MICROSATELLITE MARKERS REVEAL POPULATION GENETIC STRUCTURE OF THE TOXIC DINOFLAGELLATE ALEXANDRIUM TAMARENSE (DINOPHYCEAE) IN JAPANESE COASTAL WATERS1

This is the first report to explore the fine‐scale diversity, population genetic structure, and biogeography of a typical planktonic microbe in Japanese and Korean coastal waters and also to try to detect the impact of natural and human‐assisted dispersals on the genetic structure and gene flow in a toxic dinoflagellate species. Here we present the genetic analysis of Alexandrium tamarense (Lebour) Balech populations from 10 sites along the Japanese and Korean coasts. We used nine microsatellite loci, which varied widely in number of alleles and gene diversity across populations. The analysis revealed that Nei's genetic distance correlated significantly with geographic distance in pair‐wise comparisons, and that there was genetic differentiation in about half of 45 pair‐wise populations. These results clearly indicate genetic isolation among populations according to geographic distance and restricted gene flow via natural dispersal through tidal currents among the populations. On the other hand, high P‐values in Fisher's combined test were detected in five pair‐wise populations, suggesting similar genetic structure and a close genetic relationship between the populations. These findings suggest that the genetic structure of Japanese A. tamarense populations has been disturbed, possibly by human‐assisted dispersal, which has resulted in gene flow between geographically separated populations.     

中日5个岛屿山茶种群遗传多样性研究

运用ISSR分子标记法对中、日两国5个岛屿天然山茶种群共150个个体进行遗传多样性分析。结果表明:筛选出的20条引物扩增得到205条清晰条带,其中183条为多态性条带,多态位点百分比(PPB)为89.27%。经POPGENE软件分析,山茶种群平均多态位点百分比(PPB)为72.00%,Nei’s基因多样性指数(HE)为0.2743,Shannon信息多态性指数(H)为0.4023,种群水平遗传多样性较高。基因分化系数Gst=0.2033,表明遗传变异主要存在于种群内个体间。Mantel检验(r=0.7989,P<0.05)和UPGMA聚类表明岛屿地理隔离对山茶种群遗传分化具有重要影响。基于岛屿山茶种群遗传结构的分析,建议加强我国岛屿自然种群的就地保护力度。     

Allozyme diversity in Chinese,Seguin and American chestnut (Castanea spp.)

Allozyme genetic variability in three chestnut (Castanea) species was investigated using 19 loci from ten enzyme systems. G-tests of heterogeneity of isozymic allele distribution showed significant differences between the three species at 15 of the 19 loci, and between the 13 C. mollissima populations at 13 of the 19 loci examined. C. mollissima was found to possess a significantly-higher value of mean gene heterozygosity (H=0.3050±0.0419), the percentage of polymorphic loci (P=84.21%) and the average number of alleles per locus (A=2.05), than any other species in the Castanea section Eucastanon. When the genetic variability of populations of C. mollissima from four regions in China was investigated, the population from the Changjiang river region showed a markedly higher mean gene heterozygosity (H=0.3480±0.0436) than populations from the other regions. Genetic relationships among the four regions were assessed by Nei's genetic identity I and standard genetic distance D. An approximately-identical distance between the population from the Changjiang river region and populations from the three other regions was observed, while populations from the latter regions showed almost the same genetic distance from each other. These data, when considered with information existing prior to this study, contribute to an understanding of the possible origin and progenitor of the chestnut species.     

Genetic structure of the genera Psorophora (Diptera: Culicidae) in Columbian and North American populations using isoenzymes and ITS2 sequences

Two, a priori, Psorophora columbiae and one, a priori, Ps. toltecum populations from Colombia were studied by means of eleven isoenzyme loci. The levels of genetic diversity, Hardy-Weinberg equilibrium, gene flow estimates, isolation by distance and genetic relationships among these three populations were studied and the results were as follows: 1. The gene diversity levels were high as well as very similar among the three populations indicating no differences between them. 2. Within each population and from a hierarchical standpoint several loci were not in Hardy-Weinberg equilibrium simultaneously by excess and defect of heterozygous. This could reflect that several natural selection forces are acting upon these mosquito populations. 3. The gene flow estimate for these populations showed the existence of this event between them. This agrees quite well with the fact that the three populations really belong to a single species. In addition, maximum parsimony analyses with 16 isoenzyme for several individuals from four Colombian and two United States Psorophora populations showed that effectively only one species was present in the Colombian area surveyed, and that this species is Ps. columbiae, which hardly contrast with that previously suggested by other authors.     

Genetic and metal analyses of fragmented populations of Betula papyrifera (Marsh) in a mining reclaimed region: identification of population–diagnostic molecular marker

White birch (Betula papyrifera) is an open pollinate species that is, dominant in the Northern Ontario after land reclamation. In fact, this species represents 65% of all trees in the region. We hypothesized that the exchange of genetic information between fragmented populations by range‐wide paternal introgression is possible in wind‐pollinated species such as B. papyrifera. On the other hand, the effects of heavy metal contamination from the mining activities on plant growth and population dynamics are well documented. The main objectives of this study were (1) to assess the level of genetic variation, gene flow, and population sustainability of B. papyrifera after land reclamation; and (2) to determine the level of phytoavailable metals in soil and their accumulation in trees. We found that B. papyrifera is a Ni and Zn accumulator with a translocation factor of 6.4 and 81, respectively, and an indicator of Cu and Pb. The level of polymorphic loci, Shannon index, Nei's genetic diversity, observed number of alleles, and gene flow were determined for the fragmented populations within the targeted region. The percent of polymorphic loci ranged from 28% to 56%; the gene flow was also low with a value of 0.89, and the population differentiation was very high with a value of 0.36. Two population–diagnostic ISSR markers were identified. They were cloned, sequenced, and converted to SCAR markers. Overall, the fragmented populations of B. papyrifera in Northern Ontario are genetically sustainable based on the moderate level of intrapopulation variability.     

濒危植物三棱栎遗传多样性的RAPD分析

用随机扩增多态DNA (RAPD)标记对 5个三棱栎 (Trigonobalanusdoichangensis)居群共 99个个体进行遗传多样性和居群遗传结构分析。 16个引物共检测到 15 7个位点 ,其中多态位点 83个 ,占 5 2 87%。物种水平Shannon多样性指数I =0 2 4 31,Nei基因多样度h =0 15 95 ,种内总遗传变异量Ht=0 16 0 0 ,居群内遗传变异量Hs =0 0 74 9,居群间变异量大于居群内变异量 ,表明三棱栎的遗传变异主要存在于居群之间。与同科植物相比 ,三棱栎遗传多样性较低 ,遗传分化系数Gst =0 5 32 0 ,说明居群间的遗传变异占 5 3 2 0 % ,居群间已出现强烈的遗传分化。当地人的强烈活动造成的生境破碎化和居群隔离 ,以及三棱栎演化过程中的地史变化对其种群发展的影响等 ,可能是造成其居群间强烈的遗传分化和较低遗传多样性的原因。基于本研究结果 ,提出了三棱栎遗传多样性的保护策略。     

Genetic Variation and Differentiation of Larix decidua Populations in Swiss Alps (in English)

Genetic diversity within and among six subpopulations of Larix decidua Mill. from two altitudinal transects of Swiss Alps was investigated using 6 enzyme systems coding for 8 loci. Globally, the mean proportion of polymorphic loci was 22.9%, the average number of alleles per locus was 1.3, and the mean expected heterozygosity was 0.095. Only 5.8% of the genetic variation resided among populations. The mean genetic distance was 0.006. Several significant differences of gene frequencies were found between different age classes. Positive values of the species mean fixation index observed in this study suggested a considerable deficit of heterozygotes in the populations of L. decidua of Swiss Alps. At one of the sites (Arpette), the highest subpopulation in elevation gave the lowest level of genetic diversity (as evidenced by the lowest proportion of polymorphic loci and the lowest mean expected heterozygosity) and the largest value of genetic distance when compared to other subpopulations. The genetic differences between the highest subpopulation and the other ones suggest that the founder effect may be an important factor influencing genetic differentiation of L. decidua populations at Arpette transect.     

Molecular Markers Reveal Limited Population Genetic Structure in a North American Corvid,Clark’s Nutcracker (Nucifraga columbiana)

The genetic impact of barriers and Pleistocene glaciations on high latitude resident species has not been widely investigated. The Clark’s nutcracker is an endemic North American corvid closely associated with Pinus-dominated forests. The nutcracker’s encompasses known barriers to dispersal for other species, and glaciated and unglaciated areas. Clark’s nutcrackers also irruptively disperse long distances in search of pine seed crops, creating the potential for gene flow among populations. Using the highly variable mitochondrial DNA control region, seven microsatellite loci, and species distribution modeling, we examined the effects of glaciations and dispersal barriers on population genetic patterns and population structure of nutcrackers. We sequenced 900 bp of mitochondrial control region for 169 individuals from 15 populations and analysed seven polymorphic microsatellite loci for 13 populations across the Clark’s nutcracker range. We used species distribution modeling and a range of phylogeographic analyses to examine evolutionary history. Clark’s nutcracker populations are not highly differentiated throughout their range, suggesting high levels of gene flow among populations, though we did find some evidence of isolation by distance and peripheral isolation. Our analyses suggested expansion from a single refugium after the last glacial maximum, but patterns of genetic diversity and paleodistribution modeling of suitable habitat were inconclusive as to the location of this refugium. Potential barriers to dispersal (e.g. mountain ranges) do not appear to restrict gene flow in Clark’s nutcracker, and postglacial expansion likely occurred quickly from a single refugium located south of the ice sheets.     

Genetic analysis of the threatened American hart's-tongue fern (Asplenium scolopendrium var. americanum [Fernald] Kartesz and Gandhi): Insights into its mating system and implications for conservation

American hart's-tongue fern (AHTF) is one of the rarest ferns in the United States and concern over its conservation and management has highlighted the need for genetic analysis. Genetic analysis also provides insights into the species' mating system which contributes to our understanding of its rarity and persistence. We analyzed 88 individuals from 11 populations in NY and MI based on variations in 108 loci as revealed through ISSR markers using Nei's gene diversity index, percent polymorphic loci and other measures. Low genetic diversity predominates in the populations from NY, and even lower for the populations in MI. Our results also indicate that AHTF from NY and MI are genetically differentiated from each other, as well as the populations within them. There is no positive correlation between genetic and geographic distances, as well as between genetic distance and census population size. The significantly high among population genetic variation and low gene flow value are common indicators of a predominant inbreeding mating strategy within populations, limited spore dispersal, and genetic drift. Our results also indicate that each AHTF population is an important contributor to the overall genetic variation of the species and thus, represents a significant unit for conservation efforts.     

LONG-DISTANCE GENE FLOW IN THE SEDENTARY BUTTERFLY,EUPHILOTES ENOPTES (LEPIDOPTERA: LYCAENIDAE)

The relationship between gene flow and geographic proximity has been assessed for many insect species, but dispersal distances are poorly known for most of these. Thus, we are able to assess the concordance between vagility and gene flow for only a few species. In this study, I documented variation at six allozyme loci among Washington and Oregon populations of the sedentary, patchily distributed, lycaenid butterfly, Euphilotes enoptes (Boisduval) to assess whether the relationship between gene flow and geographic distance is consistent with the dispersal biology of this species. Both a phenogram based on genetic distances between populations and a regression analysis of gene flow estimates on geographic distances showed a pattern consistent with genetic isolation by distance. Many estimates of gene flow among pairs of populations separated by more than 100 km exceeded the equivalent of 10 individuals exchanged per generation, a value much greater than would be predicted from the limited dispersal ability of this species. However, based on the allozyme data, genetic neighborhood size was estimated to be approximately 39 individuals, a value that is consistent with poor vagility. The results of this study speak to the power of stepping-stone gene flow among populations and are compared to the results of other studies that have examined the relationship between dispersal and gene flow in sedentary insects.     

Genetic diversity and gene flow in the endangered dwarf bear poppy, Arctomecon humilis (Papaveraceae)

Arctomecon humilis is a critically endangered species endemic to the Moenkopi shale of Washington County, Utah. Recovery plans for the species would be improved by an understanding of genetic diversity and gene flow among its remaining populations. Ten variable isozyme loci were used to calculate genetic diversity statistics for study populations. Westerly populations possessed higher levels of genetic variability than other populations at the same isozyme loci. Three of the populations exhibited significant deviations from Hardy-Weinberg expectations. No correlation existed between genetic distance and geographic distance. Most of the genetic diversity was distributed among populations with little gene flow between populations, suggesting that observed genetic differences may arise from genetic drift. For the westerly populations, similar genotypes were observed in the seedling and old age classes, while intermediate age classes typically possessed an alternate set of genotypes at Pgi-2. Mean heterozygosity increased with age class across populations. Westerly populations of A. humilis shared more alleles with the nearest geographic population of A. californica than other populations. Since the westerly populations contained more genetic variability and more alleles in common with a near relative, they may be relictual. Other populations may contain less genetic diversity due to founder effects and/or genetic drift.     

GENETIC DIVERSITY AND POPULATION STRUCTURE IN CAMELLIA JAPONICA L. (THEACEAE)

Camellia japonica is a widespread and morphologically diverse tree native to parts of Japan and adjacent islands. Starch gel electrophoresis was used to score allelic variation at 20 loci in seeds collected from 60 populations distributed throughout the species range. In comparison with other plant species, the level of genetic diversity within C. japonica populations is very high: 66.2% of loci were polymorphic on average per population, with a mean number of 2.16 alleles per locus; the mean observed and panmictic heterozygosities were 0.230 and 0.265, respectively. Genotypic proportions at most loci in most populations fit Hardy-Weinberg expectations. However, small heterozygote deficiencies were commonly observed (mean population fixation index = 0.129). It is suggested that the most likely cause of the observed deficiencies is population subdivision into genetically divergent subpopulations. The overall level of population differentiation is greater than is typically observed in out-breeders: The mean genetic distance and identity (Nei's D and I) between pairs of populations were 0.073 and 0.930, respectively, and Wright's Fst was 0.144. Differences among populations appeared to be manifested as variation in gene frequencies at many loci rather than variation in allelic composition per se. However, the patterns of variation were not random. Reciprocal clinal variation of gene frequencies was observed for allele pairs at six loci. In addition, principal components analysis revealed that populations tended to genetically cluster into four regions representing the geographic areas Kyushu, Shikoku, western Honshu, and eastern Honshu. There was a significant relationship between genetic and geographic distance (r = 0.61; P < 0.01). Analysis of variance on allozyme frequencies showed that there was approximately four times as much differentiation among populations within regions, as among regions. It is likely that the observed patterns of population relationships result from the balance between genetic drift in small subpopulations and gene flow between them.     

Spatial genetic structure and clonal diversity of island populations of lady's slipper (Cypripedium calceolus) from the Biebrza National Park (northeast Poland)

Three populations of the rare and endangered plant species Cypripedium calceolus were included in a study of genetic diversity and spatial genetic structure in the Biebrza National Park, northeast Poland. Analysis of 11 allozyme loci indicate that the populations of this species contained high genetic variability (P = 45.5%, A= 1.73). On the other hand, the genetic differentiation (FST = 0.014, P < 0.05) among C. calceolus populations was very low when compared to other species with similar life history characteristics. The observed high rate of gene flow (Nm = 18) may suggest that the populations studied derived from each other in the recent past. Five polymorphic allozyme markers identified 109 multilocus genotypes in three populations and the majority of them (67%) were population-specific. One of the populations studied, characterized by particularly extensive vegetative reproduction, showed the lowest clonal diversity (G/N = 0.15) and heterozygosity (HO = 0.111) values and the highest FIS(0.380), when compared to other two populations (G/N = 0.26-0.27, HO= 0.166-178, FIS = 0.024-0.055). This may indicate that clonal reproduction has an important influence on the genetic structure of C. calceolus populations. The longevity of genets, the out-crossing breeding system and the presence of recruitment from seeds are factors maintaining genetic diversity in C. calceolus.     

Genetic variation of oaks (<Emphasis Type="Italic">Quercus</Emphasis> spp.) in Switzerland. 3. Lack of impact of postglacial recolonization history on nuclear gene loci

Quercus petraea, Quercus pubescens and Quercus robur are closely related and interfertile white oaks native to Switzerland. The three species are known to share identical cpDNA haplotypes, which are indicative of the postglacial recolonization history of populations. Only two haplotypes are common in Switzerland. We compared variation of cpDNA and of isozymes in 28 oak populations from Switzerland in order to assess the impact of the postglacial population history on current genetic structures of nuclear controlled isozyme gene loci. Species delineation was based on Principal Component Analysis of leaf morphological traits. The species status of populations was reflected at isozyme gene loci, but differentiation between populations with different cpDNA haplotypes and hence different recolonization history was very low at enzyme gene loci for all species. Thus, glacial and postglacial population history was not reflected at nuclear gene loci on the temporal and spatial scale covered by the present study. Extensive gene flow through pollen among populations is likely to have blurred a previously existing genetic differentiation at biparentally inherited gene loci that possibly evolved in the different glacial refugia of the above mentioned cpDNA haplotypes.     

利用SRAP标记研究四个暗纹东方鲀群体的遗传多样性

利用相关序列扩增多态性SRAP(Sequence-related amplified polymorphism)分子标记首次对4个暗纹东方鲀(Takifugu obscurus)群体(1个长江捕捞群体、1个放流群体和2个养殖群体)进行了遗传多样性分析。从49对引物组合中筛选出18对扩增条带清晰、稳定的引物组合对4个群体进行扩增,共获得231个位点,其中多态位点数为156个,总多态位点百分率为67.53%。4个群体内多态位点比例为54.98%—58.87%,群体的Nei’s基因多样性指数(H)为0.1992—0.2005,群体Shannon多样性指数(I)为0.2953—0.3016,群体间基因流(Nm)为4.1291。长江捕捞群体的多态位点比例、基因多样性指数、Shannon多样性指数均略高于其他三个群体。采用UPGMA法对4个群体进行聚类分析显示,上海养殖群体单独聚为一类,其余3个群体聚为另一类。AMOVA结果表明遗传变异主要来源于群体内,占总遗传变异的87.40%。以上结果表明,4个暗纹东方鲀群体具有较高的遗传多样性,群体间相似性较大,并且存在一定的基因交流。     

地被植物鹅绒委陵菜的遗传多样性研究

采用RAPD方法对不同海拔和植被盖度下的鹅绒委陵菜（Potentilla anserinaL.）6个种群的遗传多样性水平进行了研究。结果表明,13条随机引物共扩增出132条带,多态性位点为117;鹅绒委陵菜物种水平的遗传多样性较高,且种群间分化明,多态位点比率（88.64%）、Nei’s基因多样性（0.3180）、Shannon信息多样性指数（0.4732）均远高于多数克隆植物。6个种群内的遗传多样性水平变化较大,多态位点在各种群中分布不均衡,种群间分化系数（GST）为47.7%,基因流较低（Nm=0.5482）,多态位点比率在31.06%～74.24%之间,Shannon信息指数在0.1711～0.3625之间,Nei基因多样性指数在0.1164～0.2425之间。多样性水平的变化与海拔没有明显的相关性,而与生境盖度呈显著正相关。从而推断在盖度低、资源丰富的环境中,该物种可能更倾向于克隆繁殖。     

中国石榴栽培群体遗传多样性的荧光AFLP分析

以中国山东、安徽、陕西、河南、云南和新疆6个栽培石榴群体的85个品种类型为试材,利用荧光标记AFLP对中国石榴群体遗传多样性进行了研究。结果表明：8对引物组合在种级水平扩增的多态性位点数范围从135～185个不等,平均为158.25个,多态位点百分比范围为62.5%～86.11%,平均为73.26%,说明中国石榴品种遗传多样性较为丰富;石榴品种种级水平遗传多样性大于群体水平,6个群体的遗传多样性依次为河南群体〉新疆群体〉陕西群体〉安徽群体〉山东群体〉云南群体,并且具有显著性差异;群体间的遗传分化系数GST为0.2018,说明石榴遗传变异主要存在群体内,群体间的遗传变异占总变异的20.18%,根据基因分化系数,测得的基因流Nm为1.9027,说明中国石榴群体间存在适当的基因交流;UPGMA聚类分析结果表明,同一群体的大部分品种都聚在一起,但同时存在部分基因交流。所有遗传参数表明,中国石榴栽培品种遗传多样性较为丰富,其中河南群体遗传多样性显著高于其他群体,在中国石榴品种选育中具有更为广阔的应用前景。     

Strong genetic impoverishment from the centre of distribution in southern Europe to peripheral Baltic and isolated Scandinavian populations of the pearly heath butterfly

Aim Climatic changes and fluctuations in the past have strongly influenced the distribution of animal and plant species. Such fluctuations are also reflected in the patterns of genetic diversity on both local and global scales. The genetic pattern of the pearly heath butterfly, Coenonympha arcania, was used to evaluate the genetic differentiation of isolated (in north‐western Europe), peripheral (in north‐eastern Europe) and central (in southern Europe) populations in the context of post‐glacial distributional changes of the species. Location Europe (Sweden, Germany, the Baltic states, Italy, Slovenia, Hungary, Romania, Bulgaria). Thus, samples were collected from large parts of the species’ distribution representing the three categories mentioned above. Methods We analysed 18 loci of 569 individuals from 28 populations by allozyme electrophoresis. We used both individual‐based and population‐based analyses, including F‐statistics, various clustering methods and Markov chain Monte Carlo simulations. Results All loci, except Fum, were polymorphic. The mean F_ST for all samples was 0.18. The mean genetic distance among populations was 0.046. Two major genetic lineages were distinguished. Populations from the centre of the distributional range in southern Europe and the northern periphery of the distributional range differed significantly in their level of genetic variability. The central populations of south‐eastern Europe showed high levels of genetic diversity and no differentiation among populations. Main conclusions Most probably the two major genetic lineages evolved during glacial isolation in two disjunct Mediterranean refugia. The lack of genetic differentiation across south‐eastern Europe implies a continuous Würm ice age distribution in this area, thus supporting the functional existence of steppe forests throughout this region. The peripheral‐isolated populations in Sweden seem to have suffered from one or more severe bottlenecks, resulting in substantial genetic impoverishment. The peripheral‐connected eastern Baltic populations, on the other hand, are affected by post‐glacial and possibly recurrent gene flow from more central parts of the distribution.     

Population structure in the American oyster as inferred by nuclear gene genealogies

Multiple haplotypes from each of three nuclear loci were isolated and sequenced from geographic populations of the American oyster, Crassostrea virginica. In tests of alternative phylogeographic hypotheses for this species, nuclear gene genealogies constructed for these haplotypes were compared to one another, to a mitochondrial gene tree, and to patterns of allele frequency variation in nuclear restriction site polymorphisms (RFLPs) and allozymes. Oyster populations from the Atlantic versus the Gulf of Mexico are not reciprocally monophyletic in any of the nuclear gene trees, despite considerable genetic variation and despite large allele frequency differences previously reported in several other genetic assays. If these populations were separated vicariantly in the past, either insufficient time has elapsed for neutral lineage sorting to have achieved monophyly at most nuclear loci, or balancing selection may have inhibited lineage extinction, or secondary gene flow may have moved haplotypes between regions. These and other possibilities are examined in light of available genetic evidence, and it is concluded that no simple explanation can account for the great variety of population genetic patterns across loci displayed by American oysters. Regardless of the source of this heterogeneity, this study provides an empirical demonstration that different sequences of DNA within the same organismal pedigree can have quite different phylogeographic histories.      

Testing Pleistocene refugia theory: phylogeographical analysis of Desmognathus wrighti,a high-elevation salamander in the southern Appalachians

During the colder climates of the Pleistocene, the ranges of high-elevation species in unglaciated areas may have expanded, leading to increased gene flow among previously isolated populations. The phylogeography of the pygmy salamander, Desmognathus wrighti, an endemic species restricted to the highest mountain peaks of the southern Appalachians, was examined to test the hypothesis that the range of D. wrighti expanded along with other codistributed taxa during the Pleistocene. Analyses of genetic variation at 14 allozymic loci and of the 12S rRNA gene in the mtDNA genome was conducted on individuals sampled from 14 population isolates throughout the range of D. wrighti. In contrast to the genetic patterns of many other high-elevation animals and plants, genetic distances derived from both molecular markers showed significant isolation by distance and genetic structuring of populations, suggesting long-term isolation of populations. Phylogeographical analyses revealed four genetically distinct population clusters that probably remained fragmented during the Pleistocene, although there was also evidence supporting recent gene flow among some population groups. Support for isolation by distance is rare among high-elevation species in unglaciated areas of North and Middle America, although not uncommon among Plethodontid Salamanders, and this pattern suggests that populations of D. wrighti did not expand entirely into suitable habitat during the Pleistocene. We propose that intrinsic barriers to dispersal, such as species interactions with other southern Appalachian plethodontid salamanders, persisted during the Pleistocene to maintain the fragmented distribution of D. wrighti and allow for significant genetic divergence of populations by restricting gene flow.