Is the last glaciation the only relevant event for the present genetic population structure of the meadow brown butterfly Maniola jurtina (Lepidoptera: Nymphalidae)?

Phylogeographical studies are available for a considerable number of European species, but few analyses exist for temperate species with very large and fairly continuous populations that are also absent from Northern Europe. Therefore, we studied the butterfly Maniola jurtina as a model for this group. The species has two major genetic lineages (mean genetic distance between lineages: 0.033; F _CT: 0.052), most probably evolving in glacial differentiation centres in the western and eastern Mediterranean. The onset of this differentiation might have been the beginning of the last glacial stage maximum some 40 kyr bp . A hybrid zone between these two lineages exists in western Central Europe. No genetic substructures have been found within the two lineages ( F _SC: 0.017) and average genetic distances are very small. Therefore, it is highly probable that postglacial expansion was of the phalanx type. There is, at most, very limited differentiation at regional and local scales. However, the genetic diversity within populations is high (means: A : 2.68; H _E: 17.2%; P : 78%), as would be predicted for such a common species. Comparison of these results with a published allozyme analysis revealed a similar phylogeographical pattern, but lower genetic diversity in the latter. Morphological patterns of wings and genitalia show similar geographical patterns as allozyme data.   © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 85 , 419–431.     

Genetic differentiation of the marbled white butterfly, Melanargia galathea, accounts for glacial distribution patterns and postglacial range expansion in southeastern Europe

Isolation of Mediterranean species in the southern European peninsulas during the cold glacial phases often resulted in differentiation of several genetic lineages confined to the respective peninsulas. However, whilst there is good genetic evidence for multiple refugia in Iberia, there are only limited data available for the Balkans. Therefore, we wish to examine the hypothesis of a strong genetic structuring within southeastern Europe for the existence of multiple Balkan differentiation centres and/or several leading edges. As a model we use the marbled white butterfly, Melanargia galathea. We studied 18 allozyme loci of 564 individuals from 16 populations distributed over a large part of southeastern Europe. The single populations showed moderately high genetic diversity and no northward decline of genetic diversity was detected. The overall genetic differentiation between populations was considerable (F(ST) 7.0%). Cluster analysis discriminated three genetic groups: (i) a western flank in the former Yugoslavia, parts of eastern Austria and Hungary; (ii) an eastern flank with populations from Bulgaria and Romania (south of the southern Carpathians and eastern Carpathians); and (iii) the eastern Carpathian Basin. Hierarchical variance analysis distributed 53% of the variance among populations between these three groups. One sample from the Greek-Bulgarian border clustered within the eastern flank, but showed some tendency towards the eastern Carpathian Basin populations. Two populations from Carinthia clustered together with the eastern Carpathian Basin ones and a population from Styria showed an intermediate genetic composition between the three groups. Most probably, the eastern and the western flank groups are due to postglacial range expansion from the northeastern and the northwestern edges of the glacial differentiation centre (so-called leading edges). The eastern Carpathian Basin group may have resulted from postglacial expansion from northern Greece through valley systems of the central Balkan peninsula, maybe even expanding westwards north of the Balkan mountains reaching some parts of eastern Austria (e.g. Carinthia). Therefore, the Balkanic refugium of M. galathea may or may not have been continuous along the coastal areas of the Mediterranean, but must have been strongly genetically structured.     

Effects of recent and past climatic shifts on the genetic structure of the high mountain Yellow‐spotted ringlet butterfly Erebia manto (Lepidoptera,Satyrinae): a conservation problem

Mountain species have evolved important genetic differentiation due to past climatic fluctuations. The genetic uniqueness of many of these lineages is now at risk due to global warming. Here, we analyse allozyme polymorphisms of 1306 individuals (36 populations) of the mountain butterfly Erebia manto and perform Species Distribution Models (SDMs). As a consensus of analyses, we obtained six most likely genetic clusters: (i) Pyrenees with Massif Central; (ii) Vosges; (iii–v) Alps including the Slovakian Carpathians; (vi) southern Carpathians. The Vosges population showed the strongest genetic split from all other populations, being almost as strong as the split between E. manto and its sister species Erebia eriphyle. The distinctiveness of the Pyrenees‐Massif Central group and of the southern Carpathians group from all other groups is also quite high. All three groups are assumed to have survived more than one full glacial–interglacial cycle close to their current distributions with up‐hill and down‐slope shifts conforming climatic conditions. In contrast with these well‐differentiated groups, the three groups present in the Alps and the Slovakian Carpathians show a much shallower genetic structure and thus also should be of a more recent origin. As predicted by our SDM projections, rising temperatures will strongly impact the distribution of E. manto. While the populations in the Alps are predicted to shrink, the survival of the three lineages present here should not be at risk. The situation of the three other lineages is quite different. All models predict the extinction of the Vosges lineage in the wake of global warming, and also the southern Carpathians and Pyrenees‐Massif Central lineages might be at high risk to disappear. Thus, albeit global warming will therefore be unlikely to threaten E. manto as a species, an important proportion of the species’ intraspecific differentiation and thus uniqueness might be lost.     

Limited hybridization along a large contact zone between two genetic lineages of the butterfly Erebia medusa (Satyrinae, Lepidoptera) in Central Europe

Genetic lineages evolving during glacial isolation frequently come into contact as the result of postglacial range expansions. Hybridization often occurs along these contact zones. In Europe, the high mountain systems of the Alps and Pyrenees are well known for their hybrid belts. This article studies the contact zone of the Woodland Ringlet Erebia medusa in a Hercynian mountain area in the Czech-German border region not exceeding 1500 m a.s.l. Hybrid populations between an eastern and a western genetic lineage were detected by amova -based tests, principal component analysis and neighbour joining analysis. Over most of the range of the Czech-German Border Mts, the ridges separate the western and the eastern genetic lineage from each other. However, two important hybrid areas were detected: (1) the watershed of the Ohře river in the north-west of that area, a major valley system passing through these mountains and (2) the high plateaux of the Šumava Mts in the south-east, an extended area of high elevation. The location of hybrid populations in geographical vicinity to non-hybrid populations and the generally low F _IS (2.1%) make reduced fitness of hybrid individuals little likely. The hybrid populations have intermediate genetic diversity between the genetically poor western and the genetically rich eastern lineage populations.     

Genetic implications of phylogeographical patterns in the conservation of the boreal wetland butterfly Colias palaeno (Pieridae)

The boreo‐montane wetland butterfly species Colias palaeno has a European distribution from the Alps to northern Fennoscandia. Within its European range, the species’ populations have shrunk dramatically in recent historical times. Therefore, detailed baseline knowledge of the genetic makeup of the species is pivotal in planning potential conservation strategies. We collected 523 individuals from 21 populations across the entire European range and analyzed nuclear (20 allozyme loci) and mitochondrial (600 bp of the cytochrome c oxidase subunit I gene) genetic markers. The markers revealed contrasting levels of genetic diversity and divergence: higher in allozymes and lower in mitochondrial sequences. Five main groups were identified by allozymes: Alps, two Czech groups, Baltic countries, Fennoscandia, and Poland. The haplotype mitochondrial network indicates a recent range expansion. The most parsimonious interpretation for our results is the existence of a continuous Würm glacial distribution in Central Europe, with secondary disjunction during the Last Glacial Maximum into a south‐western and a north‐eastern fragment and subsequent moderate differentiation. Both groups present signs of postglacial intermixing in the Czech Republic. However, even a complete extinction in this region would not considerably affect the species’ genetic basis, as long as the source populations in the Alps and in northern Europe, comprising the most relevant evolutionary units for conservation, are surviving.     

Low diversity but high differentiation: the population genetics of Aglaope infausta (Zygaenidae: Lepidoptera)

Aim Aglaope infausta is a thermophilous Zygaenid of Atlanto-Mediterranean origin, distributed in Portugal, Spain, France and north-western Italy reaching its north-eastern distribution limit in western Germany. The local, regional and inter-regional genetic structure of this species is studied in this analysis. Location and methods The allozymes of 456 individuals from 12 populations (11 from western Germany and one from southern Portugal) were studied by electrophoresis. Results Six of the 19 loci analysed were monomorphic. Genetic differentiation between populations was high (F_ST: 0.404), while the mean genetic diversity was low (H_e: 3.4%). Most (96.5%) of the genetic variance between populations was between the Portuguese and the German samples, but also the differentiation within Germany was considerable (F_SR: 0.101). In Germany, A. infausta occurs in two major regions (middle Rhine and Nahe) that are geographically separated, and 55.5% of the genetic variance was found between these two regions. The populations of both areas do not differ in their genetic diversity, but those of the middle Rhine have significantly higher genetic distances among them than the Nahe populations (0.020 and 0.015, respectively). F_ST was also higher in the middle Rhine region than in the Nahe region (0.089 and 0.045, respectively). Main conclusions Aglaope infausta shows a very low level of genetic heterogeneity for a lepidopteran species. However, this genetic poverty is not affecting the species’ viability. During the ice ages, differentiation into two genetic lineages occurred, most probably in a south-western and a south-eastern differentiation centre in Iberia. The gap in the distribution range in Germany is clearly reflected in the genetic structure. This differentiation must have developed relatively quickly because western Germany most probably was colonized during the climatic optimum 6000 years ago.     

Multiple differentiation centres of a non-Mediterranean butterfly species in south-eastern Europe

Aim  The analysis of the phylogeographical structures of many European species reveals the importance of Mediterranean glacial refugia for many thermophilic species, but also underlines the relevance of extra-Mediterranean glacial differentiation centres for a number of temperate species. In this context, phylogeographical analyses of species from south-eastern Europe are highly important for a comprehensive understanding of Europe as a whole.
Location  Romania and Bulgaria.
Methods  We analysed 19 allozyme loci for 615 individuals of the temperate butterfly species Erebia medusa from 28 populations.
Results  These populations had an intermediate genetic diversity, but the Bulgarian populations were significantly more diverse than the ones north of the Danube in Romania. The differentiation among populations was strong, and 52.1% of the genetic variance among populations was distributed between these two countries. The genetic differentiation was considerably stronger in Romania than in Bulgaria, but several sublineages were distinguished within each of these countries.
Main conclusions  The observed genetic structure is so strong that it is most probably the result of glacial differentiation processes in south-eastern Europe and not a post-glacial structure. The strong differentiation into the two groups north and south of the Danube suggests a separating effect by this river valley. The strong differentiation accompanied with genetic impoverishment in Romania suggests the existence of several differentiation centres: at least two small ones on the southern slopes of the southern Carpathians and one in the eastern Carpathian Basin. The considerably weaker differentiation among the Bulgarian samples and their significantly higher genetic diversity imply that gene flow occurred among different regions of Bulgaria during the last ice age.     

Genetic differentiation of Diplodus sargus (Pisces: Sparidae) populations in the south-west Mediterranean

Allozyme analysis of tissue samples of 1249 white sea bream Diplodus sargus from five localities of the south-west Mediterranean revealed a high degree of genetic polymorphism. The observed heterozygosity ranged from 0.4182 (Cape of Palos) to 0.3138 (Tabarca). Several populations were characterized by unique alleles. Examination of the spatial structure was performed using Nei's distances and F- statistics, and indicated genetic differences between groups. One group, which clustered Tabarca and Guardamar, could be explained by the small geographical distance between them. Mazarrón and Cape of Palos samples showed genetic divergence from other samples (Guardamar, Tabarca and Águilas) and this difference may be as a result of local current systems and larval dispersal.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 82 , 249–261.     

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.     

Genetic differentiation between Australian and North American populations of the monarch butterfly Danaus plexippus (L.) (Lepidoptera: Nymphalidae): an exploration using allozyme electrophoresis

Allozyme analysis was used to address the question of the source of the Australian populations of the monarch butterfly Danaus plexippus (L.). The study had three major aims: (1) To compare the levels of diversity of Australian and Hawaiian populations with potential source populations. (2) To determine whether eastern and western North American populations were sufficiently divergent for the Australian populations to be aligned to a source population. (3) To compare the differentiation among regions in Australia and North America to test the prediction of greater genetic structure in Australia, as a consequence of reduced migratory behaviour. The reverse was found, with F _ST values an order of magnitude lower in Australia than in North America. Predictably, Australian and Hawaiian populations had lower allelic diversity, but unexpected higher heterozygosity values than North American populations. It was not possible to assign the Australian populations to a definitive source, although the high levels of similarity of Australian populations to each other suggest a single colonization event. The possibility that the Australian populations have not been here long enough to reach equilibrium is discussed. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 75 , 437–452.     

Strong genetic differentiation and postglacial origin of populations in the marine midge Clunio marinus (Chironomidae,Diptera)

The marine midge Clunio marinus (Chironomidae, Diptera) is characterized by a one‐dimensional distribution along the European Atlantic coast, where its lunar and circadian emergence rhythms are genetically adapted to the local tidal regimes, resulting in a series of ‘temporal races’. Clunio marinus is restricted to rocky coasts and thus the temporal races occur in different rocky patches. We studied 10 populations of Clunio marinus from five different regions, spanning the major rocky mainland coasts from Spain to Norway, using amplified fragment length polymorphisms (AFLP), microsatellites and mitochondrial cytochrome oxidase I (COI) sequences. Star‐like patterns of COI haplotypes within regions indicate postglacial colonization. A high degree of shared polymorphisms in AFLP markers suggests colonization from a single source, implying postglacial evolution of timing adaptations in relation to the local tidal regime. In contrast, no COI haplotypes are shared among regions. We hypothesize that different levels of differentiation of nuclear vs. mitochondrial markers in the source region were carried forward during postglacial expansion. Despite the recent origin of populations, all markers reveal distinct genetic differentiation between rocky coasts on a scale of 650–1150 km. Differentiation between rocky coasts is not correlated to timing adaptations, suggesting that geographic isolation is prevalent between rocky coasts and that this facilitated the evolution of local timing adaptations. At the same time there is little genetic differentiation within rocky coasts on a scale of 2–6 km; leaving open the possibility that within rocky coasts with large variation in tidal regimes, temporal adaptations evolved in the face of gene flow.     

Genetic differentiation of white sea bream within the Lion's Gulf and the Ligurian Sea (Mediterranean Sea)

An allozymic study was conducted on 190 individuals of white sea bream, Diplodus sargus sampled on five locations throughout the Lion's Gulf and the Ligurian Sea (Mediterranean Sea), in order to identify genetic structure. Electrophoretic surveys carried out on muscle and liver tissues identified 25 loci of which 12 were polymorphic ( P 0.95). G -test analysis shows significant differences on allelic frequencies between the five stations at six loci. Examination of the spatial structure was performed using Nei's distances and F statistics, and indicated significant genetic differences between three groups. A group which clustered Blanes (Spain), Marseille (France) and Livorno (Italy) where the absence of genetic difference can be explained by migration of larvae and adults along a coastal 'continuum'. The Elba sample (Italy) shows genetic divergence from other samples and this difference may result from isolation due to limited migration of larvae and adults. Banyuls (France), differs from all other stations. Several hypotheses are proposed to explain genetic patterns including local current systems, larval dispersal, geographic isolation and historical effects, and variation in the size classes of sampled individuals between sites.     

Genetic differentiation in Hippocrepis emerus (Leguminosae): allozyme and DNA fingerprint variation in disjunct Scandinavian populations

The structure of genetic variation in disjunct Scandinavian populations of Hippocrepis emerus was studied using allozymes and DNA fingerprinting. Variation in the three native regional populations in Scandinavia was compared with that in a recently introduced population in Sweden. In contrast to the recently introduced population, the native Scandinavian isolates of H. emerus showed high levels of allozyme fixation and low levels of DNA diversity. Variation in allozymes and at DNA fingerprint loci showed closely congruent patterns of geographic variation, with pronounced differentiation between the native Norwegian and Swedish isolates of the species. The structure of genetic variation in native Scandinavian H. emerus is interpreted in terms of historical population bottlenecks and founder events during the species' postglacial immigration into Scandinavia.     

Isozymes in macroalgae (seaweeds): genetic differentiation,genetic variability and applications in systematics

Use of isozymes (including allozymes) in studies of population genetics and systematics of seaweeds has increased sufficiently in the last decade to allow some generalization. Only a single locus has been observed for about half the enzymes analysed in seaweeds, compared with 29% in vascular plants. Compared with higher plants, macroalgal species generally have low amounts of electrophoretically detectable genetic variation; the lowest levels of genetic variation found in natural populations are those reported for seaweeds. Nonetheless, seaweeds show an association between levels of genetic diversity as revealed by isozymes and species-specific attributes, such as mating system and predominance of asexual versus sexual reproduction. In systematic studies, isozymes have revealed cryptic species and identified pairs of sibling taxa. The quaternary structure of enzymes appears to be conserved at the phylum level. With the current availability of improved techniques for enzyme electrophoresis and for data interpretation, we expect future studies utilizing isozyme electrophoresis to provide further insight into population and evolutionary processes in seaweeds.     

Genetic and geographic differentiation in the Rio Negro tuco-tuco (Ctenomys rionegrensis): inferring the roles of migration and drift from multiple genetic markers

Among tuco-tucos, Ctenomys rionegrensis is especially amenable to the study of the forces driving population differentiation because of the restricted geographic range it occupies in Uruguay. Within this limited area, the Rio Negro tuco-tuco is limited to sandy soils. It nonetheless exhibits remarkable variation in pelage color, including melanic, agouti, and dark-backed individuals. Two hypotheses have been put forth to explain this pattern: (1) local differentiation and fixation of alternative pelage types by genetic drift under limited gene flow; or (2) fixation by natural selection that may take place even in the presence of gene flow. A previous allozyme study rejected the genetic drift hypothesis on the basis of high inferred levels of migration. New estimates of gene flow from microsatellites and mitochondrial cytochrome b sequences were obtained for C. rionegrensis populations to further test these hypotheses. Much lower levels of gene flow were estimated with these more sensitive markers. Microsatellite-based estimates of gene flow are close to zero and may come closest to estimating current levels of migration. A lack of equilibrium between migration and genetic drift is also strongly suggested by the absence of an isolation-by-distance pattern found in all three genetic datasets. The microsatellite genotype data show that the species is strongly structured geographically, with subpopulations constituting distinct genetic entities. If current levels of gene flow are very low, as indicated by the new data, the local fixation of alternative alleles, including those responsible for pelage color polymorphism, is possible by drift alone. A scenario is thus proposed in which the species expanded in the recent past from a more restricted geographic range and has subsequently differentiated in near isolation, with genetic drift possibly playing a primary role in overall genetic differentiation. The local fixation of pelage color types could also be due to drift, but selection on this trait cannot be ruled out without direct analysis.     

The effect of geographic range and dichogamy on genetic variability and population genetic structure in Tricyrtis section Flavae

Populations of each of the four species of Tricyrtis sect. Flavae were sampled using enzyme electrophoresis to examine the effect of geographic range and dichogamy on the genetic diversity of the species. The most widespread species, T. nana, had the lowest level of genetic diversity at both the population and the species level. The depauperate genetic diversity at the population level of T. nana appears to result from the high self-fertilization of the species. The low genetic diversity at the species level of T. nana probably resulted from the bottleneck effect during the speciation process in which this species diverged from the progenitor species, T. flava. Genetic differentiation among populations was high in both adichogamous T. nana and protandrous T. flava. High self-fertilization in T. nana and the colonizing nature of T. flava are likely the main factors causing the differentiated population genetic structure. In contrast to a previous study on chloroplast DNA (cpDNA) variation in Tricyrtis sect. Flavae, T. nana was most closely related to T. flava, which corresponds to the morphological resemblance of both species.     

Genetic differentiation of a European caddisfly: past and present gene flow among fragmented larval habitats

We describe the genetic structure of a freshwater insect species and interpret it in terms of present-day population dynamics and possible postglacial colonization history. The sampling regime represented a large area of the species range in northwest Europe, particularly focusing on Britain, a region relatively neglected in molecular population genetic studies. Plectrocnemia conspersa generally showed low levels of genetic variation across the sampled populations (Nei's D = 0.0138) and subdivision was unrelated to the pattern of the drainage network. However, the results do suggest that populations across the region are not at equilibrium and that British populations still show effects of the recolonization of the species following the last glacial maximum. Levels of genetic diversity were lower in Britain than in mainland Europe. Two-dimensional scaling showed genetic differentiation between major regions and the pattern of genetic diversity indicates a more recent origin of populations in the north and west of the area compared with the south and east. We argue that, despite the highly fragmented larval habitat, dispersal over tens of kilometres is frequent. Over longer distances, however, P. conspersa does still show evidence of founder effects and postglacial range expansion into Britain.     

Population differentiation and phylogeographic pattern of a relict species, Conandron ramondioides (Gesneriaceae), revealed from sequence polymorphism and haplotypes of the CYCLOIDEA gene

Historical events are expected to affect population genetic differentiation and DNA molecular evolution,but the impact of these effects remains a matter of debate.Here,for Conandron ramondioides (Gesne...     

Maintenance of genetic differentiation across a transition zone in the sea: discordance between nuclear and cytoplasmic markers

To investigate the origin and maintenance of the genetic discontinuity between Atlantic and Mediterranean populations of the common sea bass (Dicentrarchus labrax) we analysed the genetic variation at a fragment of mitochondrial cytochrome b sequence for 18 population samples. The result were also compared with new or previously published microsatellite data. Seven mitochondrial haplotypes and an average nucleotidic divergence of 0.02 between Atlantic and Mediterranean populations that matches a Pleistocene allopatric isolation were found. The frequency variation at the cytochrome b locus was many times greater between Atlantic and Mediterranean populations (theta(C) = 0.67) than at microsatellite loci (theta(N)= 0.02). The examination of the different evolutionary forces at play suggests that a sex-biased hybrid breakdown is a likely explanation for part of the observed discrepancy between mitochondrial and nuclear loci. In addition, an analysis is made of the correlation between microsatellite loci points towards the possible existence of a hybrid zone in samples from the Alboran Sea.     

Historical vicariance and postglacial colonization effects on the evolution of genetic structure in Lophocereus,a Sonoran Desert columnar cactus

Distinguishing the historical effects of gene migration and vicariance on contemporary genetic structure is problematic without testable biogeographic hypotheses based on preexisting geological and environmental evidence. The availability of such hypotheses for North America's Sonoran Desert has contributed to our understanding of the effect of historical vicariance and dispersal events on the diversification of this region's vertebrate biota but have not yet been applied to its flora. In this paper we describe a detailed allozyme analysis of the population genetic structure and phylogeography of the Sonoran Desert columnar cactus, Lophocereus schottii (senita). Inferred phylogroup distributions reflect two historical vicariance events: (1) a middle Pliocene northward transgression of the Sea of Cortéz that is reflected in well-supported Baja California peninsular and continental phylogroups but not in current taxonomic treatments of the species; and (2) a late Pliocene transpeninsular seaway across southern Baja that is reflected in tentative support for peninsular and southern Cape Region phylogroups corresponding to taxonomic varieties L. schottii var. schottii and L. schottii var. australis, respectively. A middle Pleistocene midpeninsular seaway hypothesized to explain congruent phylogroup distributions in several vertebrate taxa is not reflected in L. s. var. schottii, nor is the distinction of a third variety, L. s. var. tenuis, from continental populations of L. s. var. schottii. Linear regression of pairwise estimates of interpopulation differentiation (M and F(ST)/[1 - F(ST)]) on interpopulation geographic distance revealed significant evidence of isolation by distance within peninsular and continental phylogroups but not between them, consistent with historical vicariance between but not within these regions. We also found significant evidence of isolation by distance between putative L. s. var. schottii and L. s. var. australis phylogroups, suggesting that reproductive isolation between peninsular and Cape Region forms is incomplete. Within peninsular, but not continental, phylogroups, northward range expansion from southern Pleistocene refugia is reflected in significant declines in genetic variation with increasing latitude and in an area phenogram in which populations are progressively nested from south (ancestral) to north (descendant) along the Baja peninsula. Although the geographic concordance of phylogenetic topologies suggests that ancient vicariance events, and not dispersal, have primarily influenced the biogeographic distributions of Baja's vertebrate biota, the phylogeographic structure of L. schottii suggests that Sonoran Desert plant species may exhibit genetic signatures of postglacial range expansion and gene flow as well as vicariance.