Mitochondrial differentiation, introgression and phylogeny of species in the Tegenaria atrica group (Araneae: Agelenidae)

The relationships between the three members of the Tegenaria atrica group ( T. atrica , T. saeva and T. gigantea ) were examined with DNA sequence data from mitochondrial CO1, 16S rRNA, tRNA^leu(CUN) and ND1 genes. Members of this group of large house spiders have overlapping distributions in western Europe and hybridize with each other to a variable degree. The close relatedness of all three species was supported by all analyses. T. saeva and T. gigantea are more closely affiliated than either is to T. atrica . Haplotypes clearly assignable to T. gigantea were also present in many specimens of T. saeva , suggesting asymmetrical introgression of mtDNA from T. gigantea into T. saeva . Molecular clock calibrations (CO1) suggest that deeper divisions within the genus Tegenaria may be in excess of 10 million years old, and that the evolutionary history of the T. atrica group has been moulded by Quaternary glacial–interglacial cycles.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 81 , 79–89.     

What can hybrid zones tell us about speciation? The case of Heliconius erato and H. himera (Lepidoptera: Nymphalidae)

To understand speciation we need to study the genetics and ecology of intermediate cases where interspecific hybridization still occurs. Two closely related species of Heliconius butterflies meet this criterion: Heliconius himera is endemic to dry forest and thorn scrub in southern Ecuador and northern Peru, while its sister species, H. erato , is ubiquitous in wet forest throughout south and central America. In three known zones of contact, the two species remain distinct, while hybrids are found at low frequency. Collections in southern Ecuador show that the contact zone is about 5 km wide, half the width of the narrowest clines between colour pattern races of H. erato. The narrowness of this dine argues that very strong selection (s ≅ 1) is maintaining the parapatric distributions of these two species. The zone is closely related with a habitat transition from wet to dry forest, which suggests that the narrow zone of parapatry is maintained primarily by ecological adaptation. Selection on colour pattern loci, assortative mating and hybrid inviability may also be important. The genetics of hybrids between the two species shows that the major gene control of pattern elements is similar to that found in previous studies of H. erato races, and some of the loci are homologous. This suggests that similar genetic processes are involved in the morphological divergence of species and races. Evidence from related Heliconius supports a hypothesis that ecological adaptation is the driving force for speciation in the group.     

Multiple routes to interspecific territoriality in sister species of North American perching birds

Behavioral interference between species can influence a wide range of ecological and evolutionary processes. Here, we test foundational hypotheses regarding the origins and maintenance of interspecific territoriality, and evaluate the role of interspecific territoriality and hybridization in shaping species distributions and transitions from parapatry to sympatry in sister species of North American perching birds (Passeriformes). We find that interspecific territoriality is pervasive among sympatric sister species pairs, and that interspecifically territorial species pairs have diverged more recently than sympatric noninterspecifically territorial pairs. None of the foundational hypotheses alone explains the observed patterns of interspecific territoriality, but our results support the idea that some cases of interspecific territoriality arise from misdirected intraspecific aggression while others are evolved responses to resource competition. The combination of interspecific territoriality and hybridization appears to be an unstable state associated with parapatry, whereas species that are interspecifically territorial and do not hybridize are able to achieve extensive fine- and coarse-scale breeding range overlap. In sum, these results suggest that interspecific territoriality has multiple origins and impacts coexistence at multiple spatial scales.     

Dispersal in cactophilic Drosophila

Three species of Drosophila each breed in necrotic tissue of specific columnar cacti endemic to the Sonoran Desert. Drosophila pachea breeds in senita ( Lophocereus schottii) , D. nigrospiracula breeds in saguaro ( Carnegiea gigantea ) or cardón ( Pachycereus pringlei ), and D. mojavensis uses organ pipe ( Stenocereus thurberi ) in Sonora, Mexico and southern Arizona. Patches of these three host cacti have very different spatial distributions, with those of senita being quite frequent and close together, while those of the other hosts are much father apart. Testing all three species simultaneously, we used capture-mark-release-recapture methods to ask if dispersal differed in these species and if differences were those predicted by the spatial availability of the host patches. D. pachea dispersed the shortest distance in all experiments. Furthermore, D. pachea was the only species showing sex-biased dispersal, with male flies exhibiting the greater propensity to disperse. The observations suggest that across similar spatial scales, D. pachea should show greater population genetic structure than the other two species, and that mitochondrial DNA, because of its maternal inheritance, might show greater evidence of structure than nuclear markers.     

Unexpected hybridization patterns in Near Eastern terrapins (Mauremys caspica,M. rivulata) indicate ancient gene flow across the Fertile Crescent

Recent studies indicate that hybridization in animals occurs more frequently than previously thought and that it may play an important evolutionary role. Chelonians are capable of extensive hybridization, raising the question how chelonian species evolve and maintain genetic integrity despite hybridization. Here, we use two sister species with parapatric distribution, Mauremys caspica and M. rivulata, as our model. These taxa are estimated to have diverged some 5.3–7.0 million years ago. Using rangewide sampling and 13 unlinked polymorphic microsatellite markers, five nuclear loci and one mitochondrial gene, we show that hybridization is rare along the contact zone of the two species in Turkey. However, we discovered an unexpected hybrid swarm in the southern Levant that has been hitherto identified with M. rivulata. This hybrid swarm is separated from the inland species M. caspica by a 700‐km‐wide distribution gap corresponding to the Syrian Desert. Ecological palaeomodelling suggests that during more humid climatic episodes in the Last Glacial Maximum and mid‐Holocene, the current contact zone extended into the southern Levant, facilitating the establishment of the now isolated hybrid swarm. Our results support that there is not necessarily a general hybridization pattern in a given species couple and that the extent of gene flow may differ considerably in different parts of the distribution range. Moreover, our results highlight that studies on hybridization should not focus only on extant contact and hybrid zones, but should use rangewide sampling to detect signals of ancient hybridization that might otherwise be missed.     

Species distribution models contribute to determine the effect of climate and interspecific interactions in moving hybrid zones

Climate is a major factor delimiting species’ distributions. However, biotic interactions may also be prominent in shaping geographical ranges, especially for parapatric species forming hybrid zones. Determining the relative effect of each factor and their interaction of the contact zone location has been difficult due to the lack of broad scale environmental data. Recent developments in species distribution modelling (SDM) now allow disentangling the relative contributions of climate and species’ interactions in hybrid zones and their responses to future climate change. We investigated the moving hybrid zone between the breeding ranges of two parapatric passerines in Europe. We conducted SDMs representing the climatic conditions during the breeding season. Our results show a large mismatch between the realized and potential distributions of the two species, suggesting that interspecific interactions, not climate, account for the present location of the contact zone. The SDM scenarios show that the southerly distributed species, Hippolais polyglotta, might lose large parts of its southern distribution under climate change, but a similar gain of novel habitat along the hybrid zone seems unlikely, because interactions with the other species (H. icterina) constrain its range expansion. Thus, whenever biotic interactions limit range expansion, species may become ‘trapped’ if range loss due to climate change is faster than the movement of the contact zone. An increasing number of moving hybrid zones are being reported, but the proximate causes of movement often remain unclear. In a global context of climate change, we call for more interest in their interactions with climate change.     

Differential effects of climate and species interactions on range limits at a hybrid zone: potential direct and indirect impacts of climate change

The relative contributions of climate versus interspecific interactions in shaping species distributions have important implications for closely related species at contact zones. When hybridization occurs within a contact zone, these factors regulate hybrid zone location and movement. While a hybrid zone''s position may depend on both climate and interactions between the hybridizing species, little is known about how these factors interact to affect hybrid zone dynamics. Here, we utilize SDM (species distribution modeling) both to characterize the factors affecting the current location of a moving North American avian hybrid zone and to predict potential direct and indirect effects of climate change on future distributions. We focus on two passerine species that hybridize where their ranges meet, the Black‐capped (Poecile atricapillus) and Carolina (P. carolinensis) chickadee. Our contemporary climate models predict the occurrence of climatically suitable habitat extending beyond the hybrid zone for P. atricapillus only, suggesting that interspecific interactions primarily regulate this range boundary in P. atricapillus, while climatic factors regulate P. carolinensis. Year 2050 climate models predict a drastic northward shift in suitable habitat for P. carolinensis. Because of the greater importance of interspecific interactions for regulating the southern range limit of P. atricapillus, these climate‐mediated shifts in the distribution of P. carolinensis may indirectly lead to a range retraction in P. atricapillus. Together, our results highlight the ways climate change can both directly and indirectly affect species distributions and hybrid zone location. In addition, our study lends support to the longstanding hypothesis that abiotic factors regulate species'' poleward range limits, while biotic factors shape equatorial range limits.     

Increased genetic divergence between two closely related fir species in areas of range overlap

Because of introgressive hybridization, closely related species can be more similar to each other in areas of range overlap (parapatry or sympatry) than in areas where they are geographically isolated from each other (allopatry). Here, we report the reverse situation based on nuclear genetic divergence between two fir species, Abies chensiensis and Abies fargesii, in China, at sites where they are parapatric relative to where they are allopatric. We examined genetic divergence across 126 amplified fragment length polymorphism (AFLP) markers in a set of 172 individuals sampled from both allopatric and parapatric populations of the two species. Our analyses demonstrated that AFLP divergence was much greater between the species when comparisons were made between parapatric populations than between allopatric populations. We suggest that selection in parapatry may have largely contributed to this increased divergence.     

Parapatry and niche complementarity of Peruvian Desert geckos (Phyllodactylus): the ambiguous role of competition

The Sechura Desert of Peru is among the most arid, barren regions of South America. Four species of nocturnal geckos (Phyllodactylus) are parapatric in part of the desert. By comparing niche associations of these species in allopatry and parapatry, I attempt to determine whether the observed parapatric distributions and niche dimension complementarity are related to competition — as is frequently assumed. While parapatry suggests a role for competition, distributional patterns can alternatively be related to adaptations of geckos to different physical environments (sandy desert and rocky foothill) that abut in the study area. Niche complementarity might also be a result of competition, but potentially contradictory evidence suggests that niche complementarity might instead be the result of adaptations developed in allopatry and having no relationship to competition. The ambiguity of these interpretations sets limits on the significance of this kind of evidence: in the absence of attempts to falsify alternative explanations, observations of parapatry or of niche dimension complementarity do not demonstrate conclusively the impact of competition as a force structuring communities.     

Sympatric serpentine endemic Monardella (Lamiaceae) species maintain habitat differences despite hybridization

Ecological differentiation and genetic isolation are thought to be critical in facilitating coexistence between related species, but the relative importance of these phenomena and the interactions between them are not well understood. Here, we examine divergence in abiotic habitat affinity and the extent of hybridization and introgression between two rare species of Monardella (Lamiaceae) that are both restricted to the same serpentine soil exposure in California. Although broadly sympatric, they are found in microhabitats that differ consistently in soil chemistry, slope, rockiness and vegetation. We identify one active hybrid zone at a site with intermediate soil and above‐ground characteristics, and we document admixture patterns indicative of extensive and asymmetric introgression from one species into the other. We find that genetic distance among heterospecific populations is related to geographic distance, such that the extent of apparent introgression is partly explained by the spatial proximity to the hybrid zone. Our work shows that plant species can maintain morphological and ecological integrity in the face of weak genetic isolation, intermediate habitats can facilitate the establishment of hybrids, and that the degree of apparent introgression a population experiences is related to its geographic location rather than its local habitat characteristics.     

Interspecific territoriality in gibbons (Hylobates lar and H. pileatus) and its effects on the dynamics of interspecies contact zones

We investigated the ecology and interspecific interactions of the two gibbon species (Hylobates lar and H. pileatus) that overlap in distribution within a narrow zone of contact in the headwaters of the Takhong River at Khao Yai National Park in central Thailand. The zone is about 10-km wide, with phenotypic hybrids comprising 6.5% of the adult population. We compared species with respect to diet, territory size, intra- and interspecific group encounters, and territory quality. The two gibbon species exploited the same types of resources within their territories despite variation in the relative abundance of food-plant species between territories. The gibbons were interspecifically territorial, and males of both species displayed aggressive behaviors at both intraspecific and interspecific territorial boundaries. There was no marked difference in the amount of overlap between territories of conspecific and heterospecific pairs of groups. Although the habitat was not homogeneous, territory quality did not vary significantly between species. The species have not diverged in habitat preference or in diet. Neither species dominated in interspecific encounters, and both were reproducing well in the contact zone. We analyzed the potential consequences of several types of interspecific interactions on individual dispersal options and on the structure of the contact zone. Interference competition through interspecific territoriality affects the dispersal of individuals into the range of the other species. In general, territorial competition coupled with limited hybridization leads to predictions of a narrow contact zone or parapatry between species; thus, behavioral and ecological interactions between species need to be considered as potential factors in explaining range borders of primate species.     

From refugia to contact: Pine processionary moth hybrid zone in a complex biogeographic setting

Contact zones occur at the crossroad between specific dispersal routes and are facilitated by biogeographic discontinuities. Here, we focused on two Lepidoptera sister species that come in contact near the Turkish Straits System (TSS). We aimed to infer their phylogeographic histories in the Eastern Mediterranean and finely analyze their co‐occurrence and hybridization patterns in this biogeographic context. We used molecular mitochondrial and nuclear markers to study 224 individuals from 42 localities. We used discordances between markers and complementary assignment methods to identify and map hybrids and parental individuals. We confirmed the parapatric distribution of Thaumetopoea pityocampa (Lepidoptera: Notodontidae) in the west and Thaumetopoea wilkinsoni in the east and identified a narrow contact zone. We identified several glacial refugia of T. wilkinsoni in southern Turkey with a strong east–west differentiation in this species. Unexpectedly, T. pityocampa crossed the TSS and occur in northern Aegean Turkey and some eastern Greek islands. We found robust evidence of introgression between the two species in a restricted zone in northwestern Turkey, but we did not identify any F₁ individuals. The identified hybrid zone was mostly bimodal. The distributions and genetic patterns of the studied species were strongly influenced both by the Quaternary climatic oscillations and the complex geological history of the Aegean region. T. pityocampa and T. wilkinsoni survived the last glacial maximum in disjoint refugia and met in western Turkey at the edge of the recolonization routes. Expanding population of T. wilkinsoni constrained T. pityocampa to the western Turkish shore. Additionally, we found evidence of recurrent introgression by T. wilkinsoni males in several T. pityocampa populations. Our results suggest that some prezygotic isolation mechanisms, such as differences in timing of the adult emergences, might be a driver of the isolation between the sister species.     

Reproductive interference, priority effects and the maintenance of parapatry in Skistodiaptomus copepods

Reproductive interactions among species, such as attempts to mate with heterospecifics, can have profound impacts on patterns of geographic range and co-occurrence. For example, several theoretical models demonstrate that reproductive interference-the negative influence of heterospecifics on the abilities of conspecifics to successfully mate-can lead to parapatry, even across homogeneous landscapes, when the parapatric species are initially allopatric. However, the potential consequences of reproductive interactions on patterns of range limits and co-occurrence have been largely ignored by ecologists. Here, I use a combination of laboratory mating experiments and a genetic survey for two parapatric species of Skistodiaptomus copepods to evaluate the potential importance of interspecific reproductive interactions on the maintenance of parapatry. The genetic survey demonstrates phylogenetic exclusivity of these species, suggesting that gene flow has not occurred between them. Moreover, laboratory crosses between both species demonstrate that their parapatric boundary is not maintained as a hybrid zone because the two species cannot form viable hybrids. However, Skistodiaptomus oregonensis and S. pygmaeus males mate indiscriminately and interfere with the ability of heterospecific females to successfully reproduce. I suggest that reproductive interference results in priority effects that strongly influence the maintenance of this parapatric boundary and stress that reproductive traits are important and often-overlooked in ecological models for species range limits.     

A morphologically intergrading population facilitates plastid introgression from diploid to tetraploid Dodecatheon (Primulaceae)

Polyploidy may promote diversification by generating reproductive isolation between ploidy levels, but this reproductive barrier may not be absolute. Several recent analyses of diploid–tetraploid contact zones have found evidence for hybridization. In these cases, inter‐cytotype gene flow is often associated with morphologically intergrading populations. In this study, we combine cytological, fitness and population genetic data to examine the evolutionary role of a morphologically intergrading population at a contact zone between species with different ploidy levels in Dodecatheon. Diploid D. frenchii and tetraploid D. meadia are usually distinguished by leaf‐shape characters. In southern Illinois, where these taxa occur in parapatry, a morphologically intergrading population includes the first documented tetraploid with D. frenchii morphology. Most plants in this intergrading population are fertile, and a nearby typical population of D. meadia has plastid DNA haplotypes that only occur in D. frenchii elsewhere in southern Illinois. These results suggest that fit neo‐tetraploids in this intergrading population have facilitated local introgression between ploidy levels. Similar patterns in other regions where these taxa co‐occur may explain weak range‐wide genetic differentiation between these species. © 2011 The Linnean Society of London, Botanical Journal of the Linnean Society, 2012, 168 , 91–100.     

Prezygotic barriers to hybridization in marine broadcast spawners: reproductive timing and mating system variation

Sympatric assemblages of congeners with incomplete reproductive barriers offer the opportunity to study the roles that ecological and non-ecological factors play in reproductive isolation. While interspecific asynchrony in gamete release and gametic incompatibility are known prezygotic barriers to hybridization, the role of mating system variation has been emphasized in plants. Reproductive isolation between the sibling brown algal species Fucus spiralis, Fucus guiryi (selfing hermaphrodite) and Fucus vesiculosus (dioecious) was studied because they form hybrids in parapatry in the rocky intertidal zone, maintain species integrity over a broad geographic range, and have contrasting mating systems. We compared reproductive synchrony (spawning overlap) between the three species at several temporal scales (yearly/seasonal, semilunar/tidal, and hourly during single tides). Interspecific patterns of egg release were coincident at seasonal (single peak in spring to early summer) to semilunar timescales. Synthesis of available data indicated that spawning is controlled by semidiurnal tidal and daily light-dark cues, and not directly by semilunar cycles. Importantly, interspecific shifts in timing detected at the hourly scale during single tides were consistent with a partial ecological prezygotic hybridization barrier. The species displayed patterns of gamete release consistent with a power law distribution, indicating a high degree of reproductive synchrony, while the hypothesis of weaker selective constraints for synchrony in selfing versus outcrossing species was supported by observed spawning in hermaphrodites over a broader range of tidal phase than in outcrossers. Synchronous gamete release is critical to the success of external fertilization, while high-energy intertidal environments may offer only limited windows of reproductive opportunity. Within these windows, however, subtle variations in reproductive timing have evolved with the potential to form ecological barriers to hybridization.     

Ecological niches and potential geographical distributions of Mediterranean fruit fly (Ceratitis capitata) and Natal fruit fly (Ceratitis rosa)

Aim To predict and compare potential geographical distributions of the Mediterranean fruit fly (Ceratitis capitata) and Natal fruit fly (Ceratitis rosa). Location Africa, southern Europe, and worldwide. Methods Two correlative ecological niche modelling techniques, genetic algorithm for rule‐set prediction (GARP) and a technique based on principal components analysis (PCA), were used to predict distributions of the two fly species using distribution records and a set of environmental predictor variables. Results The two species appear to have broadly similar potential ranges in Africa and southern Europe, with much of sub‐Saharan Africa and Madagascar predicted as highly suitable. The drier regions of Africa (central and western regions of southern Africa and Sahelian zone) were identified as being less suitable for C. rosa than for C. capitata. Overall, the proportion of the region predicted to be highly suitable is larger for C. capitata than for C. rosa under both techniques, suggesting that C. capitata may be tolerant of a wider range of climatic conditions than C. rosa. Worldwide, tropical and subtropical regions are highlighted as highly suitable for both species. Differences in overlap of predictions from the two models for these species were observed. An evaluation using independent records from the adventive range for C. capitata and comparison with other predictions suggest that GARP models offer more accurate predictions than PCA models. Main conclusions This study suggests that these species have broadly similar potential distributions worldwide (based on climate), although the potential distribution appears to be broader for C. capitata than for C. rosa. Ceratitis capitata has become invasive throughout the world, whereas C. rosa has not, despite both species having broadly similar potential distributions. Further research into the biology of these species and their ability to overcome barriers is necessary to explain this difference, and to better understand invasion risk.     

A morphometric study of a hybrid newt population (Triturus cristatus) T. carnifex): Beam Brook Nurseries, Surrey, U.K.

The crested or great crested newt ( Triturus cristatus spp) is declining and now considered threatened in many of the countries where it is present. This has resulted in the four members of the superspecies being afforded protection under local, national and international law. This study looks at a possible threat to T. cristatus in southern England through hybridization, by the introduction of a related alien species ( T. carnifex ). The study used multivariate morphometries to discriminate closely related species, and their hybrids. The character set involved both continuous and meristic data, collected through body measurements and colour pattern. The identification of the species and/or hybrids at the introduction site and surrounding areas was mapped. From the results it can be inferred that hybridization has taken place at the introduction site, but there is no morphological evidence for the spread of hybrids/aliens in to the surrounding areas.     

Phylogenetic relationships and natural hybridization among the North American woody bamboos (Poaceae: Bambusoideae: Arundinaria)

In spite of the ecological and economic importance of temperate bamboos, relatively little is known about their population biology or evolutionary history. Recently, hybridization has emerged as a potential source of diversity in this group, as well as an underlying cause of taxonomic problems. As part of a broader phylogenetic study of the temperate bamboos, we report the results of an analysis of the North American Arundinaria gigantea species complex, including estimates of genetic variation and molecular evidence of natural hybridization among A. gigantea, A. tecta, and A. appalachiana. The study involved a comparative analysis of amplified fragment length polymorphisms (AFLPs) and chloroplast DNA sequences representing diversity within and among all three species plus individuals with intermediate or unusual morphological characteristics (putative hybrids). Molecular results support the recognition of three species previously defined on the basis of morphology, anatomy, and ecology, with most of the molecular variance accounted for by among-species variation. Molecular evidence also demonstrates that A. tecta and A. appalachiana are sister species, forming a clade that is significantly divergent from A. gigantea. The role of hybridization in the phylogenetic history of Arundinaria is discussed along with implications for the evolution and taxonomy of the temperate woody bamboos.     

A hybrid zone between hydrothermal vent mussels (Bivalvia: Mytilidae) from the Mid-Atlantic Ridge

This study provides the first example of a hybrid zone between animal taxa distributed along the mid-ocean ridge system. We examined the distribution and genetic structure of deep-sea hydrothermal vent mussels (Bivalvia: Mytilidae) along a 2888-km portion of the Mid-Atlantic Ridge between 37 degrees 50' N and 14 degrees 45' N latitude. Mitochondrial DNA (mtDNA), allozymes and multivariate-morphometric evidence discriminated between individuals of a northern species, Bathymodiolus azoricus, and a southern species, B. puteoserpentis, that were separated by an intermediate ridge segment almost devoid of mussels. A small sample of mussels from Broken Spur, a vent locality along this intermediate zone, revealed a mixed population with gene frequencies and morphology that were broadly intermediate to those of the northern and southern species. Multilocus clines in mtDNA and allozyme frequencies were centred over the intermediate zone. We consider intrinsic and extrinsic processes that might limit genetic exchange across this hybrid zone.