The genetics and ecology of sympatric speciation: A case study

Oviposition stimulants underlying different preferences between host races in the leaf-mining moth Acrocercops transecta (Lepidoptera: Gracillariidae)

The importance of plant chemistry in the host specialization of phytophagous insects has been emphasized. However, only a few chemicals associated with host shifting have been characterized. Herein, we focus on the leaf-mining moth Acrocercops transecta (Gracillariidae) consisting of ancestral Juglans (Juglandaceae)- and derived Lyonia (Ericaceae)-associated host races. The females of the Lyonia race laid eggs on a cover glass treated with an L. ovalifolia leaf extract; the extract was fractionated using silica gel and ODS column chromatography to isolate the oviposition stimulants. From a separated fraction, two analogous Lyonia-specific triterpenoid glycosides were characterized as oviposition stimulants. Furthermore, we observed probable contact chemosensilla on the distal portion of the female antennae. Lyonia race females laid their eggs on the non-host Juglans after the leaves were treated with a Lyonia-specific oviposition stimulant, although they do not lay eggs on Juglans. These results suggest that Lyonia race females do not lay eggs on Juglans leaves because the leaves do not contain specific oviposition stimulant(s). Otherwise, the activity of the oviposition stimulants overcomes oviposition deterrents contained in Juglans leaves. This paper describes the roles of plant chemicals in the different preferences between host races associated with distantly related plant taxa.

Hybridization, Behavioral Patterns, and Pre- and Postzygotic Isolation Between Two Recently Derived Species of Walnut-Infesting Rhagoletis Fruit Flies in the Highlands of Mexico

Episodes of isolation and secondary contact among populations of insects of Nearctic origin during Pleistocene glacial/postglacial climatic cycles had a strong evolutionary influence on the diversity of flies in the genus Rhagoletis in mountainous areas of Mexico. As a series of experiments undertaken to gather support for phylogenetic hypotheses on the origin of three walnut-infesting species in the suavis group, we examined pre- and postzygotic isolation between Rhagoletis completa Cresson, 1929 and R. ramosae Hernández-Ortiz, 1985. Despite morphological, biological, and behavioral differences, these two species were found to be capable of hybridization. Mating experiments in large enclosures revealed asymmetric sexual isolation. There were notable differences in male sexual behavior. While R. ramosae males mated exclusively on host fruit, R. completa males used fruit and alternative mating locations. During fruit-guarding and male-male contests, R. completa and R. ramosae males adopted markedly different wing postures. R. completa females were more reluctant to copulate with heterospecific males than R. ramosae females. During no choice crosses in small enclosures, there was a reduction of egg hatch for the hybrid cross of R. completa males × R. ramosae females. Our results and previous studies on reproductive isolation between other species pairs in the suavis group support a clade in which R. ramosae, R. zoqui Bush, 1966, and R. completa are close relatives all still capable of hybridizing.

Revision of the Ropalopus ungaricus/insubricus group (Coleoptera: Cerambycidae: Callidiini) from the western Palaearctic region

This paper revises the taxonomy and phylogeny of Ropalopus (Coleoptera: Cerambycidae: Callidiini) species belonging to the taxonomically intricate Ropalopus ungaricus/insubricus group from the western Palaearctic. Specimens from all taxa were studied and compared. The key characters, including the male terminalia, were examined by means of scanning electron microscopy. High-quality stacked photographs of the habitus of the specimens (dorsal, ventral) are presented. The phylogenetic analyses were based on 34 adult morphological characters. Both the strict and majority consensus trees revealed the monophyly of the revised group. Identification keys are provided for every taxon from this group, and their geographical distributions are also mapped. All European populations are reduced to subspecies of Ropalopus ungaricus. Additionally, a new subspecies from Greece, Ropalopus ungaricus ossae subsp. nov., is described and illustrated. A new synonymy for Ropalopus insubricus fischeri is proposed: Callidium insubricum = Callidium fischeri. Ropalopus nataliyae, which was described based on only a single female, is herein redescribed owing to the collection of abundant new material. Apart from geographical barriers, the main differentiating factor in this group is proved to be the elevation above sea level. Taxa of the R. ungaricus/insubricus group are therefore allopatric or, when parapatric, are isolated by elevation.

Old residents and new arrivals of Rhagoletis species in Europe

The genus Rhagoletis (Diptera: Tephritidae) comprises more than 65 species distributed throughout Europe, Asia and America, including many species of high economic importance. Currently, there are three Rhagoletis species that infest fruits and nuts in Europe. The European cherry fruit fly, Rhagoletis cerasi (may have invaded Europe a long time ago from the Caucasian area of West Asia), and two invasive species (recently introduced from North America): the eastern American cherry fruit fly, R. cingulata, and the walnut husk fly, R. completa. The presence of different Rhagoletis species may enhance population dynamics and establish an unpredictable economic risk for several fruit and nut crops in Europe. Despite their excessive economic importance, little is known on population dynamics, genetics and symbiotic associations for making sound pest control decisions in terms of species-specific, environmental friendly pest control methods. To this end, the current paper (a) summarizes recently accumulated genetic and population data for the European Rhagoletis species and their association with the endosymbiont Wolbachia pipientis, and (b) explores the possibility of using the current knowledge for implementing the innovative biological control methods of sterile insect technique and incompatible insect technique.

Climatic niche evolution in the viviparous Sceloporus torquatus group (Squamata: Phrynosomatidae)

The cold-climate hypothesis maintains that viviparity arose as a means to prevent increased egg mortality in nests owing to low temperatures, and this hypothesis represents the primary and most strongly supported explanation for the evolution of viviparity in reptiles. In this regard, certain authors have stated that viviparous species will exhibit speciation via climatic niche conservatism, with similar climatic niches being observed in allopatric sister species. However, this prediction remains to be tested with bioclimatic variables relevant to each viviparous group. In the present study, we examined climatic niche evolution in a group of North American viviparous lizards to determine whether their diversification is linked to phylogenetic niche conservatism (PNC). We evaluated the phylogenetic signal and trait evolution of individual bioclimatic variables and principal component (PC) scores of a PC analysis, along with reconstructions of ancestral climate tolerances. The results suggest that diversification of the Sceloporus torquatus group species is associated with both niche differentiation and PNC. Furthermore, we did not observe PNC across nearly all bioclimatic variables and in PC2 and PC3. However, in Precipitation Seasonality (Bio15), in Precipitation of Coldest Quarter (Bio19) and in PC1 (weakly associated with variability of temperature), we did observe PNC. Additionally, variation of the scores along the phylogeny and Pagel's delta (δ) >1 of PC3 suggests a fast, recent evolution to dry conditions in the clade that sustains S. serrifer.

Climatic niche evolution is faster in sympatric than allopatric lineages of the butterfly genus Pyrgus

Understanding how speciation relates to ecological divergence has long fascinated biologists. It is assumed that ecological divergence is essential to sympatric speciation, as a mechanism to avoid competition and eventually lead to reproductive isolation, while divergence in allopatry is not necessarily associated with niche differentiation. The impact of the spatial context of divergence on the evolutionary rates of abiotic dimensions of the ecological niche has rarely been explored for an entire clade. Here, we compare the magnitude of climatic niche shifts between sympatric versus allopatric divergence of lineages in butterflies. By combining next-generation sequencing, parametric biogeography and ecological niche analyses applied to a genus-wide phylogeny of Palaearctic Pyrgus butterflies, we compare evolutionary rates along eight climatic dimensions across sister lineages that diverged in large-scale sympatry versus allopatry. In order to examine the possible effects of the spatial scale at which sympatry is defined, we considered three sets of biogeographic assignments, ranging from narrow to broad definition. Our findings suggest higher rates of niche evolution along all climatic dimensions for sister lineages that diverge in sympatry, when using a narrow delineation of biogeographic areas. This result contrasts with significantly lower rates of climatic niche evolution found in cases of allopatric speciation, despite the biogeographic regions defined here being characterized by significantly different climates. Higher rates in allopatry are retrieved when biogeographic areas are too widely defined-in such a case allopatric events may be recorded as sympatric. Our results reveal the macro-evolutionary significance of abiotic niche differentiation involved in speciation processes within biogeographic regions, and illustrate the importance of the spatial scale chosen to define areas when applying parametric biogeographic analyses.

How populations differentiate despite gene flow: sexual and natural selection drive phenotypic divergence within a land fish, the Pacific leaping blenny

Background

Divergence between populations in reproductively important features is often vital for speciation. Many studies attempt to identify the cause of population differentiation in phenotype through the study of a specific selection pressure. Holistic studies that consider the interaction of several contrasting forms of selection are more rare. Most studies also fail to consider the history of connectivity among populations and the potential for genetic drift or gene flow to facilitate or limit phenotypic divergence. We examined the interacting effects of natural selection, sexual selection and the history of connectivity on phenotypic differentiation among five populations of the Pacific leaping blenny (Alticus arnoldorum), a land fish endemic to the island of Guam.

Results

We found key differences among populations in two male ornaments—the size of a prominent head crest and conspicuousness of a coloured dorsal fin—that reflected a trade-off between the intensity of sexual selection (male biased sex ratios) and natural selection (exposure to predators). This differentiation in ornamentation has occurred despite evidence suggesting extensive gene flow among populations, which implies that the change in ornament expression has been recent (and potentially plastic).

Conclusions

Our study provides an early snapshot of divergence in reproductively important features that, regardless of whether it reflects genetic or plastic changes in phenotype, could ultimately form a reproductive barrier among populations.

Malaria infections reinforce competitive asymmetry between two Ficedula flycatchers in a recent contact zone

Parasites may influence the outcome of interspecific competition between closely related host species through lower parasite virulence in the host with which they share the longer evolutionary history. We tested this idea by comparing the prevalence of avian malaria (Haemosporidia) lineages and their association with survival in pied and collared flycatchers (Ficedula hypoleuca and F. albicollis) breeding in a recent contact zone on the Swedish island of Öland. A nested PCR protocol amplifying haemosporidian fragments of mtDNA was used to screen the presence of malaria lineages in 1048 blood samples collected during 6 years. Competitively inferior pied flycatchers had a higher prevalence of blood parasites, including the lineages that were shared between the two flycatcher species. Multistate mark-recapture models revealed a lower survival of infected versus uninfected female pied flycatchers, while no such effects were detected in male pied flycatchers or in collared flycatchers of either sex. Our results show that a comparatively new host, the collared flycatcher, appears to be less susceptible to a local northern European malarial lineage where the collared flycatchers have recently expanded their distribution. Pied flycatchers experience strong reproductive interference from collared flycatchers, and the additional impact of species-specific blood parasite effects adds to this competitive exclusion. These results support the idea that parasites can strongly influence the outcome of interspecific competition between closely related host species, but that the invading species need not necessarily be more susceptible to local parasites.

Abundance of apple maggot, Rhagoletis pomonella, across different areas in central Washington, with special reference to black-fruited hawthorns

The apple maggot fly, Rhagoletis pomonella (Walsh) (Diptera: Tephritidae), infests non-commercial apple (Malus domestica (Borkh.) Borkh.) and native black-fruited hawthorns (mostly Crataegus douglasii Lindl.) in central Washington, but little has been published on the abundance of the fly in this region. In this paper, the abundance of R. pomonella across different sites near apple-growing areas in central Washington is documented in order to assess the threat of the fly to commercial apple orchards. The fly was first detected on traps in Klickitat, Yakima, and Kittitas Counties in 1981, 1995, and 1997, respectively. From 1981-2010 in Kittitas and Yakima Counties, only 0 to 4.7% of traps on apple, crabapple, and hawthorn trees were positive for flies, whereas in Klickitat County, located farther from commercial apple orchards, 0 to 41.9% of traps were positive. In 2008, in Yakima County and Goldendale in Klickitat County, 7.8% of black-fruited hawthorn trees were infested, with 0 to 0.00054 larvae per fruit. In 2010, in Kittitas and Yakima Counties and Goldendale in Klickitat County, 25.0% of C. douglasii trees were infested, with 0.00042 to 0.00248 larvae per fruit. In 2010, in a remote forested area of Klickitat County far from commercial apple orchards, 94.7% of C. douglasii trees were infested, with 0.20813 larvae per fruit. Overall results suggest R. pomonella is unlikely to develop high populations rapidly near major commercial apple-growing areas in central Washington, including in black-fruited hawthorns, increasing chances it can be kept out of commercial orchards.

Post-colonization temporal genetic variation of an introduced fly, Rhagoletis completa

Evolutionary biologists have been puzzled by the success of introduced species: despite founder effects that reduce genetic variability, invasive species are still successful at colonizing new environments. It is possible that the evolutionary processes during the post-colonization period may increase the genetic diversity and gene flow among invasive populations over time, facilitating their long-term success. Therefore, genetic diversity and population structure would be expected to show greater temporal variation for successful introduced populations than for native populations. We studied the population genetics of the walnut husk fly, Rhagoletis completa, which was introduced into California from the Midwestern US in the early 1900s. We used microsatellites and allozymes to genotype current and historic fly populations, providing a rare perspective on temporal variability in population genetic parameters. We found that introduced populations showed greater temporal fluctuations in allele frequencies than native populations. Some introduced populations also showed an increase in genetic diversity over time, indicating multiple introductions had occurred. Population genetic structure decreased in both native and introduced populations over time. Our study demonstrates that introduced species are not at equilibrium and post-colonization processes may be important in ameliorating the loss of genetic diversity associated with biological invasions.

On the elusiveness of enemy-free space: spatial, temporal, and host-plant-related variation in parasitoid attack rates on three gallmakers of goldenrods

Host shifting by phytophagous insects may play an important role in generating insect diversity by initiating host-race formation and speciation. Models of the host shifting process often invoke reduced rates of natural enemy attack on a novel host in order to balance the maladaptation expected following the shift. Such "enemy-free space" has been documented for some insects, at some times and places, but few studies have assessed the occurrence of enemy-free space across years, among sites, or among insect species. We measured parasitoid attack rates on three insect herbivores of two goldenrods (Solidago altissima L. and Solidago gigantea Ait.), with data from multiple sites and multiple years for each herbivore. For each insect herbivore, there were times and sites at which parasitoid attack rates differed strongly and significantly between host plants (that is, enemy-free space existed on one host plant or the other). However, the extent and even the direction of the attack-rate difference varied strongly among sites and even among years at the same site. There was no evidence of consistent enemy-free space for any herbivore on either host plant. Our data suggest that enemy-free space, like many ecological and evolutionary forces, is likely to operate as a geographic and temporal mosaic, and that conceptual models of host shifting that include enemy-free space as a consequence of host novelty are likely too simple.

Phylogeographic patterns of host-race evolution in Tephritis conura (Diptera: Tephritidae)

Host-race evolution is a prime candidate for sympatric speciation because host shifts must take place in the presence of both hosts. However, the geographic context in which the shift takes place may have strong allopatric or peripatric components if the primary host within a localized area is scarce or even goes extinct. Inference of the relative importance of the geographic mode of speciation may be gained from phylogeographic imprints. Here, we investigate the phylogeography of host races of the tephritid fly Tephritis conura from sympatric, parapatric and allopatric populations of Cirsium heterophyllum and Cirsium oleraceum (Asteraceae) in Europe, for addressing the age and direction, and the geographic context of host-race formation. Haplotype networks of the host races differed significantly in molecular diversity and topology. However, host-race haplotypes were nested within the same network, with a central haplotype H1 being the most common haplotype in both host races. C. heterophyllum flies were genetically more diverse and substructured than flies from C. oleraceum, suggesting that the latter is the derived race. The phylogeographic imprint indicates either that C. heterophyllum flies colonized C. oleraceum during range expansion or that heterophyllum flies went extinct in an area where oleraceum flies persisted (followed by re-immigration). These imprints are in concordance with peripatric diversification, probably in the European Alps and related to the last ice age, where the host-race diversification was largely completed before the postglacial range expansion on C. oleraceum to current areas of sympatry and parapatry with C. heterophyllum.

Genetic and morphological differentiation in Tephritis bardanae (Diptera: Tephritidae): evidence for host-race formation

The fruit fly Tephritis bardanae infests flower heads of two burdock hosts, Arctium tomentosum and A. minus. Observations suggest host-associated mating and behavioural differences at oviposition indicating host-race status. Previously, flies from each host plant were found to differ slightly in allozyme allele frequencies, but these differences could as well be explained by geographical separation of host plants. In the present study, we explicitly test whether genetic and morphological variance among T. bardanae are explained best by host-plant association or by geographical location, and if this pattern is stable over a 10-year period. Populations of A. tomentosum flies differed significantly from those of A. minus flies in (i) allozyme allele frequencies at the loci Pep-A and Pgd, (ii) mtDNA haplotype frequencies and (iii) wing size. In contrast, geographical location had no significant influence on the variance estimates. While it remains uncertain whether morphometric differentiation reflects genotypic variability or phenotypic plasticity, allozyme and mtDNA differentiation is genetically determined. This provides strong evidence for host-race formation in T. bardanae. However, the levels of differentiation are relatively low indicating that the system is in an early stage of divergence. This might be due to a lack of time (i.e. the host shift occurred recently) or due to relatively high gene flow preventing much differentiation at loci not experiencing selection.

Sequential radiation of unrelated organisms: the gall fly Eurosta solidaginis and the tumbling flower beetle Mordellistena convicta

Host shifts and the formation of insect-host races are likely common processes in the speciation of herbivorous insects. The interactions of goldenrods Solidago (Compositae), the gall fly Eurosta solidaginis (Diptera: Tephritidae) and the beetle Mordellistena convicta (Coleoptera: Mordellidae) provide behavioural, ecological and genetic evidence of host races that may represent incipient species forming via sympatric speciation. We summarize evidence for Eurosta host races and show that M. convicta has radiated from goldenrod stems to Eurosta galls to form host-part races and, having exploited the galler's host shift, has begun to differentiate into host races within galls. Thus, host-race formation has occurred in two interacting, but unrelated organisms representing two trophic levels, resulting in 'sequential radiation' (escalation of biodiversity up the trophic system). Distributions of host races and their behavioural isolating mechanisms suggest sympatric differentiation. Such differentiation suggests host-race formation and subsequent speciation may be an important source of biodiversity.

Reproductive character displacement and the genetics of gamete recognition in tropical sea urchins

Reproductive character displacement occurs when sympatric and allopatric populations of a species differ in traits crucial to reproduction, and it is commonly thought of as a signal of selection acting to limit hybridization. Most documented cases of reproductive character displacement involve characters that are poorly understood at the genetic level, and rejecting alternative hypotheses for biogeographic shifts in reproductive traits is often very difficult. In sea urchins, the gamete recognition protein bindin evolves under positive selection when species are broadly sympatric, suggesting character displacement may be operating in this system. We sampled sympatric and allopatric populations of two species in the sea urchin genus Echinometra for variation in bindin and for the mitochondrial cytochrome oxidase I to examine patterns of population differentiation and molecular evolution at a reproductive gene. We found a major shift in bindin alleles between central Pacific (allopatric) and western Pacific (sympatric) populations of E. oblonga. Allopatric populations of E. oblonga are polyphyletic with E. sp. C at bindin, whereas sympatric populations of the two species are reciprocally monophyletic. There is a strong signal of positive selection (P(N)/P(S) = 4.5) in the variable region of the first exon of bindin, which is associated with alleles found in sympatric populations of E. oblonga. These results indicate that there is a strong pattern of reproductive character displacement between E. oblonga and E. sp. C and that the divergence is driven by selection. There is much higher population structure in sympatric populations at the bindin locus than at the neutral mitochondrial locus, but this difference is not seen in allopatric populations. These data suggest a pattern of speciation driven by selection for local gamete coevolution as a result of interactions between sympatric species. Although this pattern is highly suggestive of speciation by reinforcement, further research into hybrid fitness and egg-sperm interactions is required to address this potential mechanism for character displacement.

Host races in plant-feeding insects and their importance in sympatric speciation

The existence of a continuous array of sympatric biotypes - from polymorphisms, through ecological or host races with increasing reproductive isolation, to good species - can provide strong evidence for a continuous route to sympatric speciation via natural selection. Host races in plant-feeding insects, in particular, have often been used as evidence for the probability of sympatric speciation. Here, we provide verifiable criteria to distinguish host races from other biotypes: in brief, host races are genetically differentiated, sympatric populations of parasites that use different hosts and between which there is appreciable gene flow. We recognize host races as kinds of species that regularly exchange genes with other species at a rate of more than ca. 1% per generation, rather than as fundamentally distinct taxa. Host races provide a convenient, although admittedly somewhat arbitrary intermediate stage along the speciation continuum. They are a heuristic device to aid in evaluating the probability of speciation by natural selection, particularly in sympatry. Speciation is thereby envisaged as having two phases: (i) the evolution of host races from within polymorphic, panmictic populations; and (ii) further reduction of gene flow between host races until the diverging populations can become generally accepted as species. We apply this criterion to 21 putative host race systems. Of these, only three are unambiguously classified as host races, but a further eight are strong candidates that merely lack accurate information on rates of hybridization or gene flow. Thus, over one-half of the cases that we review are probably or certainly host races, under our definition. Our review of the data favours the idea of sympatric speciation via host shift for three major reasons: (i) the evolution of assortative mating as a pleiotropic by-product of adaptation to a new host seems likely, even in cases where mating occurs away from the host; (ii) stable genetic differences in half of the cases attest to the power of natural selection to maintain multilocus polymorphisms with substantial linkage disequilibrium, in spite of probable gene flow; and (iii) this linkage disequilibrium should permit additional host adaptation, leading to further reproductive isolation via pleiotropy, and also provides conditions suitable for adaptive evolution of mate choice (reinforcement) to cause still further reductions in gene flow. Current data are too sparse to rule out a cryptic discontinuity in the apparently stable sympatric route from host-associated polymorphism to host-associated species, but such a hiatus seems unlikely on present evidence. Finally, we discuss applications of an understanding of host races in conservation and in managing adaptation by pests to control strategies, including those involving biological control or transgenic parasite-resistant plants.

Developmental-behavioral initiation of evolutionary change

The traditional approach to evolutionary psychology relies entirely on natural selection as the cause of the evolution of adaptations. Exclusive reliance on natural selection overlooks the fact that changes in development are a necessary prerequisite for evolutionary change. These developmental changes provide the material for natural selection to work on. In the neo-Darwinian scenario, the mechanisms of evolution are mutation or genetic recombination, selection, migration, and eventual reproductive isolation. In the spirit of evolutionary pluralism, the author describes a different 3-stage scenario in which migration (the invasion of new niches or habitats) may occur without mutation or genetic recombination and selection first initiating a change in genes or gene frequencies.

Sympatric speciation in phytophagous insects: moving beyond controversy?

Sympatric speciation is the splitting of one evolutionary lineage into two without the occurrence of geographic isolation. The concept has been intimately tied to entomology since the 1860s, when Benjamin Walsh proposed that many host-specific phytophagous insects originate by shifting and adapting to new host plant species. If true, sympatric speciation would have tremendous implications for our understanding of species and their origins, biodiversity (25-40% of all animals are thought to be phytophagous specialists), insect-plant coevolution, community ecology, phylogenetics, and systematics, as well as practical significance for the management of insect pests. During much of the twentieth century sympatric speciation was viewed as much less plausible than geographic (allopatric) speciation. However, empirical field studies, laboratory experiments, developments in population genetics theory, and phylogenetic and biogeographic data have all recently combined to shed a more favorable light on the process. We review the evidence for sympatric speciation via host shifting for phytophagous insects and propose a set of testable predictions for distinguishing geographic mode (allopatric versus sympatric) of divergence. Our conclusion is that sympatric speciation is a viable hypothesis. We highlight areas where more thorough testing is needed to move sympatric speciation into the realm of accepted scientific theory.

Sympatric speciation in animals: the ugly duckling grows up

Sympatric speciation has become increasingly accepted in the past decade, as a result of new models substantiating its plausibility and new evidence that the conditions specified by the models are met in many natural populations. Retrospective phylogenetic and population genetic signatures of sympatric speciation have also been derived, and these are beginning to be tested. This new work has helped increase the acceptance of sympatric speciation as a plausible process, although it remains difficult to show conclusively that specific pairs of taxa have speciated through sympatric processes alone. It might be time for a re-evaluation of the geographical classification of speciation modes in favor of one based primarily on evolutionary mechanisms

Phylogenetics and speciation

Species-level phylogenies derived from molecular data provide an indirect record of the speciation events that have led to extant species. This offers enormous potential for investigating the general causes and rates of speciation within clades. To make the most of this potential, we should ideally sample all the species in a higher group, such as a genus, ensure that those species reflect evolutionary entities within the group, and rule out the effects of other processes, such as extinction, as explanations for observed patterns. We discuss recent practical and theoretical advances in this area and outline how future work should benefit from incorporating data from genealogical and phylogeographical scales.

Parallel speciation: a key to sympatric divergence

Until recently, our view of speciation was that reproductive isolation evolves during long periods of allopatry through the accumulation of genetic differences that result in genetic incompatibility. This view now contrasts with new findings that characters affecting reproductive isolation can diverge rapidly in sympatry as a result of natural selection. Recent studies combining research on phylogeny and ecology of natural populations cast new light on patterns, timescales and mechanisms, and emphasize the role of ecological factors in speciation. Studies of parallel speciation provide a strong case for sympatric speciation and for natural selection generating reproductive barriers.

Mitochondrial sequence analysis of Salamandra taxa suggests old splits of major lineages and postglacial recolonizations of central Europe from distinct source populations of Salamandra salamandra

Representatives of the genus Salamandra occur in Europe, Northern Africa and the Near East. Many local variants are known but species and subspecies status of these is still a matter of dispute. We have analysed samples from locations covering the whole expansion range of Salamandra by sequence analysis of mitochondrial D-loop regions. In addition, we have calibrated the rate of divergence of the D-loop on the basis of geologically dated splits of the closely related genus Euproctus. Phylogenetic analysis of the sequences suggests that six major monophyletic groups exist (S. salamandra, S. algira, S. infraimmaculata, S. corsica, S. atra and S. lanzai) which have split between 5 and 13 million years ago (Ma). We find that each of the Salamandra species occupies a distinct geographical area, with the exception of S. salamandra. This species occurs all over Europe from Spain to Greece, suggesting that it was the only species that has recolonized Central Europe after the last glaciation. The occurrence of specific east and west European haplotypes, as well as allozyme alleles in the S. salamandra populations suggests that this recolonization has started from at least two source populations, possibly originating in the Iberian peninsula and the Balkans. Two subpopulations of S. salamandra were found that are genetically very distinct from the other populations. One lives in northern Spain (S. s. bernardezi) and one in southern Italy (S. s. gigliolii). Surprisingly, the mitochondrial lineages of these subpopulations group closer together than the remainder S. salamandra lineages. We suggest that these populations are remnants of a large homogeneous population that had colonized Central Europe in a previous interglacial period, approximately 500 000 years ago. Animals from these populations were apparently not successful in later recolonizations. Still, they have maintained their separate genetic identity in their areas, although they are not separated by geographical barriers from very closely related neighbouring populations.