa, Eurasia, East Asia, Oceania, and America. There is a similar decrease in the frequency of private alleles. With multidimensional scaling, populations belonging to the same major geographic region cluster together, and some regions permit a finer resolution of populations. When a stepwise mutation model is used, a population tree based on TD estimates of divergence time suggests that the branches leading to the present sub-Saharan African populations of hunter-gatherers were the first to diverge from a common ancestral population (approximately 71-142 thousand years ago). The branches corresponding to sub-Saharan African farming populations and those that left Africa diverge next, with subsequent splits of branches for Eurasia, Oceania, East Asia, and America. African hunter-gatherer populations and populations of Oceania and America exhibit no statistically significant signature of growth. The features of population subdivision and growth are discussed in the context of the ancient expansion of modern humans.     

Phylogeography of Hemibarbus labeo (Cyprinidae): secondary contact of ancient lineages of mtDNA

Lin, C.-J., Lin, H.-D., Wang, J.-P., Chao, S.-C. & Chiang, T.-Y. (2007). Phylogeography of Hemibarbus labeo (Cyprinidae): secondary contact of ancient lineages of mtDNA. —Zoologica Scripta, 39, 23–35. Nucleotide sequences of the D-loop control region of mtDNA were used to assess the genetic structure and phylogeography of Hemibarbus labeo in Mainland China and Taiwan. A hierarchical analysis of molecular variance of populations in 11 major streams from three geographical regions revealed significant structuring among populations and geographical regions. High levels of nucleotide diversity (π = 1.88%) and haplotype diversity (h = 96 ± 0.009%) suggest a large effective population size. A maximum likelihood tree based on mtDNA variation identified two ancient mtDNA lineages, which split approximately 3.39 million years ago. Overlapping distribution of the major lineages displayed low correspondence with geographical regions and reflects a scenario of secondary mergence after long isolation. Gene genealogy further revealed a unidirectional migration. Nevertheless, there existed a phylogeographical structure that mostly agreed with a biogeographical hypothesis. That is, within each lineage, a close phylogeny between populations of the River-Campagna and East-Pacific regions was supported by the mtDNA gene genealogy, although monophyly of each geographical region was not supported. The degree of genetic differentiation was correlated with geographical distances between populations, displaying a pattern of ‘isolation by distance’. Gene genealogy of mtDNA revealed that Yangtzejiang population may act as a divergence centre of H. labeo. In addition, Taiwan population was colonized via a recent a founder event, likely from population Yangtzejiang River about 131 000 years before present. Low haplotype number and genetic variability also suggested possible bottleneck events in the Rongjiang and Dazhangjiang populations.     

Evolution of population structure in a highly social top predator, the killer whale

Intraspecific resource partitioning and social affiliations both have the potential to structure populations, though it is rarely possible to directly assess the impact of these mechanisms on genetic diversity and population divergence. Here, we address this for killer whales (Orcinus orca), which specialize on prey species and hunting strategy and have long-term social affiliations involving both males and females. We used genetic markers to assess the structure and demographic history of regional populations and test the hypothesis that known foraging specializations and matrifocal sociality contributed significantly to the evolution of population structure. We find genetic structure in sympatry between populations of foraging specialists (ecotypes) and evidence for isolation by distance within an ecotype. Fitting of an isolation with migration model suggested ongoing, low-level migration between regional populations (within and between ecotypes) and small effective sizes for extant local populations. The founding of local populations by matrifocal social groups was indicated by the pattern of fixed mtDNA haplotypes in regional populations. Simulations indicate that this occurred within the last 20,000 years (after the last glacial maximum). Our data indicate a key role for social and foraging behavior in the evolution of genetic structure among conspecific populations of the killer whale.     

Endangered species in small habitat patches can possess high genetic diversity: the case of the Tana River red colobus and mangabey

We used mtDNA sequence data from the Tana River red colobus and mangabey to determine how their population genetic structure was influenced by dispersal and habitat fragmentation. The colobus and mangabey are critically endangered primates endemic to gallery forests in eastern Kenya. The forests are a Pliocene–Pleistocene refugium that has recently undergone significant habitat loss and fragmentation due to human activities. We expected both primates to exhibit low levels of genetic diversity due to elevated genetic drift in their small populations, and to show a strong correspondence between genetic and geographic distance due to disruption of gene flow between forests by habitat fragmentation. Additionally, because mangabey females are philopatric, we expected their mtDNA variation to be homogeneous within forest patches but to be heterogeneous between patches. In contrast, colobus have a female-biased dispersal and so we expected their mtDNA variation to be homogeneous within and between forest patches. We found high levels of haplotype and nucleotide diversity as well as high levels of sequence divergence between haplotype groups in both species. The red colobus had significantly higher genetic variation than the mangabey did. Most of the genetic variation in both primates was found within forest fragments. Although both species showed strong inter-forest patch genetic structure we found no correspondence between genetic and geographic distances for the two primates. We attributed the high genetic diversity to recent high effective population size, and high sequence divergence and strong genetic structures to long-term habitat changes in the landscape.     

Indo-West Pacific patterns of genetic differentiation in the high-dispersal starfish Linckia laevigata

Genetic variation in four natural populations of the starfish Linckia laevigata from the Indo-West Pacific was examined using restriction fragment analysis of a portion of the mtDNA including the control region. Digestion with seven restriction enzymes identified 47 haplotypes in a sample of 326 individuals. Samples collected from reef sites within each location were not significantly differentiated based on Φ_ST or spatial distribution of haplotypes, indicating that dispersal is high over short to moderate distances. Evidence of gene flow is further supported by the low divergence among haplotypes and the lack of any clear geographical structuring among different haplotypes in the gene phylogeny. However, analysis of molecular variance ( AMOVA ), Φ_ST and contingency χ² analyses of the spatial distribution of haplotypes demonstrate the presence of significant broad scale population genetic structure among the four widespread locations examined. RFLP data are consistent with high gene flow between the Philippines and Western Australia and moderate gene flow between the Great Barrier Reef (GBR) and Fiji, but only limited gene flow between either the Philippines or Western Australia and either the GBR or Fiji. The presence of mtDNA structure contrasts with previous allozyme data which suggest that dispersal among widely separated locations is equivalent to dispersal among populations within the highly connected GBR studies. This discordance between patterns of gene flow inferred from these two markers cannot be fully accounted for by differences in effective population size for mtDNA. This might suggest that while mtDNA variation may represent contemporary patterns of gene flow, allozyme variation among populations is yet to reach equilibrium between drift and migration over the range surveyed.     

The structure of allozyme and leaf shape variation in isolated, range-margin populations of the shrub Hippocrepis emerus (Leguminosae)

We investigated allozyme variation and phenotypic variation in leaf shape in 15 populations of the shrub Hippocrepis emerus (Leguminosae) from the three isolated, regional populations in Scandinavia and analysed patterns of differentiation and the hierarchical structuring of diversity on different geographic scales There are pronounced geographic differences between the Norwegian and Swedish isolates of the species and most of the polymorphic allozyme loci show reciprocal fixation in the two isolates The Scandinavian populations of H emerus are not only disjunct on a macrogeographic scale but also show considerable disjunction within regions Within the Oland regional population, a central group of populations shows low levels of Inter-population differentiation the Gotland group of populations is related to this core group of Oland populations The geographically marginal populations on Oland are spatially isolated and show a higher degree of divergence between populations than does the central group of Oland populations We interpret genetic divergence between the marginal populations in terms of genetic drift - as a result of historical fluctuations in habitat availability and population size