Parapatric Distribution Research Articles

THE CONTRIBUTION OF EDAPHIC HETEROGENEITY TO THE EVOLUTION AND DIVERSITY OF BURSERACEAE TREES IN THE WESTERN AMAZON

Environmental heterogeneity in the tropics is thought to lead to specialization in plants and thereby contribute to the diversity of the tropical flora. We examine this idea with data on the habitat specificity of 35 western Amazonian species from the genera Protium, Crepidospermum, and Tetragastris in the monophyletic tribe Protieae (Burseraceae) mapped on a molecular-based phylogeny. We surveyed three edaphic habitats that occur throughout terra firme Amazonia: white-sand, clay, and terrace soils in eight forests across more than 2000 km in the western Amazon. Twenty-six of the 35 species were found to be associated with only one of three soil types, and no species was associated with all three habitats; this pattern of edaphic specialization was consistent across the entire region. Habitat association mapped onto the phylogenetic tree shows association with terrace soils to be the probable ancestral state in the group, with subsequent speciation events onto clay and white-sand soils. The repeated gain of clay association within the clade likely coincides with the emergence of large areas of clay soils in the Miocene deposited during the Andean uplift. Character optimizations revealed that soil association was not phylogenetically clustered for white-sand and clay specialists, suggesting repeated independent evolution of soil specificity is common within the Protieae. This phylogenetic analysis also showed that multiple cases of putative sister taxa with parapatric distributions differ in their edaphic associations, suggesting that edaphic heterogeneity was an important driver of speciation in the Protieae in the Amazon basin.

The community context of species’ borders: ecological and evolutionary perspectives

Species distributional limits may coincide with hard dispersal barriers or physiological thresholds along environmental gradients, but they may also be influenced by species interactions. We explore a number of models of interspecific interactions that lead to (sometimes abrupt) distribution limits in the presence and absence of environmental gradients. We find that gradients in competitive ability can lead to spatial segregation of competitors into distinct ranges, but that spatial movement tends to broaden the region of sympatry between the two species, and that Allee effects tend to sharpen these boundaries. We generalize these simple models to include metapopulation dynamics and other types of interactions including predator–prey and host–parasite interactions. We derive conditions for range limits in each case. We also consider models that include coevolution and gene flow and find that character displacement along environmental gradients can lead to stable parapatric distributions. We conclude that it is essential to consider coevolved species interactions as a potential mechanism limiting species distributions, particularly when barriers to dispersal are weak and environmental gradients are gradual.

Diploid and polyploid cytotype distribution inMelampodium cinereumandM. leucanthum(Asteraceae, Heliantheae)

Previous chromosomal studies within Melampodium (Asteraceae, Heliantheae) of Mexico and Central America have documented chromosome numbers n = 9, 10, 11, 12, 18, 20, 23, 25 ± 1, 27, 30, and 33. Some species also have been shown to exhibit infra- and interpopulational polyploidy. The presence of cytotype mixtures is especially pronounced in the white-rayed complex, which occurs in the southwestern United States and adjacent Mexico. This group includes M. cinereum (n = 10 and 20), M. leucanthum (n = 10 and 20), and M. argophyllum (n = 30). Cytotype distribution has been newly analyzed in 415 plants from 152 populations and added to existing data from 185 plants from 113 populations, yielding information from a total of 600 individuals from 265 populations. Within M. cinereum and M. leucanthum are parapatric distributions of cytotypes, with tetraploids centered in the eastern and diploids in the western portions of their ranges. Tetraploids are most likely of autopolyploid origin, forming recurrently, with adaptations that allow colonization and establishment in new ecological regions. Contact zones are relatively narrow and only two triploid individuals have been detected. The tetraploid cytotypes probably extended eastward into central and southern Texas to the natural barriers at the edge of the Edward's Plateau in M. leucanthum and the low sandy plains in M. cinereum. The hexaploid M. argophyllum is interpreted as a relict surviving in the low mountains of northern Mexico; it may be an allopolyploid of hybrid origin between ancestors of the evolutionary lines that eventually yielded M. cinereum and M. leucanthum.

Population dynamics across a parapatric range boundary: Ulex gallii and Ulex minor

Summary 1 Ulex gallii and U. minor show a parapatric range distribution across southern England. Their population dynamics were explored along an environmental gradient that encompasses the range limits of the two species. Matrix population models were constructed to provide estimates of population growth rate (λ) at various sites. 2 Contrary to expectation no decline in λ was observed for populations of either species at their distribution limit. Density-independent models gave values considerably in excess of unity at the range margin of both species, indicating potential for expansion into each other's range. 3 Inclusion of density-dependent seedling emergence and flowering within the matrix models alters the relative importance of the elasticity values, emphasizing the role played by the mortality of adults in determining population size and structure, through modulating the flux of individuals from the seedling stage through the intermediate size classes. 4 Estimates of λ obtained from the generalized density-dependent model run at low density were approximately 22% and 63% higher for U. gallii and U. minor , respectively, than the minimum λ -values obtained from the site-specific density-independent models. 5 The predicted stable stage distribution differs between the density-independent and equilibrium models (approximately log-linear vs. U-shaped). Size structures seen in the field were intermediate between the two extremes, suggesting that the observed populations are subject to density-dependence but are not at equilibrium.

Molecular phylogeny of the lionfish genera Dendrochirus and Pterois (Scorpaenidae, Pteroinae) based on mitochondrial DNA sequences

This study investigates the molecular phylogeny of seven lionfishes of the genera Dendrochirus and Pterois. MP, ML, and NJ phylogenetic analysis based on 964 bp of partial mitochondrial DNA sequences (cytochrome b and 16S rDNA) revealed two main clades: (1) “ Pterois” clade ( Pterois miles and Pterois volitans), and (2) “ Pteropterus–Dendrochirus” clade (remainder of the sampled species). The position of Dendrochirus brachypterus either basal to the main clades or in the “ Pteropterus–Dendrochirus” clade cannot be resolved. However, the molecular phylogeny did not support the current separation of the genera Pterois and Dendrochirus. The siblings P. miles and P. volitans are clearly separated and our results support the proposed allopatric or parapatric distribution in the Indian and Pacific Ocean. However, the present analysis cannot reveal if P. miles and P. volitans are separate species or two populations of a single species, because the observed separation in different clades can be either explained by speciation or lineage sorting. Molecular clock estimates for the siblings P. miles and P. volitans suggest a divergence time of 2.4–8.3 mya, which coincide with geological events that created vicariance between populations of the Indian and Pacific Ocean.

A spatially explicit individual-based model of reinforcement in hybrid zones.

An individual-based model consisting of two dioecious populations in a two-dimensional environmental grid was constructed. Each population began with, and never exceeded, 1000 individuals; extinction was allowed. Genomes consisting of 30 biallelic loci for male sexual advertisement call, female mate preference, and population origin were constructed, and lineages of each individual in the starting populations were followed for 2000 generations. Type and level of hybrid disadvantage, initial population distribution, patchiness of environmental resources, and level of mate choice were varied. Persistence of bimodal hybrid zones was nonexistent at low levels of hybrid disadvantage and universal at high levels of hybrid disadvantage, with a narrow threshold in which persistence was unpredictable. Persistence occurred at lower levels of hybrid disadvantage when populations were initially parapatric rather than sympatric, and environments were patchy rather than homogeneous. Increased divergence in mating systems occurred when hybrid disadvantage was high, hybrids were infertile, populations were initially parapatric, and increased female choice was allowed. Mating system divergence was much higher in interacting populations compared with noninteracting populations, indicating that reinforcement caused most of the observed divergence. When hybrids were infertile, reinforcement contributed to speciation, because under hybrid infertility the probability of persistence at low levels of hybrid disadvantage was positively related to mate choice. The results agree with previous one-dimensional spatial models in finding that population persistence is more likely in parapatric and patchy population distributions. In addition, the results show that hybrid infertility may facilitate the process of reinforcement and speciation.

A SPATIALLY EXPLICIT INDIVIDUAL-BASED MODEL OF REINFORCEMENT IN HYBRID ZONES

An individual-based model consisting of two dioecious populations in a two-dimensional environmental grid was constructed. Each population began with, and never exceeded, 1000 individuals; extinction was allowed. Genomes consisting of 30 biallelic loci for male sexual advertisement call, female mate preference, and population origin were constructed, and lineages of each individual in the starting populations were followed for 2000 generations. Type and level of hybrid disadvantage, initial population distribution, patchiness of environmental resources, and level of mate choice were varied. Persistence of bimodal hybrid zones was nonexistent at low levels of hybrid disadvantage and universal at high levels of hybrid disadvantage, with a narrow threshold in which persistence was unpredictable. Persistence occurred at lower levels of hybrid disadvantage when populations were initially parapatric rather than sympatric, and environments were patchy rather than homogeneous. Increased divergence in mating systems occurred when hybrid disadvantage was high, hybrids were infertile, populations were initially parapatric, and increased female choice was allowed. Mating system divergence was much higher in interacting populations compared with noninteracting populations, indicating that reinforcement caused most of the observed divergence. When hybrids were infertile, reinforcement contributed to speciation, because under hybrid infertility the probability of persistence at low levels of hybrid disadvantage was positively related to mate choice. The results agree with previous one-dimensional spatial models in finding that population persistence is more likely in parapatric and patchy population distributions. In addition, the results show that hybrid infertility may facilitate the process of reinforcement and speciation.

Evolution in Hawaiian cave-adapted isopods (Oniscidea: Philosciidae): vicariant speciation or adaptive shifts?

We assessed evolutionary relationships among Hawaiian cave-adapted isopods using a maximum-likelihood criterion to analyze cytochrome oxidase I nucleotide sequences. Results support morphological data that two genera of philosciid isopods have invaded caves independently in the islands. In the genus Littorophiloscia, a sister relationship between a surface-dwelling species, L. hawaiiensis, and an undescribed cave species was corroborated. This evidence, along with the known parapatric distributions between species, supports a speciation event by an adaptive shift on the island of Hawaii from a marine littoral to a terrestrial subterranean habitat. The monophyletic genus Hawaiioscia contains four known obligate cave-dwelling species, each of which occurs on a separate island. However, despite present-day allopatric distributions between Hawaiioscia species, the geographic and phylogenetic patterns are not sufficient to support a vicariant mode of speciation. Instead, we believe that the known species of Hawaiioscia evolved from a widespread ancestral surface species or a group of closely related species through multiple, independent adaptive shifts on each of the islands of Kauai, Oahu, Molokai, and Maui. This is the first molecular investigation of evolutionary relationships between surface-dwelling and cavernicolous arthropods in Hawaii and it suggests that simple vicariance is insufficient to explain the evolution of troglobites in tropical zones.

DOES DIFFERENTIAL SUSCEPTIBILITY TO PREDATION IN TADPOLES STABILIZE THE BOMBINA HYBRID ZONE?

Despite substantial divergence, the European toads Bombina bombina and Bombina variegata (Anura: Discoglossidae) interbreed freely wherever their parapatric distributions adjoin. Natural selection that stabilizes the resulting hybrid zones should rest in part on the adaptation to different breeding habitat of the pure taxa. While B. bombina lays its eggs in semipermanent ponds, B. variegata is a typical puddle breeder. Here, we investigate whether selection for rapid larval development in B. variegata has resulted in the loss of effective antipredator defenses, thus excluding this species from predator-rich ponds. We collected adults from four populations in Romania (two for each taxon) and reared the offspring from four crosses per population in the laboratory either in the presence or in the absence of caged odonate predators (Aeshna cyanea). In predation trials, we found no taxon difference in mortality rate among tadpoles that had been reared with predators. The resilience of B. variegata tadpoles may have been due to their remarkable phenotypic plasticity. In both taxa, predator presence led to the development of a higher tail fin, which has been shown to reduce predation rates in other amphibians. This response was much stronger in B. variegata than in B. bombina. Moreover, differences between the two B. variegata populations in terms of laboratory predation rates and levels of plasticity correlated with predator abundance at the collection sites so as to suggest local adaptation in predator defenses. Finally, delayed metamorphosis in the predator-induced morphs of both taxa implies a cost to the defense. Given the heterogeneity of temporary habitat in terms of desiccation rate and predator occurrence, the greater amount of phenotypic plasticity in B. variegata fits predictions of life history theory. At the same time, our results leave the question unresolved as to why this species avoids ponds.

The vertical distribution of pronophiline butterflies (Nymphalidae, Satyrinae) along an elevational transect in Monte Zerpa (Cordillera de Mérida, Venezuela) with remarks on their diversity and parapatric distribution

AbstractAim The distribution of neotropical butterflies of the tribe Pronophilini (Lepidoptera, Satyridae) was analysed with the aim of determining (i) the elevational ranges of distribution of each species (ii) the elevational gradient in diversity, and (iii) the existence of parapatric distributions of some closely related species pairs.Location The field study was carried out in the middle and upper section of the valley of the Río Albarregas, a tributary of the Río Chama on the southern slopes of the Serranía de la Culata, in the central part of the Cordillera de Mérida, Venezuela.Methods The material was collected along an elevational transect set on a trail leading from Merida to the Páramo de Los Conejos in the area known as Monte Zerpa. It consisted of a series of 32 collection sites set at every 25 m of altitude and covered an elevation from 2250 m to 3025 m.Results The ranges of Lymanopoda obsoleta, L. albocincta, L. zapatoza, Corades chelonis, C. pannonia, C. medeba, Mygona irmina, Pedaliodes epidipnis, P. japhleta, P. montagna and P. panyasis were all restricted to the lower half of the cloud forest zone and L. diezti, C. pax, P. polla, P. ornata and P. ferratilis were all restricted to the upper part. Two species, Erethris porphyria and Steroma bega, crossed the entire elevational range of the cloud forest zone.The biodiversity, as measured by Shannon’s index, attained a maximum at 2700 m, beyond which it gradually decreased. A parapatric distribution was confirmed for three pairs of closely related species: L. obsoleta and L. diezti, C. chelonis and C. pax and P. montagna and P. ferratilis.Main conclusions The peak in diversity at middle elevations seems to result from the overlapping ranges of species distributed over lower and upper parts of the cloud forest. Correlation of species composition at particular transect sites with elevation showed that increase in elevation was associated with an increase in species composition difference. Post‐mating isolation was suggested as a primary factor responsible for maintaining the parapatric distributions of species occupying the upper and lower zones and preventing their respective distributions from expanding.

Evidence for historical introgression along a contact zone between two species of char (Pisces: Salmonidae) in northwestern North America.

Phylogeographic analyses can yield valuable insights into the geographic and historical contexts of contact and hybridization between taxa. Two species of char (Salmonidae), Dolly Varden (Salvelinus malma) and bull trout (S. confluentus) have largely parapatric distributions in watersheds of northwestern North America. They are, however, sympatric in several localities and hybridization and some introgression occurs across a broad area of contact. We conducted a comparative phylogenetic analysis of Dolly Varden and bull trout to gain a historical perspective of hybridization between these species and to test for footprints of historical introgression. We resolved two major Dolly Varden mitochondrial DNA (mtDNA) clades (with 1.4-2.2% sequence divergence between haplotypes) that had different geographical distributions. Clade N is distributed across most of the range of Dolly Varden, from southern British Columbia through to the Kuril Islands in Asia. Clade S had a much more limited distribution, from Washington state, at the southern limit of the Dolly Varden range, to the middle of Vancouver Island. The distribution and inferred ages of the mtDNA clades suggested that Dolly Varden survived the Wisconsinan glaciation in a previously unsuspected refuge south of the ice sheet, and that Dolly Varden and bull trout were probably in continuous contact over most of the last 100,000 years. When bull trout were included in the phylogenetic analysis, however, the mtDNA of neither species was monophyletic: Clade S Dolly Varden clustered within the bull trout mtDNA clade. This pattern was discordant with two nuclear phylogenies produced (growth hormone 2 and rRNA internal transcribed sequence 1), in which Dolly Varden and bull trout were reciprocally monophyletic. This discordance between mtDNA- and nDNA-based phylogenies indicates that historical introgression of bull trout mtDNA into Dolly Varden occurred. Percent sequence divergence within these introgressed Dolly Varden (clade S) was 0.2-0.6%, implying that the introgression occurred prior to the most recent glaciation. Our analysis and other evidence of contact between divergent lineages in northwestern North America strongly suggests that the area may be the site of previously unsuspected suture zones of aquatic biotas.

Distribution of Karyotypes of the <I>Cryptocercus punctulatus</I> Species Complex (Dictyoptera: Cryptocercidae) in the Southern Appalachians: Relation to Habitat and History

The distributional pattern of the four known karyotypes (male 2n = 37, 39, 43, 45) of the Cryptocercus punctulatus Scudder species complex is reported, based on 71 sites in the Southern Appalachian Mountains with an emphasis on western North Carolina. Populations with different karyotypes are geographically structured in a mosaic, with at least one karyotype occurring in two disjunct regions. Abrupt geographic transitions between karyotypes suggest a parapatric distribution. We found no overlap in the distribution of the different karyotypes, as recently suggested. Although the boundary zones between karyotypes do not appear to coincide with physical or ecological barriers to dispersal, several transitions between karyotypes occur on or near the highest mountains in the southern Appalachians. We suggest that the different karyotypes arose by vicariance, with current boundaries formed by secondary contact when populations isolated in glacial refugia subsequently spread into high-mountain habitats. Because of their dependence on mature mesic forests, populations of the cockroach likely advance up and down mountainsides in cycles of advances and retreats dictated by climatic oscillations that raise and lower the timberline. We discuss the taxonomic status of the different karyotype groups in the C. punctulatus complex, and conclude that more exacting evidence is required to establish if species-level status is warranted. The conclusions of certain earlier studies are weak because, among other things, karyology was not examined in the sampled specimens, including those designated as types.

The geographical distribution of populations of the large-billed subspecies of reed bunting matches that of its main winter food

We investigated the effects of resource distribution on the population structure and distribution of a polytypic bird species. We compared the presence of insect remains (mainly dormant larvae) in the winter diet of two reed bunting subspecies, small billed E. s. schoeniclus and large billed E. s. intermedia and studied the distribution of this resource within reed (Phragmites sp.) stems in seven north Italian localities where the two subspecies breed (three schoeniclus and four intermedia populations). We also tested if the distributions of winter insect resources and breeding populations of the large bill subspecies overlapped. The distribution of the larvae in reed stems matched closely the distribution of large billed breeding populations. The winter diets of the two subspecies were significantly different in terms of frequency of insect remains. These results, when compared to theoretical models of para-patric distribution, suggest that the two subspecies may be subject to ecological (vicariant) selection maintaining their reproductive isolation.

Reproductive assurance and the evolutionary ecology of self‐pollination in Clarkia xantiana (Onagraceae)

The reproductive assurance hypothesis posits that selection favors self-pollination in flowering plants where mates and/or pollinators are scarce. A corollary is that self-pollinating populations are expected to be superior colonizers of mate- and pollinator-scarce environments. The California annual Clarkia xantiana includes outcrossing populations (ssp. xantiana) and autogamously self-pollinating populations (ssp. parviflora). Outcrossing is ancestral, and the subspecies have parapatric distributions with a narrow contact zone. We tested aspects of the reproductive assurance hypothesis by examining geographic and subspecies variation in the densities of mates and pollinators (native bees) and the density dependence of pollinator visitation and pollen receipt. Plant and flower densities, pollinator density, and pollinator visitation rates were lowest in the region of exclusively self-pollinating populations. Pollinator assemblages there lacked Clarkia-associated pollinator taxa that were common elsewhere. Self-pollinating populations in the contact zone generally had densities and visitation rates intermediate between allopatric self-pollinating populations and outcrossing populations. Visitation rate and pollen receipt increased significantly with plant density. These findings suggest that selection for reproductive assurance influenced the origin of self-pollination and/or that reproductive assurance influenced the geographic distribution of self-pollination. Geographic variation in pollinator assemblages may have generated variation in the value of reproductive assurance.

Intraspecific variation and evolutionary trends in Capparis spinosa L. (Capparaceae)

In order to investigate the variability of the polymorphic Capparis spinosa L., a comparative study was carried out in Sicily on subsp. spinosa and subsp. rupestris. Autecology, phenology, quantitative morphology, plant architecture and leaf development of several populations were examined. These data reveal a parapatric distribution of the two subspecies and support their present taxonomic treatment. Subsp. spinosa is widespread on clay soils and shows remarkable intrapopulational variation. It is characterized by shortened reproductive and vegetative periods, high shoot growth rate, winter-desiccating shoot system and stipular thorns. Subsp. rupestris, less variable and linked with carbonatic and volcanic outcrops, shows prolonged phenological patterns, slow growth rate, woody habit and caducous stipules. A noteworthy phenotypic convergence with subsp. rupestris was observed in individuals of subsp. spinosa growing in rocky habitats. The evolutionary trends of Capparisspinosa in the Mediterranean region are discussed.

Allozyme segregation and inter-cytotype reproductive barriers in the polyploid complex Centaurea jacea.

In eastern Belgium, diploid and tetraploid knapweeds (Centaurea jacea L. s. l.) show a parapatric distribution with a contact zone. To compare the success of inter- and intra-cytotype crosses, and to investigate the pattern of allozyme segregation, controlled crosses involving plants from this contact zone were performed. Polysomic inheritance at two loci was observed in tetraploids, suggesting an autopolyploid origin. Two crosses allowed the detection of double reduction events in tetraploids at one locus, but no such event was detected among the 217 progenies. Null alleles were detected in tetraploids at two loci. Both cytotypes were highly self-incompatible. Inter-cytotype crosses were much less successful than intra-cytotype crosses in terms of seed set (five-fold reduction) and germination rates (three-fold reduction), suggesting a 'triploid block' effect. Progenies from inter-cytotype crosses most often had the maternal cytotype and resulted from selfing or pollen contamination. However, six triploids were detected. The significance of these results for the understanding of the geographical distribution of the cytotypes and inter-cytotype gene flow is discussed.

Phylogeography of lateral plate dimorphism in the freshwater type of ninespine sticklebacks, genus Pungitius

The freshwater type of ninespine sticklebacks, genus Pungitius, is widely distributed in northern Japan and reproductively isolated from other genetically divergent types endemic to small regions in Japan. This type expresses dimorphism in its lateral plate morphology: complete and partial row morphs. The two morphs show a parapatric distribution in Japan. To clarify the process involving the distribution of these two morphs, we examined their phylogeography based on restriction fragment length polymorphism of an entire mitochondrial DNA (mtDNA). The survey was carried out with seven restriction enzymes on the populations of the freshwater type collected from 41 localities across the distribution range in Japan, and 6 further Pungitius populations from the Okhotsk Sea coast of Russia were appended. An unweighted pair-group method with arithmetic averages (UPGMA) tree among 54 mtDNA haplotypes resolved eight clustering groups that differed in sequence divergence by approximately 1.3%–2.1%. Two of the eight groups were found only in Russia. mtDNA phylogenies constructed by neighbor-joining and Wagner parsimony methods suggested that the haplotypes of each plate morph were polyphyletic. The geographic distribution pattern of these groups suggests that they should be classified into two broad categories, one with extensive distribution and the other with localized distribution of the constituent haplotypes within a group. The former groups were found mainly in the populations with the completely plated morph and the latter groups with the partially plated morph. It is supposed that twice dispersals of dimorphic or complete plated ancestors and genetic differentiation during the interglacial played an important role in the formation of the present distribution of the two morphs in Japan.

Geographical Variations in Chromosomes of the Greater Japanese Shrew-Mole, Urotrichus talpoides (Mammalia: Insectivora)

Karyotypes of the greater Japanese shrew-mole, Urotrichus talpoides (Talpidae), collected from 57 localities in Japan were studied by conventional, G- and C-band staining techniques to elucidate geographical chromosomal variations, the mechanism of changes in chromosomes, and the distribution patterns of karyotypic races. Shrew-mole samples examined could be divided geographically into two slightly divergent chromosomal forms designated as the western and eastern races. These two karyotypic races had the same diploid number (34) and fundamental number (64), but they had autosomes with different makeups. A comparison of conventional karyotypes showed a distinct intraspecific variation in shape of autosomal pair no. 14 which was classified as subtelocentric in the western race and as metacentric in the eastern race. G- and C-banding analyses revealed that karyotypic variation found in no. 14 pair was involved in pericentric inversion and quantitative changes in constitutive heterochromatin. Intraspecific and geographical variation in chromosomes caused by such karyological events is rare and unique among members of the family Talpidae examined so far. Furthermore, our results demonstrated that the clear boundary between the two parapatric karyotypic races was actually located along the Kurobe-Fuji line in the central part of Honshu, but not along the Owari-Tsuruga isthmian line previously postulated by Tsuchiya (1987, 1988). Zoogeographical implications of the boundary of parapatric distribution in U. talpoides are also discussed.

Speciation in Chitaura grasshoppers (Acrididae: Oxyinae) on the island of Sulawesi: colour patterns, morphology and contact zones

We investigate colour pattern and morphological variation in Chitaura grasshoppers on the Indonesian island of Sulawesi, and examine the relationship between divergence in these two sets of characters and population history, as reflected by variation in mitochondrial DNA. Analysis of colour pattern variation identifies a total of 21 distinct Operational Taxonomic Units in Sulawesi, the majority of which have parapatric distributions. Patterns of phenotypic variation at two contact zones in north Sulawesi suggest genetic independence between three of these colour forms, indicating that speciation has occurred. Despite this, colour pattern divergence is only coincident with morphological differentiation at one of these contact zones. In addition, neither type of phenotypic divergence is associated with geographical structuring in mitochondrial DNA, suggesting that historical isolation has had little influence on evolutionary diversification. Instead, divergence in colour pattern and morphology appears to have occurred rapidly or under conditions of continued gene flow, possibly in response to spatially variable natural selection. This result has implications for the identification of conservation units based solely on molecular markers.

Sympatry and hybridization between the clawed frogs Xenopus laevis laevis and Xenopus muelleri (Pipidae)

Though most Xenopus species are polyploid and can be hybridized in the laboratory, natural hybridization seems to be restricted to hybrids of X. laevis laevis×X. gilli. A parapatric distribution pattern between X. l. laevis and X. muelleri seems to prevent interbreeding. We found X. l. laevis and X. muelleri living sympatrically in water bodies in the Republic of South Africa west of Kruger Park and report a natural hybrid of X. l. laevis×X. muelleri. Natural occurrence of such a hybrid has not been reported previously. The hybrid character was confirmed by analysis of its advertisement calls, which were intermediate between those of the parental species, by analysis of its serum albumin pattern, and by determination of the nuclear DNA content of its erythrocytes. The results indicate that this specimen is an F1 hybrid between X. l. laevis and X. muelleri. Backcrossing with both parental species indicated that the hybrid is sterile. It is discussed why sympatry between X. l. laevis and X. muelleri though possible might be rare.