EVOLUTION OF THE PYGMY-FOREST EDAPHIC SUBSPECIES OF PINUS CONTORT A ACROSS AN ECOLOGICAL STAIRCASE

Patterns of allozyme variation within and between two of the subspecies of Pinus contorta were examined for the evolutionary relationship between them. In coastal northern California, these subspecies are parapatric. Pinus contorta ssp. contorta occurs on grassy coastal bluffs on the lowest and youngest of a sequence of five marine terraces; P. contorta ssp. bolanderi is endemic to a pygmy forest ecosystem that occurs on the increasingly older and harsher soils of the third, fourth, and fifth terraces. The soils of the upper three terraces are characterized by extreme podzolization, low pH, low nutrient availability, summer drought (with periodic fires), and winter surface flooding above the hardpan. Dune and cliff soils support a tall redwood and Douglas-fir forest between the terraces. Analyses of seeds collected from 11 pygmy-forest and 6 coastal populations showed ssp. bolanderi to have significantly less allozyme variation than spp. contorta. The two subspecies did not show the phylogenetic dichotomy in allozyme allelic constitutions expected for subspecific classification. Within ssp. bolanderi, the pattern of genetic distances correlated better with edaphic differences among sites than with geographic distance. It appears that ssp. bolanderi is a recently evolved derivative of ssp. contorta, and that the low degree of allozyme differentiation among the bolanderi populations may be due to colonization of the sites by small numbers of individuals, or to hitchhiking of allozyme loci linked to loci undergoing strong selection imposed by the severe edaphic conditions typical of bolanderi sites.     

Glacial vicariance and historical biogeography of rock ptarmigan (Lagopus mutus) in the Bering region

In this paper, we address alternative hypotheses for the evolution of subspecies of rock ptarmigan (Lagopus mutus) endemic to the Aleutian Archipelago. To do this we examined patterns of genetic differentiation among populations of rock ptarmigan in the Aleutian Islands and parts of both Alaska and Siberia. Variation in mitochondrial control region sequences of 105 rock ptarmigan from 10 subspecies within the Bering region revealed three major phylogenetic lineages, two of which are endemic to the Aleutian Islands. Accordingly, haplotype and nucleotide diversities of rock ptarmigan within the archipelago are much higher than within mainland Alaska or Siberia. For Aleutian rock ptarmigan, analyses of molecular variance indicated significant genetic structuring and low estimates of gene flow among populations, despite small interisland distances within the archipelago. However, isolation by distance did not describe the pattern of gene flow or differentiation at this scale. Our estimates of divergence times of lineages suggest that Aleutian rock ptarmigan became isolated prior to the most recent Pleistocene glaciation event (late Wisconsin Stade) and that current patterns of genetic variation reflect the postglacial redistribution of divergent lineages and subsequent limited gene flow. In addition, genetic divergence among lineages was concordant with the distribution of plumage types among subspecies. The patterns of genetic variation described here for rock ptarmigan provide evidence for the role of glacial vicariance in contributing to genetic diversity within this and other Bering region species.     

Rapid divergence and postglacial colonization in western North American Steller's jays (Cyanocitta stelleri)

Post-Pleistocene avian colonization of deglaciated North America occurred from multiple refugia, including a coastal refugium in the northwest. The location of a Pacific Coastal refugium is controversial; however, multiple lines of evidence suggest that it was located near the Queen Charlotte Islands (also known as Haida Gwaii). The Queen Charlotte Islands contain a disproportionately large number of endemic plants and animals including the Steller's jay Cyanocitta stelleri carlottae. Using five highly variable microsatellite markers, we studied population structure among eight populations of Steller's jay (N = 150) from geographical areas representing three subspecies in western North America: C. s. carlottae, C. s. stelleri and C. s. annectens. Microsatellite analyses revealed genetic differentiation between each of the three subspecies, although more extensive sampling of additional C. s. annectens populations is needed to clarify the level of subspecies differentiation. High levels of population structure were found among C. s. stelleri populations with significant differences in all but two pairwise comparisons. A significant isolation by distance pattern was observed amongst populations in the Pacific Northwest and Alaska. In the C. s. carlottae population, there was evidence of reduced genetic variation, higher number of private alleles than northern C. s. stelleri populations and higher levels of divergence between Queen Charlotte Island and other populations. We were unable to reject the hypothesis that the Queen Charlotte Islands served as a refugium during the Pleistocene. Steller's jay may have colonized the Queen Charlotte Islands near the end of the last glaciation or persisted throughout the Pleistocene, and this subspecies may thus represent a glacial relic. The larger number of private alleles, despite reduced genetic variation, morphological distinctiveness and high divergence from other populations suggests that the Queen Charlotte Island colonization pre-dates that of the mainland. Furthermore, our results show rapid divergence in Steller's jay populations on the mainland following the retreat of the ice sheets.     

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     

What can morphology and isozymes tell us about the history of the Dryas integrifolia–octopetala complex?

An earlier study of the morphological variation in the circumpolar genus Dryas in Greenland revealed that populations in east Greenland are highly variable and this variation was interpreted as a hybrid zone between D. octopetala and D. integrifolia. It was proposed that populations of Dryas in east Greenland immigrated postglacially from Svalbard. The purpose of our study was to investigate the population structure of the Dryas integrifolia-octopetala complex using a combination of morphological and genetic characters. Populations of Dryas were sampled in Greenland, Svalbard, Iceland and north Scandinavia (Sweden). The morphological traits clearly separated west Greenland from Svalbard, Iceland and north Scandinavia. A large number of individuals from east Greenland and some from north Greenland could be interpreted as hybrids between the morphs. The genetic pattern was, however, partly different as the greatest amount of differentiation among regions was found between west and north Greenland (FRT = 0.65). The highest degree of genetic variation appeared within east Greenland. This was in accordance with the putative hybrid origin of the populations, as concluded based on the morphological results. The total amount of genetic differentiation found among the six regions (FRT = 0.61) was larger than between the two species (FRT = 0.44). The genetic pattern matched the proposed postglacial migration routes.     

Ecotype differentiation and coexistence of two parapatric tetraploid subspecies of cocksfoot (Dactylis glomerata) in the Alps

Two tetraploid subspecies of Dactylis glomerata L., subsp. reichenbachii (Hausm.) Stebbins et Zohary and subsp. glomerata , occur in the French Alps. The former is confined to dolomitic, south-facing, alpine lawns above 2000 m, whereas the latter occurs in non-dolomitic habitats in subalpine meadows mainly below 1900 m. Previous studies of allozyme variation have shown that genetic introgression between the two subspecies occurs over large areas. By contrast, morphologically intermediate individuals only occur in an extremely narrow area, suggesting that the morphological and physiological differences between the two subspecies is of adaptive significance. A reciprocal clone transplant experiment was set up to examine (1) any genetic differences between subspecies indicative of ecotypic differentiation in relation to habitat characteristics and (2) the level of phenotypic plasticity in the two subspecies. Genetic differentiation was confirmed by a statistically significant taxon × site interaction effect in anova for all traits studied. The glomerata populations produced more tillers, longer leaves and higher culms in all sites, especially in their home environment. However, reichenbachii populations produced more seeds than the glomerata populations in the original reichenbachii environment, suggesting ecotypic differentiation between the two subspecies. This result might also explain why the glomerata subspecies is unable to colonize dolomitic habitats occupied by the reichenbachii subspecies. The reichenbachii populations showed less plasticity than the glomerata populations for leaf length and floriferous tiller number, a result which is discussed in the context of the response of plants from productive and non-productive habitats to environmental variation.     

Patterns of genetic diversity in related taxa of Antirrhinum L. assessed using allozymes

Allozyme diversity was studied within and among populations of five related taxa of Antirrhinum L. endemic to the Iberian Peninsula ( A. graniticum Rothm. ssp. graniticum , ssp. brachycalyx Sutton and ssp. ambiguum (Lange) Mateu & Segarra, A. boissieri Rothm. and A. onubensis (Fdez. Casas) Fdez. Casas). All of the studied taxa are obligate outcrossing endemic perennial herbs which form isolated populations. However, the taxa vary in range and population sizes, and are found on different soil types. The level and distribution of allozyme diversity differed widely between taxa: A. graniticum ssp. brachycalyx had the lowest level of allozyme diversity (H_T = 0.09), whilst the highest level was detected in A. boissieri (H_T = 0.25). Total variation was partitioned into within- and among-population variation. The proportion attributable to variation within populations varied from about 67% up to 84.3% and 89.5% in A. graniticum ssp. brachycalyx and A. graniticum ssp. ambiguum , respectively. Both these subspecies also showed little population divergence (G_ST = 0.10 and 0.09, respectively) and had high levels of estimated gene flow (Nm = 2.18 and 2.62, respectively). These results are discussed in relation to geographical proximity of populations and habitat continuity. Isolation by distance was not detected in any of the studied taxa. This result suggests that divergence among populations is due to random genetic drift.  © 2003 The Linnean Society of London . Biological Journal of the Linnean Society , 2003, 79 , 299–307.     

Concordance of genetic divergence among sockeye salmon populations at allozyme, nuclear DNA, and mitochondrial DNA markers

Abstract.— We examined genetic variation at 21 polymorphic allozyme loci, 15 nuclear DNA loci, and mitochondrial DNA in four spawning populations of sockeye salmon ( Oncorhynchus nerka ) from Cook Inlet, Alaska, to test for differences in the patterns of divergence among different types of markers. We were specifically interested in testing the suggestion that natural selection at allozyme loci compromises the effectiveness of these markers for describing the amount and patterns of gene flow among populations. We found concordance among markers in the amount of genetic variation within and among populations, with the striking exception of one allozyme locus ( sAH ), which exhibited more than three times the amount of among-population differentiation as other loci. A consideration of reports of discordance between allozymes and other loci indicates that these differences usually result from one or two exceptional loci. We conclude that it is important to examine many loci when estimating genetic differentiation to infer historical amounts of gene flow and patterns of genetic exchange among populations. It is less important whether those loci are allozymes or nuclear DNA markers.     

Phylogeography of the Alpine salamander, Salamandra atra (Salamandridae) and the influence of the Pleistocene climatic oscillations on population divergence

Fifty individuals of the endemic Alpine salamander, Salamandra atra, representing 13 populations throughout the range of the two currently recognized subspecies, atra and aurorae, were examined for sequence variation in a large portion (1050 bp) of the mitochondrial cytochrome b gene. We revealed a large number of mitochondrial DNA (mtDNA) haplotypes (10). Interpopulation sequence divergence was very low, ranging from 0 to 3.1%. The relationships among haplotypes were poorly resolved. The divergence time estimate between several mtDNA haplotypes suggested a pre-Pleistocene differentiation approximately 3 million years ago. Moreover, the impact of the Pleistocene glaciations on the phylogeographical patterns appears to have been secondary, although a somewhat reduced genetic variability was found in populations living in areas that were directly affected by the glaciation.     

Allozyme diversity in the endangered pitcher plant Sarracenia rubra ssp. Alabamensis (Sarraceniaceae) and its close relative S. rubra ssp. rubra

Genetic variability in the federally endangered pitcher plant Sarracenia rubra ssp. alabamensis was assessed in eight Alabama populations using starch gel electrophoresis. Ten populations of the more widespread Sarracenia rubra ssp. rubra were sampled in the southeastern United States for comparison. Fifteen allozyme loci representing 13 enzyme systems were scored for each species. In contrast to S. oreophila and S. jonesii, two previously analyzed endangered pitcher plants, genetic diversity was high for both S. rubra subspecies. Within ssp. alabamensis the percentage polymorphic loci (Ps) was 80.0, the mean number of alleles per polymorphic locus was (APs) = 2.58, and expected heterozygosity (Hes) was 0.209. Genetic diversity was slightly lower for ssp. rubra (Ps = 73.3, APs = 2.91, and Hes = 0.177). The proportion of total genetic diversity found among populations was fairly low for both species (GST = 0.09 for ssp. alabamensis and 0.14 for ssp. rubra). Little genetic divergence has occurred between the two subspecies as indicated by the lack of diagnostic alleles, the proportion of total genetic diversity between taxa (GST = 0.09), and the genetic identity estimate (I = 0.90). The relatively high genetic diversity found for ssp. alabamensis indicates that the maintenance of its evolutionary potential is possible if population sizes are maintained or increased. Low levels of genetic diversity found within small Georgia ssp. rubra populations indicate that genetic erosion may increase extinction risks for these populations.     

Genetic structure and indirect estimates of gene flow in three taxa of Cucurbita (Cucurbitaceae) in western Mexico

Cultivated squash (Cucurbita argyrosperma ssp. argyrosperma and C. moschata) are important in the Mexican traditional agroecosystem. They are typically cultivated within maize fields where adjacent populations of a wild, close relative, C. argyrosperma ssp. sororia, occur. Consequently, there are ample opportunities for gene flow between domesticated and free-living Cucurbita populations. We used allozymes to examine genetic variation and gene flow among these three Cucurbita taxa in the state of Jalisco in Western Mexico. Twelve polymorphic allozyme loci were used to calculate genetic diversity for 16 populations of Cucurbita. We found high levels of genetic variation: polymorphism of 0.96, mean allelic diversity of 2.08, average expected heterozygosity 0.407, and little differentiation among conspecific populations (D = 0.081; F(ST) = 0.087; N(e)m = 5.22). These findings indicate that Cucurbita possess a high pollen dispersal potential, a somewhat surprising result considering they have specialist pollinators. Unweighted pair group method with arithmetic means (UPGMA) analysis of allozymes suggests the existence of at least two distinct groups of populations, one consisting of both subspecies of C. argyrosperma and another consisting of C. moschata.     

Genetic variability in Plantago species in relation to their ecology

Summary Genetic variation in leaf and inflorescence morphology and in generative development within the species Plantago major has been analysed by means of crosses between members of two different subspecies. The variable characters chosen are supposed to be important for determining the ecological differences between the subspecies and other ecotypes. The analyses of F₂'s indicated that a substantial number of loci controlling the above mentioned characters are situated near the Pgm-1 locus, forming a gene complex. This gene complex can exist in at least three different forms in ssp. pleiosperma, ssp. major lawn type and ssp. major roadside type, respectively. In addition, some important factors for ecotypic differentiation are situated in the neighbourhood of the Got-1 locus and in a linkage group containing three other allozyme loci. These linkages between allozyme loci and fitness-affecting loci can explain the restriction of some enzyme alleles to a particular subspecies.Grassland Species Research Group Publication No. 50     

Responses of Dryas octopetala to ITEX environmental manipulations: a synthesis with circumpolar comparisons

We have examined organismic responses of Dryas octopetala to simulated changes in the summer climate at four tundra sites as part of the International Tundra Experiment (ITEX). Our study sites are located in the High Arctic, on Svalbard, Norway, in the Low Arctic at Abisko, Sweden, and at Toolik Lake, Alaska, USA and our temperate alpine site is at Niwot Ridge, Colorado, USA. These sites represent a range of tundra temperature and precipitation regimes, being generally cold and dry in the High Arctic and warmer and wetter at Toolik Lake and Niwot Ridge. Results from our studies indicate organismic attributes such as flowering shoot length varies by 30% between low and high arctic populations and that experimental warming results in significant increases in shoot height at three of four sites. We find that phenological development of Dryas is accelerated under experimentally warmed conditions which corresponds with a lengthening of the growing season in autumn, greater degrees of seed set and a higher likelihood of colonization of bare ground. We also observe that Dryas dominated ecosystems which are exposed to experimental manipulations are capable of exhibiting net carbon sequestration in late autumn, and that Dryas photosynthesis and green leaf biomass is significantly greater under warmer as opposed to ambient temperature conditions. Dryas leaf nitrogen is also significantly lowered under warmer conditions resulting in senescent leaves having a higher C:N ratio than those under ambient conditions. Together these findings indicate that Dryas phenology and carbon flux may be altered to the greatest degree in spring and again in autumn by higher summer temperatures and that simultaneously both positive and negative feedback effects may result from changes in plant and ecosystem performance.     

The historical biogeography of two Caribbean butterflies (Lepidoptera: Heliconiidae) as inferred from genetic variation at multiple loci

Mitochondrial DNA and allozyme variation was examined in populations of two Neotropical butterflies, Heliconius charithonia and Dryas iulia. On the mainland, both species showed evidence of considerable gene flow over huge distances. The island populations, however, revealed significant genetic divergence across some, but not all, ocean passages. Despite the phylogenetic relatedness and broadly similar ecologies of these two butterflies, their intraspecific biogeography clearly differed. Phylogenetic analyses of mitochondrial DNA sequences revealed that populations of D. iulia north of St. Vincent are monophyletic and were probably derived from South America. By contrast, the Jamaican subspecies of H. charithonia rendered West Indian H. charithonia polyphyletic with respect to the mainland populations; thus, H. charithonia seems to have colonized the Greater Antilles on at least two separate occasions from Central America. Colonization velocity does not correlate with subsequent levels of gene flow in either species. Even where range expansion seems to have been instantaneous on a geological timescale, significant allele frequency differences at allozyme loci demonstrate that gene flow is severely curtailed across narrow ocean passages. Stochastic extinction, rapid (re)colonization, but low gene flow probably explain why, in the same species, some islands support genetically distinct and nonexpanding populations, while nearby a single lineage is distributed across several islands. Despite the differences, some common biogeographic patterns were evident between these butterflies and other West Indian taxa; such congruence suggests that intraspecific evolution in the West Indies has been somewhat constrained by earth history events, such as changes in sea level.     

Lack of congruence between morphometric evolution and genetic differentiation suggests a recent dispersal and local habitat adaptation of the Madeiran lizard Lacerta dugesii

Genetic differentiation among nine populations of the endemic lizard Lacerta dugesii Milne-Edwards 1829 (Lacertidae) from four groups of islands constituting the Archipelago of Madeira, was investigated by protein electrophoresis at 23 enzyme loci. Among twenty polymorphic loci, the total genetic diversity was due primarily to intra-population variation. The allele and genotypic frequencies among populations showed some heterogeneity, allowing the species to present a structuring pattern compatible with their geographical clustering. Some evidence suggests that selection acting on some loci in different ecological conditions may be responsible for the clustering of the populations studied. There was no apparent isolation effect expected under an "island" model of population divergence, and no correlation was found between genetic and geographic distances among populations. Morphological variation of the proposed three L. dugesii subspecies is not congruent with the allozyme analysis. This most probably suggests a rapid colonization of the islands followed by a strong effect of selection operating over the morphological characters used to define the subspecies.     

Detecting Holocene divergence in the anadromous-freshwater three-spined stickleback (Gasterosteus aculeatus) system

The anadromous-freshwater three-spined stickleback (Gasterosteus aculeatus) system allows for inferring the role of adaptation in speciation with a high level of accuracy because the freshwater ecotype has evolved multiple times from a uniform anadromous ancestor. A cause for concern is that independent evolution among drainages is not guaranteed in areas with a poorly resolved glacial history. This is the case for the west European great rivers, whose downstream valleys flanked the southern limit of the late Pleistocene ice sheet. We tested for independent and postglacial colonization of these valleys hypothesizing that the relationships among anadromous and freshwater sticklebacks correspond to a raceme structure. We compared the reduction in plate number accompanying this colonization to the genetic differentiation using 13 allozyme and five microsatellite loci in 350 individuals. Overall microsatellite differentiation (F(ST) = 0.147) was twice as large as allozyme differentiation (F(ST) = 0.066). Although habitat-specific gene flow may mask the ancestral relationships among both ecotypes, levels of microsatellite differentiation supported the hypothesis of raceme-like divergence, reflecting independent colonizations rather than the presence of two distinct evolutionary clades. Under an infinite alleles model and in the absence of gene flow, the observed freshwater divergence might be reached after 440 (microsatellites) to 4500 (allozymes) generations. Hence, the anadromous-freshwater stickleback system most likely diverged postglacially. We conclude that the reduction in plate number in two freshwater basins probably occurred independently, and that its considerable variation among populations is not in agreement with the time since divergence.     

Morphological differentiation among closely related species with disjunct distributions: a case study of Mediterranean Cyclamen L. subgen. Psilanthum Schwarz (Primulaceae)

In Cyclamen subgenus Psilanthum , previous authors have recognized three species ( C. repandum, C. balearicum and C. creticum ), with C. repandum subdivided into three subspecies (ssp. repandum , ssp. peloponnesiacum and ssp. rhodense ), and two varieties (var. vividum and var. peloponnesiacum ) within C. repandum ssp. peloponnesiacum . To examine the validity of this classification and the degree of morphological differentiation among closely related endemic taxa with disjunct distributions, we undertook analyses of three quantitative and eight qualitative morphological traits of flowers and leaves, most of which have previously been used without statistical comparison in the classification of the different taxa. Cyclamen balearicum showed high levels of morphological differentiation from all other taxa in subgenus Psilanthum consistent with its specific status. In contrast, morphological differentiation between C. creticum and subspecies of C. repandum was similar to that among the three subspecies of C. repandum . When compared with a recent molecular phylogeny of this subgenus, our results suggest that C. creticum may best be described as a geographical subspecies of C. repandum . The two varieties of C. repandum ssp. peloponnesiacum showed few significant differences for individual traits and their overall morphology is very similar to each other. The morphology of plants at three sites on Corsica strongly suggest, consistent with molecular data, the occurrence of hybrid populations between C. balearicum (outside of its previously recognized distribution) and local C. repandum .  © 2002 The Linnean Society of London, Botanical Journal of the Linnean Society , 2002, 139 , 133–144.     

Comparison of genetic diversity in the two arctic–alpine plants Diapensia lapponica var. obovata (Diapensiaceae) and Empetrum nigrum var. japonicum (Empetraceae) between Sakhalin in Russian Far East and Jeju Island in Korea, the southernmost edge of their distribution range

We compared allozyme variation in the two arctic–alpine plants Diapensia lapponica var. obovata and Empetrum nigrum var. japonicum between Sakhalin Island in Russian Far East, within their range core, and the Korean island of Jeju, their world’s southernmost distribution. For D. lapponica var. obovata, Sakhalin populations harbored moderate levels of within-population genetic variation and low among-population divergence, whereas extremely low levels of within-population genetic diversity and high among-population differentiation were found in Jeju Island populations. In contrast, we found moderate levels of within-population variation and low among-population differentiation in E. nigrum var. japonicum in both northern populations (those of Sakhalin and an additional population from northern Japan) and Jeju Island populations. Under a similar scenario of immigration history of arctic–alpine plants on Jeju Island during the glacial periods of the Pleistocene and local persistence through glacial/interglacial cycles, the contrasting genetic structure between D. lapponica var. obovata and E. nigrum var. japonicum is mainly attributable to their different life-history, ecological, and demographic traits: (1) hermaphroditic versus monoecious, dioecious or polygamous, (2) seeds with no adaptations for long-distance dispersal versus berry-like drupes dispersed by animals and birds, and (3) a very small patch near the peak of Mt. Halla with a few hundred individuals versus a relatively continuous distribution around the peak of Mt. Halla with numerous individuals. From a conservation perspective, in situ and ex situ conservation measures should be strengthened for D. lapponica var. obovata on Jeju Island given their extreme rarity there.     

Genetic variation in the allotetraploid Dryopteris corleyi (Dryopteridaceae) and its diploid parental species in the Iberian Peninsula

Studies on genetic diversity help us to unveil the evolutionary processes of species and populations and can explain several traits of diploid-polyploid complexes such as their distributions, their breeding systems, and the origin of polyploids. We examined the allozyme variation of Dryopteris aemula and D. oreades, diploid ferns with highly fragmented habitats, and the allotetraploid D. corleyi to (1) analyze the putative relationship between both diploids and the tetraploid, (2) compare the levels of genetic variation among species and determine their causes, and (3) assess the breeding system of these taxa. The allozymic pattern of D. corleyi confirms that it derived from D. aemula and D. oreades. The lack of genetic diversity in D. aemula, a species of lowland habitats, may be due to genetic drift associated with the contraction of populations in the last glaciation. By contrast, the alpine D. oreades had moderate intrapopulation genetic variation, which may derive from the expansion of populations during the last glaciation. In the latter species, low interpopulational variation suggested effective gene flow (spore exchange), and genotype frequencies in Hardy-Weinberg equilibrium indicated cross-fertilization of gametophytes. Evolutionary history appears to be an essential element in the interpretation of genetic variation of highly fragmented populations.     

Genetic divergence and introgressive hybridization betweenAlnus sinuata andA. crispa (Betulaceae)

The actinorhizal genusAlnus contains numerous taxa that have been morphologically classified into different subgenera, species and subspecies. The genetic divergence has been evaluated within subg.Alnobetula between the parapatric taxaAlnus sinuata andA. crispa, using diversity of allozyme markers at 15 structural loci among 20 populations. Evidence for introgressive hybridization at the overlap of their ranges was noted in three populations. However, the width of the hybrid zone appeared tenuous. The average genetic distance derived from the comparisons of conspecific populations was much smaller than the interspecific distance (D = 0.047). This allelic divergence was also paralleled with larger amounts of allelic and genotypic diversity within and among populations ofA. sinuata, which are occupying a more heterogenous ecological niche. It is proposed that the repeated advances and retreats of the ice sheet during the Pleistocene may have promoted the divergence and allopatric evolution of these subspecies, and that secondary contact may have occurred repeatedly during the interglacial periods. The dynamic-equilibrium model would predict in such cases that narrow hybrid zones, formed at the contact of parapatric ranges, would impede gene exchange between parental taxa by selection against hybrids. The results obtained in this study seemed concordant with this hypothesis, as they were also in agreement with the existent taxonomical treatment of these taxa based on morphology.