Allopatric Differentiation Research Articles

Phylogeography of the genus Cardiandra based on genetic variation in cpDNA sequences

We investigated the phylogenetic relationships within the genus Cardiandra based on plastid DNA sequences. The phylogenetic tree showed that Cardiandra populations from the Ryukyu Islands (Japan) and Taiwan were monophyletic (Ryukyu-Taiwan clade), whereas taxa from China and mainland Japan were sisters to this clade. The divergence time between the Ryukyu-Taiwan clade and the other species was estimated to be 0.082 MYA, i.e., the late Pleistocene. The infrageneric and/or infraspecific differentiation of Cardiandra is estimated to have depended largely on allopatric differentiation caused by the presence or division of the past landbridge of the Ryukyu Islands, which connected mainland Japan to the Asian Continent during the Quaternary.

Panbiogeography of Nothofagus (Nothofagaceae): analysis of the main species massings

AbstractAim The aim of this paper is to analyse the biogeography of Nothofagus and its subgenera in the light of molecular phylogenies and revisions of fossil taxa.Location Cooler parts of the South Pacific: Australia, Tasmania, New Zealand, montane New Guinea and New Caledonia, and southern South America.Methods Panbiogeographical analysis is used. This involves comparative study of the geographic distributions of the Nothofagus taxa and other organisms in the region, and correlation of the main patterns with historical geology.Results The four subgenera of Nothofagus have their main massings of extant species in the same localities as the main massings of all (fossil plus extant) species. These main massings are vicariant, with subgen. Lophozonia most diverse in southern South America (north of Chiloé I.), subgen. Fuscospora in New Zealand, subgen. Nothofagus in southern South America (south of Valdivia), and subgen. Brassospora in New Guinea and New Caledonia. The main massings of subgen. Brassospora and of the clade subgen. Brassospora/subgen. Nothofagus (New Guinea–New Caledonia–southern South America) conform to standard biogeographical patterns.Main conclusions The vicariant main massings of the four subgenera are compatible with largely allopatric differentiation and no substantial dispersal since at least the Upper Cretaceous (Upper Campanian), by which time the fossil record shows that the four subgenera had evolved. The New Guinea–New Caledonia distribution of subgenus Brassospora is equivalent to its total main massing through geological time and is explained by different respective relationships of different component terranes of the two countries. Global vicariance at family level suggests that Nothofagaceae/Nothofagus evolved largely as the South Pacific/Antarctic vicariant in the breakup of a world‐wide Fagales ancestor.

History of the Crested Lark in the Mediterranean region as revealed by mtDNA sequences and morphology

The Crested Lark has a very complex taxonomy, partly as a result of a strong variation in plumage ground color seemingly linked with environmental factors. However, large variations in body size and bill shape further complicate the situation in the Maghreb. In this paper, we first present a set of hypotheses to explain patterns of morphological variation around the Mediterranean Sea. A phylogeographical analysis covering all major biogeographical areas in the species’ range is then performed to test these scenarios. Three mtDNA groups with distinct geographical distribution were identified. The randonii clade (= G. (c.) randonii) is endemic from central Maghreb and is phylogenetically basal relative to cristata and senegallensis. These two latter groups are much more widespread. The cristata clade is found in NW Morocco, throughout Europe and W Asia and in NE Africa, while senegallensis regroups the populations sampled in the Western Sub-Saharan Africa and in NE Maghreb (E Algeria, Tunisia). A combination of genetic and paleoenvironmental evidences supports a scenario of allopatric differentiation of these two lineages outside the Maghreb, with subsequent range expansion leading to their secondary presence in the Maghreb. However, the alternative hypothesis of differentiation in two, or even three separate Maghreb refuges cannot be completely dismissed with the present data. Interestingly, the Sahara desert and the Gibraltar Strait did not act as permanent barriers to dispersal in this species. In addition, the populations in the Maghreb are consistently longer-billed than their closest relatives, suggesting a role for natural selection or phenotypic plasticity.

Mitochondrial DNA provides an insight into the mechanisms driving diversification in the ithomiine butterfly Hyposcada anchiala (Lepidoptera: Nymphalidae: Ithomiinae)

Geographic subspecies of several ithomiine butterflies on the lower east Andean slopes display a black and orange "melanic tiger" aposematic wing pattern that occurs from Colombia to Bolivia, while geographically adjacent lowland subspecies typically bear a coloured, "tiger" pattern. However, it is not clear whether subspecies with similar wing patterns in different regions have arisen through independent events of convergent adaptation, possibly through parapatric differentiation, or result from allopatric differentiation, as proposed by the refuge hypothesis. Here, we examine geographic patterns of divergence in the widespread and common ithomiine butterfly Hyposcada anchiala. We present phylogenetic hypotheses for 5 subspecies of H. anchiala, based on 1567 bp mitochondrial DNA. All topologies indicated that a single switch in mimetic pattern best explained the wing patterning of the H. anchiala studied here. This finding suggests that the subspecies of H. anchiala studied here result from at least two stages of differentiation, and is consistent with a single colonisation into a novel altitudinal zone coincident with a wing pattern switch, followed by subsequent divergence within, rather than across altitudinal zones. The subspecies divergences indicated diversifications were consistent with the Pleistocene. Furthermore, the lowland subspecies were more recently derived than the montane taxa, in contrast to predictions of the "Andean species pump" hypothesis.

Pre‐reproductive isolation as a consequence of allopatric differentiation between populations of Drosophila melanogaster

While pre-reproductive isolations are more and more frequently described between closely related species or within species, very little is known about their conditions of emergence. In Brazzaville, two populations (Kronenbourg and Loua) of Drosophila melanogaster show a premating isolation. Two hypotheses were proposed to explain such a situation: a local sympatric differentiation or an allopatric divergence followed by a secondary contact. A microsatellite analysis, using markers on all chromosomes, strongly suggests that the Kronenbourg population has a European origin. Therefore, the allopatric divergence between Kronenbourg and Loua populations is probably responsible for the sexual isolation observed today in sympatry, after a recent introduction of a European propagule in Brazzaville.

Molecular systematics of the endemic Leptaxini (Gastropoda: Pulmonata) on the Azores islands

The Azorean representatives of the Leptaxini (Pulmonata) are single island endemics, where a high-spired shell distinguishes the monotypic genus Helixena from two slightly different low-spired forms within Leptaxis ( azorica and caldeirarum type). We studied the evolutionary history of putative taxa and the three shell-types using 12 allozyme loci and sequences of nuclear (ITS-1 and ITS-2) and mitochondrial DNA (COI and 16S rRNA). While little variation was found in both ITS genes, allozyme and mtDNA divergence was among the highest reported for pulmonate land snails. Generally, phylogeographic patterns are indicative of allopatric differentiation via the successive colonization of (younger) islands, while a major role for adaptive evolution is not supported. The azorica shell-type is monophyletic and has no common history with other sympatric shell-types on the same islands. The (ambiguous) position of Helixena sanctaemariae makes Leptaxis paraphyletic on the Azores and possibly also the caldeirarum shell-type. Helixena can therefore not be distinguished as a separate genus on the Azores. Following a lineage-based concept, representatives on all (ancient) islands should be considered distinct species.

The importance of time scale and multiple refugia: Incipient speciation and admixture of lineages in the butterfly Erebia triaria (Nymphalidae)

Evidence of four different Quaternary glacial stages has been found in NW Iberia. The different magnitude of these episodes probably conditioned the distribution of currently montane species. We examined if the population history of NW Iberian Erebia triaria butterflies reflected such an influence of different glacial stages. We also investigated whether these populations were post-glacially colonised from a single refugial area or several sources. For this, we performed phylogenetic analyses and coalescence simulations on mitochondrial DNA sequences of individuals from five NW Iberian locations. We analysed three additional populations as reference, i.e., Central Spain, the Pyrenees, and the Alps. One of the NW Iberian populations, a subspecies endemic to a particular mountain range (Xistral), showed a high level of genetic divergence from all other populations, regardless of their geographic distance. Isolation after an ancient glacial stage, and followed by allopatric differentiation, may account for such high differentiation. The genetic pattern shown by the rest of the NW Iberian population samples consisted of two lineages, likely reflecting that the ancestors of these populations sought for refuge in at least two different areas during a subsequent glacial stage. We showed evidence of both temporal and spatial divisions in the phylogeography of these butterflies. The congruence of this pattern with the geological history suggests that the Iberian Peninsula hosted several refugial areas that differed in area and location depending on the glacial stage.

“Species” Radiations of Symbiotic Dinoflagellates in the Atlantic and Indo-Pacific Since the Miocene-Pliocene Transition

Endosymbiotic dinoflagellates, or "zooxanthellae," are required for the survival of a diverse community of invertebrates that construct and dominate shallow, tropical coral reef ecosystems. Molecular systematics applied to this once understudied symbiont partner, Symbiodinium spp., divide the group into divergent lineages or subgeneric "clades." Within each clade, numerous closely related "types," or species, exhibit distinctive host taxon, geographic, and/or environmental distributions. This diversity is greatest in clade C, which dominates the Indo-Pacific host fauna and shares dominance in the Atlantic-Caribbean with clade B. Two "living" ancestors in this group, C1 and C3, are common to both the Indo-Pacific and Atlantic-Caribbean. With these exceptions, each ocean possesses a diverse clade C assemblage that appears to have independently evolved (adaptively radiated) through host specialization and allopatric differentiation. This phylogeographic evidence suggests that a worldwide selective sweep of C1/C3, or their progenitor, must have occurred before both oceans separated. The probable timing of this event corresponds with the major climactic changes and low CO(2) levels of the late Miocene and/or early Pliocene. Subsequent bursts of diversification have proceeded in each ocean since this transition. An ecoevolutionary expansion to numerous and taxonomically diverse hosts by a select host-generalist symbiont followed by the onset of rapid diversification suggests a radical process through which coral-algal symbioses respond and persist through the vicissitudes of planetary climate change.

The close relationship between estimated divergent selection and observed differentiation supports the selective origin of a marine snail hybrid zone

To study the role of divergent selection in the differentiation of the two morphs in a hybrid zone of the intertidal snail Littorina saxatilis, we compared the strength of the divergent selection acting on a series of shell characters (as estimated by the viability of snails in a reciprocal transplant experiment) with the contribution of these characters to the phenotypic differences between the morphs. We found a close correlation between selection and differentiation, which suggests a cause-effect relationship, i.e. that all present differentiation is the result of past divergent selection. In addition, divergent selection was a very important component of the total natural selection acting on shell measures. These novel results support previous evidence, based on allozyme analysis, of a parapatric origin for this hybrid zone. We discuss possible limitations of this interpretation and the circumstances under which allopatric differentiation would produce the same results. Phenotypic analysis of divergent selection may be a useful method of investigating the evolutionary mechanisms involved in differentiation processes.

Italian Cenozoic crocodilians: taxa, timing and palaeobiogeographic implications

Crocodylian remains are collected in 39 fossil-bearing localities but only in seven localities specimens with reliable taxonomic attributions, at least to genus level have been collected. Three species have been reported from the early Lutetian Purga di Bolca site: Pristichampsus cf. Pristichampsus rollinati, Asiatosuchus sp., Hassiacosuchus sp. (= Allognathosuchus sp.). The three crocodilians discovered at Purga di Bolca have been reported also from Geiseltal and Messel (Middle Eocene, Germany). Bolca at that time was part of a Tethysian archipelago and no mammals have been found there till now. Crocodilians and turtles clearly arrived from the European mainland across a marine water barrier. Among the other fossiliferous localities of Veneto, very interesting is the Monte Zuello site, of late Middle Eocene age, yielding a longirostrine crocodilian, Megadontosuchus arduini, a tomistomine species. Tomistomines are known in contemporaneous sediments of both Europe and Africa, but the European forms Dollosuchus and Kentisuchus seem the closest taxa. Remains of Oligocene age have been collected in Veneto and Liguria, but the fossils discovered in the second region are teeth or fragmented bones. The fossil crocodilians of Monteviale (Veneto), of Early Oligocene age, have been assigned to two species but they have been recently all identified as Diplocynodon ratelii, known from several European sites of Late Eocene, Oligocene and Miocene age. This species arrived in the Monteviale area from the European mainland across a narrow sea. Several crocodilian fossils of Miocene age are very fragmentary or represented by isolated teeth. In the Middle and Late Miocene of Sardinia, a well-established species, Tomistoma calaritanum is present. Remains of Tomistoma of the same age have been reported in some localities in Tuscany, Apulia, Sicily and Malta. In the Mediterranean area, the genus is known from European and African sites (of older age). The colonisation of Europe by this genus is the result of a dispersion from Africa (or less probably from Asia). During Late Miocene Sardinia and Tuscany belong to the same palaeobioprovince characterized by the Oreopithecus– Maremmia fauna. In Tuscany, a crocodilian identified as Crocodylus bambolii is present in the late Miocene site of Monte Bamboli. If the generic attribution of this form is correct, its ancestors must have arrived from Africa. Another fossil assemblage of Late Miocene age characterizes the Apulia–Abruzzi palaeobioprovince ( Hoplitomeryx– Microtia fauna) and testifies complete isolation between the two palaeobioprovinces. In this last area, remains of Crocodylus sp. have been collected in coastal sandstones at Scontrone (Abruzzi) and in several fissure fillings of Gargano of slightly younger age. The ancestors of this species arrived from Africa while no African elements are present among the mammalian fauna. The dispersion of the genus Crocodylus in the Italian palaeoislands may have taken place once, with allopatric differentiation of the two populations (Tuscany–Sardinia and Apulia–Abruzzi) or twice with independent colonisation of each area.

Preliminary genetic analysis of koaro (Galaxias brevipinnis) in New Zealand lakes: Evidence for allopatric differentiation among lakes but little population subdivision within lakes

Lake systems are often notable for the diversification of freshwater fish species. This has yet to be documented in New Zealand, although a possible case has been described in Galaxias brevipinnis from Lake Okareka, where lake‐ and stream‐resident fish show morphological and ecological differences. Samples of G. brevipinnis collected from three New Zealand lakes (Wakatipu, Wanaka, and Okareka) were examined using microsatellite DNA analysis to look for evidence of genetic structuring among samples within lakes (possibly indicative of incipient sympatric speciation or natal stream loyalty). Comparison among lakes was made to provide a context for genetic structuring within lakes. There was evidence of genetic differentiation among lake populations, reflecting original founder effect and subsequent genetic drift. However, there was little evidence of restricted gene flow among samples from different parts of lakes and their tributaries. Namely, there is no genetic evidence of incipient speciation in Lake Okareka, and little or no evidence of stream loyalty in Lake Wakatipu. However, the situation in Lake Okareka warrants further analysis using more microsatellite loci and wider sampling.

Phylogeography of Hyphoderma setigerum (Basidiomycota) in the Northern Hemisphere

Previous studies of morphological variation in the homobasidiomycete Hyphoderma setigerum have lead to suspicions of a species complex. This study explores variation in DNA sequences from the nuclear ribosomal ITS region of 45 specimens from America, Asia, and Europe in a phylogeographic context. Based on molecular analysis, morphological studies, and crossing tests, nine preliminary taxa are shown to exist inside the species complex, and the two previously described segregate species H. subsetigerum and H. nudicephalum are confirmed. The molecular analysis shows evidence of allopatric differentiation over intercontinental distances. Only one of the nine well-supported clades has a geographic distribution spanning more than one continent, probably indicating the importance of vicariance in the evolution of this species complex. The basionym of H. setigerum, Thelephora setigera, is neotypified to fix the application of that name.

Evolutionary and biogeographical implications of the karyological variations in the oviparous and viviparous forms of the lizard Lacerta (Zootoca) vivipara

The lizard Lacerta (Zootoca)vivipara has two modes of reproduction and is variable karyologically. We describe its karyological variation from literature data and from new data on two viviparous populations from France, on two oviparous populations from the Pyrenees in south-western France and on three oviparous populations recently discovered in Slovenia. Males have 36 chromosomes, whereas females have only 35 chromosomes in all viviparous populations and in the Pyrenean oviparous populations. This karyotype has been interpreted to result from a fusion of an ancestral sexual W chromosome with an autosome from the Zl or from the Z2 pair. The karyotype formula is 32 autosomes + ZIZ2W for the female and 32 autosomes + Z1Z1Z2Z2 for the male. The karyotype of the Slovenian oviparous populations, 34 autosomes + ZW in the male and 34 autosomes +ZW in the female, represents an evolutionary stage that preceded the chromosomal fusion. There is minor karyological variation, mainly concerning the W and Z2 chromosomes, within the Pyrenean oviparous populations. This parallels the geographic variation of the W-linked alleles of the MPI enzyme and suggests that allopatric differentiation of these oviparous populations might have occurred in the vicinity of the Pyrenees during the Pleistocene. The viviparous populations from western Europe carry a metacentric W chromosome, whereas oviparous populations from south-western Europe and eastern viviparous populations both show an acrocentrie, or a subtelocentrie. W chromosome. This suggests that the acrocentric-subtelocentric W is a primitive character and that viviparity probably arose in an eastern lineage of the species.

Evolutionary and biogeographical implications of the karyological variations in the oviparous and viviparous forms of the lizard Lacerta (Zootoca) vivipara

The lizard Lacerta (Zootoca) vivipara has two modes of reproduction and is variable karyologically. We describe its karyological variation from literature data and from new data on two viviparous populations from France, on two oviparous populations from the Pyrénées in south‐western France and on three oviparous populations recently discovered in Slovenia. Males have 36 chromosomes, whereas females have only 35 chromosomes in all viviparous populations and in the Pyrenean oviparous populations. This karyotype has been interpreted to result from a fusion of an ancestral sexual W chromosome with an autosome from the Z1 or from the Z2 pair. The karyotype formula is 32 autosomes +Z1Z2W for the female and 32 autosomes+Z1Z1Z2Z2 for the male. The karyotype of the Slovenian oviparous populations, 34 autosomes +ZZ in the male and 34 autosomes +ZW in the female, represents an evolutionary stage that preceded the chromosomal fusion. There is minor karyological variation, mainly concerning the W and Z2 chromosomes, within the Pyrenean oviparous populations. This parallels the geographic variation of the W‐linked alleles of the MPI enzyme and suggests that allopatric differentiation of these oviparous populations might have occurred in the vicinity of the Pyrénées during the Pleistocene. The viviparous populations from western Europe carry a metacentric W chromosome, whereas oviparous populations from south‐western Europe and eastern viviparous populations both show an acrocentric, or a subtelocentric, W chromosome. This suggests that the acrocentric‐subtelocentric W is a primitive character and that viviparity probably arose in an eastern lineage of the species.

Intraspecific Phylogeography of Lacerta vivipara and the Evolution of Viviparity

The lacertid lizard Lacerta vivipara is one of the few squamate species with two reproductive modes. We present the intraspecific phylogeny obtained from neighbor-joining and maximum-parsimony analyses of the mtDNA cytochrome b sequences for 15 individuals from Slovenian oviparous populations, 34 individuals from western oviparous populations of southern France and northern Spain, 92 specimens from European and Russian viviparous populations, and 3 specimens of the viviparous subspecies L. v. pannonica. The phylogeny indicates that the evolutionary transition from oviparity to viviparity probably occurred once in L. vivipara. The western oviparous group from Spain and southern France is phylogenetically most closely related to the viviparous clade. However, the biarmed W chromosome characterizing the western viviparous populations is an apomorphic character, whereas the uniarmed W chromosome, existing both in the western oviparous populations and in the geographically distant eastern viviparous populations, is a plesiomorphic character. This suggests an eastern origin of viviparity. Various estimates suggest that the oviparous and viviparous clades of L. vivipara split during the Pleistocene. Our results are discussed in the framework of general evolutionary models: the concept of an oviparity–viviparity continuum in squamates, the cold climate model of selection for viviparity in squamates, and the contraction–expansion of ranges in the Pleistocene resulting in allopatric differentiation.

Intraspecific variability in Peniophora aurantiaca (Basidiomycetes)

Intraspecific, genetic variation was studied in Peniophora aurantiaca using the internal transcribed spacer regions of the nuclear ribosomal RNA region (ITS). Specimens originating from Canada, Greenland and Switzerland were compared. The ITS sequence data were similar with only 11 phylogenetically informative characters, but an allopatric differentiation was obvious. Overall variability is high within the North American specimens compared with the Swiss specimens which had almost no variable characters.

A phylogenetic study of the genus Schizopora (Basidiomycota) based on ITS DNA sequences

A phylogenetic analysis based on internal transcribed spacer regions of the nuclear ribosomal RNA region (ITS) sequences has been undertaken for species in the genus Schizopora. Four known species were studied, viz. S. radula, S. paradoxa, S. flavipora, S. nothofagi, and a hitherto undescribed species from Taiwan. Allopatric differentiation was apparent in S. radula, for which several geographically representive samples were available. Among these, ITS sequence data obtained from S. radula isolates from British Columbia and Australasia were identical. All previously recognised Schizopora species were well separated from each other with respect to their ITS sequences. Mating compatibility tests also showed complete reproductive separation between species, but intercompatibility between cultures from the same species, irrespective of geographical origin. Overall, the morphology of basidiomes of S. radula from New Zealand differs only in minor respects from those of European and Argentinean origin, but shows a higher degree of variability than described for European specimens.

The allopatric phase of speciation: the sharp-beaked ground finch (Geospiza difficilis) on the Galápagos islands

Using microsatellite DNA variation, morphological measurements and sonagrams made from tape-recordings in the field, we examine the allopatric differentiation of six populations of the sharp-beaked ground finch, Geospiza difficilis, in the Galápagos archipelago. We ask how and why the populations became differentiated, and consider what the differences imply about speciation. An important factor is time G. difficilis is one of the phylogenetically oldest species. Populations became differentiated by adapting in beak morphology to different food supplies. Since beak size and shape also function in conspecific mate recognition and choice, the potential for reproductive isolation from sister and parental taxa arose as a correlated effect of natural selection on resource-exploiting traits. This conforms to a standard explanation for the origin of pre-mating isolation as a byproduct of adaptive changes in allopatry without being adaptive itself. However, this explanation is incomplete. Vocal characteristics also differentiated, partly as a result of natural and sexual selection independent of beak size change in environments with different sound transmitting properties. An additional element of chance is indicated by a comparison of closely-related populations on two islands, Wolf and Darwin, that are geographically close, and topographically and floristically similar. The populations have markedly different vocalizations. Morphological characters, vocalizations and genetic traits do not vary concordantly among the six populations. One population (Genovesa) is genetically more similar to other congeneric species. Phenotypic similarities with G. difficilis are the result of a uniquely long retention of shared ancestral traits. Arguments under the phylogenetic species concept justify recognizing this population as a separate species, Geospiza acutirostris. Under the biological species concept it should remain as currently classified,G. difficilis .

The allopatric phase of speciation: the sharp-beaked ground finch (Geospiza difficilis) on the Galápagos islands

Abstract Using microsatellite DNA variation, morphological measurements and sonagrams made from tape-recordings in the field, we examine the allopatric differentiation of six populations of the sharp-beaked ground finch, Geospiza difficilis , in the Galapagos archipelago. We ask how and why the populations became differentiated, and consider what the differences imply about speciation. An important factor is time G. difficilis is one of the phylogenetically oldest species. Populations became differentiated by adapting in beak morphology to different food supplies. Since beak size and shape also function in conspecific mate recognition and choice, the potential for reproductive isolation from sister and parental taxa arose as a correlated effect of natural selection on resource-exploiting traits. This conforms to a standard explanation for the origin of pre-mating isolation as a byproduct of adaptive changes in allopatry without being adaptive itself. However, this explanation is incomplete. Vocal characteristics also differentiated, partly as a result of natural and sexual selection independent of beak size change in environments with different sound transmitting properties. An additional element of chance is indicated by a comparison of closely-related populations on two islands, Wolf and Darwin, that are geographically close, and topographically and floristically similar. The populations have markedly different vocalizations. Morphological characters, vocalizations and genetic traits do not vary concordantly among the six populations. One population (Genovesa) is genetically more similar to other congeneric species. Phenotypic similarities with G. difficilis are the result of a uniquely long retention of shared ancestral traits. Arguments under the phylogenetic species concept justify recognizing this population as a separate species, Geospiza acutirostris. Under the biological species concept it should remain as currently classified, G. difficilis .

POPULATION GENETIC STRUCTURE AND GENE FLOW ACROSS ARID VERSUS MESIC ENVIRONMENTS: A COMPARATIVE STUDY OF TWO PARAPATRIC SENECIO SPECIES FROM THE NEAR EAST.

To shed light on the potential effects of xeric/arid versus mesic environments on plant population genetic structure and patterns of gene flow, we have compared allozyme and cpDNA haplotype variation in populations of two closely related, highly outcrossed, and largely wind-dispersed winter annuals of Senecio (Asteraceae). The species form a distinctive zone of parapatric distribution in the Near East by differing in their ecogeographical regimes. Senecio vernalis mainly thrives in the mesic Mediterranean life zone of Israel, whereas S. glaucus inhabits either xeric maritime or arid (semi-) desert sites. Significant differences in allozymic population subdivision among S. vernalis (θn = 0.04; Nmn = 5.85) and S. glaucus (θn = 0.12; Nmn = 1.85) largely resulted from topogeographical substructuring present within the latter species. Because of the similarity of within-region estimates of population structure for S. glaucus with those measured among populations of S. vernalis, it appears unlikely that ecological "aridity" factors per se are important in influencing levels of population differentiation in these species. Based on hierarchical F-statistics and tests of isolation by distance, we further conclude that geographical topologies influence the level and mode of nuclear gene flow (via pollen and/or seed) among and within subsets of S. glaucus populations, although without providing a complete barrier to interregional dispersal (dNmreg = 2.16) and without promoting allopatric differentiation via drift. The allozymic data further suggested that S. vernalis and S. glaucus form a zone of secondary contact in the Near East, accompanied by an almost complete interspecific barrier to nuclear gene flow (dnNmsp = 0.253). However, to account for the considerable sharing of cpDNA haplotypes, both at the intra- and interspecific level, it is necessary to invoke either (1) selection acting against alien nuclear but not cytoplasmic DNA; or (2) the sporadic immigration of cpDNA via seed with large homogenizing effects on cytoplasmic population structure over time.