COMPETITION AND THE EVOLUTION OF AGGRESSIVE BEHAVIOR IN TWO SPECIES OF TERRESTRIAL SALAMANDERS

The effects of competition on the evolution of interspecific interference mechanisms were studied by comparing the aggressive behavior of two terrestrial salamander species from two localities that differ in the intensity of interspecific competition. Plethodon jordani and P. glutinosus are closely related, ecologically similar species that are sympatric at intermediate elevations in the southern Appalachian Mountains. Previous removal and transplant experiments showed that interspecific competition is more intense in the northeastern Great Smoky Mountains, where the species are narrowly sympatric, than in the nearby Balsam Mountains, where sympatry is broader. In laboratory encounters, P. glutinosus from the Great Smoky Mountains were more aggressive to heterospecific and conspecific intruders than were P. glutinosus from the Balsam Mountains. For P. jordani, however, the variation in interspecific and intraspecific aggressive behavior among individuals within populations was as great as the variation between populations. Alpha-selection (i.e., improved competitive ability by the acquisition of interspecific interference mechanisms) has occurred in populations of P. glutinosus under conditions of intense interspecific competition. The evolution of aggressive behavior appears to have been influenced by the intensity of intraspecific competition as well.     

Patterns of differential introgression in a salamander hybrid zone: inferences from genetic data and ecological niche modelling

Hybrid zones have yielded considerable insight into many evolutionary processes, including speciation and the maintenance of species boundaries. Presented here are analyses from a hybrid zone that occurs among three salamanders –Plethodon jordani, Plethodon metcalfi and Plethodon teyahalee– from the southern Appalachian Mountains. Using a novel statistical approach for analysis of non‐clinal, multispecies hybrid zones, we examined spatial patterns of variation at four markers: single‐nucleotide polymorphisms (SNPs) located in the mtDNA ND2 gene and the nuclear DNA ILF3 gene, and the morphological markers of red cheek pigmentation and white flecks. Concordance of the ILF3 marker and both morphological markers across four transects is observed. In three of the four transects, however, the pattern of mtDNA is discordant from all other markers, with a higher representation of P. metcalfi mtDNA in the northern and lower elevation localities than is expected given the ILF3 marker and morphology. To explore whether climate plays a role in the position of the hybrid zone, we created ecological niche models for P. jordani and P. metcalfi. Modelling results suggest that hybrid zone position is not determined by steep gradients in climatic suitability for either species. Instead, the hybrid zone lies in a climatically homogenous region that is broadly suitable for both P. jordani and P. metcalfi. We discuss various selective (natural selection associated with climate) and behavioural processes (sex‐biased dispersal, asymmetric reproductive isolation) that might explain the discordance in the extent to which mtDNA and nuclear DNA and colour‐pattern traits have moved across this hybrid zone.     

Secondary contact between Lycaeides idas and L. melissa in the Rocky Mountains: extensive admixture and a patchy hybrid zone

Studies of hybridization have increased our understanding of the nature of species boundaries, the process of speciation, and the effects of hybridization on the evolution of populations and species. In the present study we use genetic and morphological data to determine the outcome and consequences of secondary contact and hybridization between the butterfly species Lycaeides idas and L. melissa in the Rocky Mountains. Admixture proportions estimated from structure and geographical cline analysis indicate L. idas and L. melissa have hybridized extensively in the Rocky Mountains and that reproductive isolation was insufficient to prevent introgression for much of the genome. Geographical patterns of admixture suggest that hybridization between L. idas and L. melissa has led to the formation of a hybrid zone. The hybrid zone is relatively wide, given estimates of dispersal for Lycaeides butterflies, and does not show strong evidence of cline concordance among characters. We believe the structure of the Lycaeides hybrid zone might be best explained by the patchy distribution of Lycaeides, local extinction and colonization of habitat patches, environmental variation and weak overall selection against hybrids. We found no evidence that hybridization in the Rocky Mountains has resulted in the formation of independent hybrid species, in contrast to the outcome of hybridization between L. idas and L. melissa in the Sierra Nevada. Finally, our results suggest that differences in male morphology between L. idas and L. melissa might contribute to isolation, or perhaps even that selection has favoured the spread of L. melissa male genitalia alleles.     

THE GENETIC STRUCTURE OF A HYBRID ZONE BETWEEN TWO CHROMOSOME RACES OF THE SCELOPORUS GRAMMICUS COMPLEX (SAURIA,PHRYNOSOMATIDAE) IN CENTRAL MEXICO

The F5 (2n = 34) and FM2 (2n = 44–46) chromosome races of the Sceloporus grammicus complex form a parapatric hybrid zone in the Mexican state of Hidalgo, characterized by steep concordant clines among three diagnostic chromosome markers across a straight-line distance of about 2 km. Here, we show that this zone is actually structured into local patches in which hybridization extends over an extremely irregular front. The distribution of hybrid-index (HI) scores across the transect reveals some hybridization at almost all localities mapped in a central 7 km × 3 km area. Pooling the central samples produces both a strong heterozygote deficit for all diagnostic markers and strong linkage disequilibria between all pairwise combinations of these (unlinked) markers. Moreover, a highly significant association exists between the habitat on which each individual was caught and its karyotype (F5 chromosomes are more likely to be found on oak). Analysis of genotype frequencies over a range of spatial scales shows that there is no significant heterozygote deficit or habitat association within local areas of less than about 200 m; however, there is significant linkage disequilibrium over the smallest scales (R = D (pquv)^1/2 = 0.29, support limits, 0.18–0.36) over 100 m. These patterns suggest that lizards mate and choose habitats randomly within local patches. This conclusion is supported by mark-recapture estimates of dispersal (≈ 80 m in a generation) and by inference of matings from embryo and maternal karyotypes. Closer examination of the two-dimensional pattern reveals a convoluted cline for all three markers, with a width of 830 m (support limits 770 m–930 m). This cline width, combined with the strength of local linkage disequilibrium, implies a dispersal rate of σ = 160 m in a generation and an effective selection pressure of 30% on each chromosome marker. The proportion of inviable embryos is greater in females from the center of the hybrid zone; this is caused by effects associated with both karyotype and location. The hybrid zone is likely to be maintained by selection against chromosomal heterozygotes, by other kinds of selection against hybrids, and by selection adapting the chromosome races to different habitats. The structure of the contact may be caused by both random drift and by selection in relation to habitat.     

The evolution of competing species of terrestrial salamanders: niche partitioning or interference?

Summary One of the central assumptions of evolutionary ecology is that interspecific competition is a potent evolutionary force acting on coexisting species. There are few animal species that provide an opportunity for an experimental analysis of the evolutionary consequences of the phenomenon. We have taken advantage of the fact that two species of terrestrial salamander,Plethodon glutinosus andP. jordani, have different altitudinal distributions on two mountain ranges in North Carolina. Field removal experiments showed that interspecfic competition was much stronger in the Great Smoky Mountains than in the Balsam Mountains, and transplant experiments between the two mountain ranges showed that neither species from the Balsam Mountains had a measurable effect on its congener in the Smokies, although both species from the Smokies had strong negative effects on the Balsam congeners. Other experiments were conducted on the behavioral and ecological changes that have (or have not) evolved in the two areas. Our studies show that increased interspecific interference was the major evolutionary response of these largePlethodon species to interspecific competition, and that partitioning of food or microhabitat was not involved.     

Recognition of individual,sex and species odours by salamanders of the Plethodon glutinosus-P. Jordani complex

Salamanders of the Plethodon glutinosus-P. jordani complex were tested for the ability to distinguish conspecific, sex-specific, and heterospecific chemical cues. Male and female P. glutinosus preferred substrates previously occupied by conspecifics over their own, but randomly chose between substrates marked by male or female conspecifics. This suggests that while these salamanders are able to distinguish between their own and conspecific substrate odours, they are unable to identify sex by means of substrate odours. Experiments using an olfactometer showed that male P. jordani, male and female P. glutinosus, and an electrophoretically distinct and non-hybridizing sympatric phenotype in the P. glutinosus complex (here called species A), all preferred female airborne odours over male airborne odours. This demonstrates that these salamanders can identify sex by means of airborne odours. Male P. glutinosus and species A both preferred conspecific female odours over heterospecific female odours in olfactometer experiments. These results suggest an important role for olfaction in the sexual and social behaviour of these salamanders, particularly as a pre-mating isolating mechanism.     

Concordance of genetic and phenotypic characters across a sapsucker hybrid zone

Hybridization has presented a challenge for taxonomists and conservation biologists, since hybridizing forms could be stable evolutionary entities or ephemeral forms that are blending together. However, hybrid zones also provide a unique opportunity for evolutionary biologists who study the interaction between gene flow and reproductive isolation in speciation. Three forms of woodpeckers (sapsuckers; genus Sphyrapicus) in North America that are mostly geographically separated but hybridize with each other where they come into contact present a remarkable system for the study of hybridization. We provide the first comprehensive analysis of phenotypic and genetic variation across a hybrid zone between two of these forms, the red‐breasted Sphyrapicus ruber and yellow‐bellied S. varius sapsuckers. The objective was to infer whether selection maintains the differences between forms. Our analysis of eight morphometric and 20 plumage traits, and two molecular markers showed clear differences between the forms and roughly concordant clinal variation across a narrow hybrid zone. Thirty percent of sampled birds in the hybrid zone had mixed west/east genotypes at the genetic markers examined. The center of the genetic cline was located 20 km west of the crest of the Rocky Mountains. The width of the zone was 122 km, narrower than would be expected under neutral blending given reasonable estimates of the age of the zone and individual dispersal distances. Heterozygote deficit and cytonuclear disequilibrium at the centre of the hybrid zone suggested nonrandom mating or limited hybridization. Given these patterns and lack of evidence for habitat segregation we conclude that this hybrid zone is maintained by selection, most likely in the form of hybrid inferiority. This study provides an illustrative example of extensive hybridization between stable entities, providing additional evidence against the historical practice of treating hybridizing forms as members of the same species.     

STRUCTURE OF AN ASYMMETRIC HYBRID ZONE BETWEEN TWO WATER STRIDER SPECIES (HEMIPTERA: GERRIDAE: LIMNOPORUS)

The water stricter species Limnoporus dissortis and L. notabilis hybridize across a broad zone in western Canada. Body length and alleles at four allozyme loci show a steep cline along the east slope of the Rocky Mountains in western Alberta, while in central British Columbia the parental phenotypes coexist without merging fully. One sex-linked locus shows little introgression, while there is apparently considerable gene flow at three autosomal loci. Although the hybrid zone has characteristics of a broad tension zone, the spatial distribution of introgression suggests that habitat patchiness and differential habitat associations of the two species also contribute to the pattern of hybridization. Asymmetry in interspecific mating success and incompatibilities of sex chromosomes with each other or with cytoplasmic factors appear to account for the occurrence of L. dissortis genotypes within the range of L. notabilis, and the lack of L. notabilis genotypes within the range of L. dissortis. The genetic structure of this hybrid zone supports the importance of sex-linked traits in maintaining the integrity of species, while its spatial structure suggests that extrinsic habitat features can combine with intrinsic genetic incompatibilities to produce complex hybrid interactions.     

The role of climate in the dynamics of a hybrid zone in Appalachian salamanders

I examined the potential influence of climate change on the dynamics of a previously studied hybrid zone between a pair of terrestrial salamanders at the Coweeta Hydrologic Laboratory, U.S. Forest Service, in the Nantahala Mountains of North Carolina, USA. A 16‐year study led by Nelson G. Hairston, Sr. revealed that Plethodon teyahalee and Plethodon shermani hybridized at intermediate elevations, forming a cline between ‘pure’ parental P. teyahalee at lower elevations and ‘pure’ parental P. shermani at higher elevations. From 1974 to 1990 the proportion of salamanders at the higher elevation scored as ‘pure’P. shermani declined significantly, indicating that the hybrid zone was spreading upward. To date there have been no rigorous tests of hypotheses for the movement of this hybrid zone. Using temperature and precipitation data from Coweeta, I re‐analyzed Hairston's data to examine whether the observed elevational shift was correlated with variation in either air temperature or precipitation from the same time period. For temperature, my analysis tracked the results of the original study: the proportion of ‘pure’P. shermani at the higher elevation declined significantly with increasing mean annual temperature, whereas the proportion of ‘pure’P. teyahalee at lower elevations did not. There was no discernable relationship between proportions of ‘pure’ individuals of either species with variation in precipitation. From 1974 to 1990, low‐elevation air temperatures at the Coweeta Laboratory ranged from annual means of 11.8 to 14.2 °C, compared with a 55‐year average (1936–1990) of 12.6 °C. My re‐analyses indicate that the upward spread of the hybrid zone is correlated with increasing air temperatures, but not precipitation, and provide an empirical test of a hypothesis for one factor that may have influenced this movement. My results aid in understanding the potential impact that climate change may have on the ecology and evolution of terrestrial salamanders in montane regions.     

ELECTROPHORETIC CONFIRMATION OF INTERSPECIFIC HYBRIDIZATION IN AESCULUS (HIPPOCASTANACEAE) AND THE GENETIC STRUCTURE OF A BROAD HYBRID ZONE

Within a broad (>200 km wide) hybrid zone involving three parapatric species of Aesculus, we observed coincident clines in allele frequency for 6 of 14 electrophoretic loci. The cooccurrence of alleles characteristic of A. pavia, A. sylvatica, and A. flava was used to estimate genetic admixtures in 48 populations involving various hybrids between these taxa in the southeastern United States. High levels of allelic polymorphism (up to 40% greater than the parental taxa) were observed in hybrid populations and also in some populations bordering the hybrid zone. A detailed analysis of a portion of the hybrid zone involving A. pavia and A. sylvatica revealed a highly asymmetrical pattern of gene flow, predominantly from Coastal Plain populations of A. pavia into Piedmont populations of A. sylvatica. Computer simulations were used to generate expected genotypic arrays for parental, F₁; and backcross individuals, which were compared with natural populations using a character index scoring system. In these comparisons, hybrid individuals could be distinguished from either parent, but F₁ and backcross progeny could not be distinguished from each other. Most hybrid populations were found to include hybrids and one of the parental taxa, but never both parents. Three populations appeared to be predominantly hybrids with no identifiable parental individuals. Hybrids occurred commonly at least 150 km beyond the range of A. pavia, but usually not more than 25 km beyond the range of A. sylvatica. Introgression, suggested by genetically hybrid individuals and significant gene admixtures of two or more species in populations lacking morphological evidence of hybridization, may extend the hybrid zone further in both directions. The absence of one or both parental species from hybrid populations implies a selective disadvantage to parentals in the hybrid zone and/or that hybridization has occurred through long-distance gene flow via pollen, primarily from A. pavia into A. sylvatica. Long-distance pollen movement in plants may generate hybrid zones of qualitatively different structure than those observed in animals, where gene flow involves dispersal of individuals.     

ESTIMATING THE MATING BEHAVIOR OF A PAIR OF HYBRIDIZING HELICONIUS SPECIES IN THE WILD

Premating isolation between incipient species is rarely studied in nature, even though mating tests in captivity may give an inaccurate picture of natural hybridization. We studied premating barriers between the warningly colored butterflies Heliconius erato and H. himera (Lepidoptera) in a narrow contact zone in Ecuador, where hybrids are found at low frequency. Eggs obtained from wild-mated females, supplemented with eggs and young larvae collected from the wild, were reared to adulthood. Adult color patterns of these progeny were then used to infer how their parents must have mated. Likelihood was used to estimate both the frequencies of potential parental genotypes from adult phenotypes collected in the wild, and the degree of assortative mating from the inferred parents. The frequencies of parental genotypes varied across the hybrid zone, but our statistical method allowed estimates of hybrid deficit and assortative mating to be integrated across all sites sampled. The best estimate of the frequency of F₁ and backcross hybrid adults in the center of the hybrid zone was 10%, with support limits (7.1%, 13.0%; support limits are asymptotically equivalent to 95% confidence limits). Mating was highly assortative: in the center of the hybrid zone the cross-mating probability between H. erato and H. himera was only 5% (0.3%, 21.4%). Wild hybrids themselves mated with both pure forms, and the probabilities that they mated in any direction were not significantly lower than those among conspecifics. These results are consistent with earlier laboratory studies on mate choice, and suggest that selection against hybrids must be strong to prevent formation of a hybrid swarm. Unfortunately, the wide support limits on mating behavior precluded a measure of the strength of selection from these data alone. Our statistical approach provides a useful general method for estimating mate choice in the wild.     

Low levels of hybridization across two contact zones among three species of woodpeckers (Sphyrapicus sapsuckers)

Three species of closely related woodpeckers (sapsuckers; Sphyrapicus) hybridize where they come into contact, presenting a rare ‘λ‐shape’ meeting of hybrid zones. Two of the three arms of this hybrid zone are located on either side of the Interior Plateau of British Columbia, Canada bordering the foothills of the Coast Mountains and the Rocky Mountains. The third arm is located in the eastern foothills of the Rocky Mountains. The zones of hybridization present high variability of phenotypes and alleles in relatively small areas and provide an opportunity to examine levels of reproductive isolation between the taxa involved. We examined phenotypes (morphometric traits and plumage) and genotypes of 175 live birds across the two hybrid zones. We used the Genotyping By Sequencing (GBS) method to identify 180 partially diagnostic single nucleotide polymorphisms (SNPs) to generate a genetic hybrid index (GHI) for each bird. Phenotypically diverged S. ruber and S. nuchalis are genetically closely related, while S. nuchalis and S. varius have similar plumage but are well separated at the genetic markers studied. The width of both hybrid zones is narrower than expected under neutrality, and analyses of both genotypes and phenotypes indicate that hybrids are rare in the hybrid zone. Rarity of hybrids indicates assortative mating and/or some form of fitness reduction in hybrids, which might maintain the species complex despite close genetic distance and introgression. These findings further support the treatment of the three taxa as distinct species.     

Introgressive hybridization of Senecio hercynicus and S. ovatus (Compositae,Senecioneae) along an altitudinal gradient in Harz National Park (Germany)

Introgressive hybridization of Senecio hercynicus and S. ovatus (Compositae, Senecioneae) was studied in a hybrid zone on the southern slopes of Mt Brocken (Harz Mountains, Germany). A total of 415 plants representing 10 stands along an altitudinal gradient were investigated using multivariate statistical analyses of morphological characters and molecular markers (random amplified polymorphic DNA[RAPD]). Both types of traits detected pure S. hercynicus stands on the summit plateau, pure S. ovatus stands at the lowest elevations, and hybrid swarms at intermediate elevations. While morphological and molecular patterns coincided, some individuals in hybrid stands combined morphological patterns typical of S. ovatus with RAPD patterns typical of S. hercynicus, and vice versa. In general, introgression was symmetrical within stands, though one stand combined S. ovatus characters with the glandular hair typical for S. hercynicus, and two stands combined a S. hercynicus typical RAPD genotype with morphological characters shifted towards S. ovatus. Because pure stands of S. hercynicus occurred only on the summit plateau of Mt Brocken, and markers typical for S. ovatus were detectable in stands up to 1040 m a.s.l., future fusion or assimilation of the rare form, S. hercynicus, by the more widespread S. ovatus appears possible at Mt Brocken.     

PATTERNS OF HYBRIDIZATION IN THE PIRIQUETA CAROLINIANA COMPLEX IN CENTRAL FLORIDA: EVIDENCE FOR AN EXPANDING HYBRID ZONE

We have identified a broad zone of hybridization between two morphologically and ecologically distinct herbaceous perennial taxa (morphotypes) within the Piriqueta caroliniana complex, which extends more than 300 km across the central Florida peninsula. Phylogeographic analyses indicate that the caroliniana morphotype has been present in north and central Florida since the early Pleistocene and that the viridis morphotype has immigrated into southern Florida much more recently. We examine the distribution of diagnostic morphological characters and nuclear genetic markers to assess the extent and patterns of introgression in this system. The results are consistent with the hypothesis that this hybrid zone has expanded north in recent history as viridis alleles have introgressed into regions that were previously occupied by populations of caroliniana. Genetic markers diagnostic for caroliniana have consistently high frequencies across the hybrid zone, whereas markers for viridis are extremely variable among populations with frequency reversals in adjacent populations. The latter pattern is probably the result of the combined stochastic effects of dispersal and drift on viridis alleles as they introgressed northward. Additional evidence for the recent expansion of this hybrid zone comes from patterns of variation for morphological and genetic markers. As expected for an expanding hybrid zone, within-population morphological variation was greatest toward the advancing front of introgression and levels of genetic variation for neutral diagnostic markers were greatest in the region of initial contact and lower in areas of recent expansion. The observed patterns of variation suggest that at least some hybrid genotypes have high fitnesses, which has led to the expansion of the hybrid zone via the displacement of parental genotypes in central Florida.     

THE EFFECTS OF HOST-PLANT GENOTYPE,HYBRIDIZATION, AND ENVIRONMENT ON GALL-APHID ATTACK AND SURVIVAL IN COTTONWOOD: THE IMPORTANCE OF GENETIC STUDIES AND THE UTILITY OF RFLPS

Using restriction fragment length polymorphisms (RFLPs) we show how host-plant genotype and hybridization in cotton wood, Populus sp., affects the attack and survival of the gall-forming aphid, Pemphigus betae. Fremont cottonwoods, hybrid ???F1's and backcross ???1's were found to be highly resistant, while backcross 2's, 3's, 4's and pure narrowleafs ranged from highly resistant to highly susceptible with only a few trees being highly resistant. Results from our genetic analysis also suggest that resistance is likely polygenic, not the result of single gene resistance. In addition, genetic analysis showed that studies based upon leaf morphology alone give an inaccurate assessment of host-plant genotype ???and the extent of hybridization, altering previous views of the relationship between plant hybridization and pest attack. Previous studies assumed that narrowleafs were more resistant than backcross genotypes based upon comparisons of overall levels of resistance between the hybrid zone and the “pure” narrowleaf zone. Results from RFLP analyses, however, show that there are no significant differences in the levels of resistance between backcross genotypes (BC2's-4's) and pure narrowleafs. Furthermore, results show that the “pure” narrowleaf zone is in fact a mixture of pure and backcross genotypes, extending the zone of introgression previously reported. Experiments in combination with RFLP analyses suggest that resistance traits are differentially expressed along an environmental gradient partially explaining the previously reported differences in resistance between these two regions. In light of our results it is clear that genetic studies will be necessary to discern the true relationship between hybridization and pest resistance. Until such studies are widely conducted generalizations regarding the effects of hybridization on the structure and dynamics of pest populations will be premature at best.     

Hybridization and mitochondrial genome introgression between Rana chensinensis and R. kukunoris

Mitochondrial genome (mito‐genome) introgression among metazoans is commonplace, and several biological processes may promote such introgression. We examined two proposed processes for the mito‐genome introgression between Rana chensinensis and R. kukunoris: natural hybridization and sex‐biased dispersal. We sampled 477 individuals from 28 sites in the potential hybrid zone in the western Tsinling Mountains. Mitochondrial gene (cyt‐b) trees were used to examine the introgression events. Microsatellite DNA loci, cyt‐b and morphological data were used to identify hybrids and to examine the extent of natural hybridization. We detected rampant bidirectional introgressions, both ancient and recent, between the two species. Furthermore, we found a wide hybrid zone, and frequent and asymmetric hybridization. The hybrid zone cline analysis revealed a clear mitochondrial–nuclear discordance; while most nuclear markers displayed similar and steep clines, cyt‐b had a displaced cline centre and a more gradual and wider cline. We also detected strong and asymmetric historical maternal gene flow across the hybrid zone. This widespread hybridization and detected low mito‐nuclear conflicts may, at least partially, explain the high frequency of introgression. Lastly, microsatellite data and population genetic methods were used to assess sex‐biased dispersal. A weak pattern of female‐biased dispersal was detected in both species, suggesting it may not play an important role in the observed introgression. Our data are consistent with the hybridization hypothesis, but support for the sex‐biased dispersal hypothesis is weak. We further suggest that selective advantages of the R. kukunoris‐type mito‐genome in thermal adaptation may also contribute to the introgression between the two species.     

Hybridization between Townsend's Dendroica townsendi and black‐throated green warblers D. virens in an avian suture zone

Hybrid zones between species provide natural systems for the study of processes involved in divergence, reproductive isolation and speciation. Townsend's Dendroica townsendi and black‐throated green D. virens warblers are phenotypically and genetically divergent groups that occur in western and eastern North America respectively, with potential for range contact in the Rocky Mountains of British Columbia, where other west–east avian pairs come into contact. Although one potential hybrid (a phenotypic Townsend's warbler with the black‐throated green mitochondrial DNA) has been previously reported, there have been no studies of interactions between the taxa in potential areas of sympatry. To determine whether interbreeding between these species is a regular occurrence we examined variation in individuals across the area of putative range overlap. Analysis of plumage, morphology, and mitochondrial (COI) and nuclear molecular markers (CHD1Z and numt‐Dco1) shows surprisingly extensive hybridization between these species, with at least 38% of individuals in the hybrid zone being either hybrids or backcrosses. Each of the traits displays a sigmoidal cline centred along the eastern slope of the Rocky Mountains (molecular cline centres averaging 50 km east of the crest of the Rockies, ranging from 41 to 56 km). The clines are narrow (average molecular cline width is 60 km, ranging from 40 to 87 km) relative to the dispersal distance of related warbler species, suggesting that selection is maintaining the hybrid zone; we discuss possible sources of selection. Given the narrowness of the zone we recommend the two forms should continue to be treated as separate taxonomic species. Townsend's warblers also form an extensively studied hybrid zone with their more closely related southern relative, the hermit warbler D. occidentalis. The combined system of three discrete forms separated by narrow hybrid zones provides an excellent system for the study of hybridization, reproductive isolation and speciation.     

ABRUPT CLINE FOR SEX CHROMOSOMES IN A HYBRID ZONE BETWEEN TWO SPECIES OF MICE

We compared the patterns of movement of sex chromosomal and autosomal loci along a 160 km transect across a zone of hybridization between M. domesticus and M. musculus in southern Germany and western Austria using seven genetic markers. These included one Y-specific DNA sequence (YB10), two X-specific loci (DXWas68 and DXWas31), and four autosomal isozyme loci (Es-10, Es-1, Mpi-1, and Np-1). Random effects logistic regression analysis enabled us to examine the relationship between M. domesticus allele frequency and geographic distance from the western edge of the hybrid zone and allowed statistical evaluation of differences in cline midpoint and width among loci. More limited movement was observed for all three sex chromosomal markers across the zone compared with three of the four autosomal markers. If differential movement reflects fitness differences of specific alleles (or alleles at closely linked loci) on a hybrid background, then alleles that move to a limited extent across a hybrid zone may contribute to hybrid breakdown between two species. The limited flow of both X- and Y-specific alleles suggest that sex chromosomes have played an important role in Mus speciation.     

Testing an hypothesis of hybrid zone movement for toads in France

Hybrid zone movement may result in substantial unidirectional introgression of selectively neutral material from the local to the advancing species, leaving a genetic footprint. This genetic footprint is represented by a trail of asymmetric tails and displaced cline centres in the wake of the moving hybrid zone. A peak of admixture linkage disequilibrium is predicted to exist ahead of the centre of the moving hybrid zone. We test these predictions of the movement hypothesis in a hybrid zone between common (Bufo bufo) and spined toads (B. spinosus), using 31 nuclear and one mtDNA SNPs along a transect in the northwest of France. Average effective selection in Bufo hybrids is low and clines vary in shape and centre. A weak pattern of asymmetric introgression is inferred from cline discordance of seven nuclear markers. The dominant direction of gene flow is from B. spinosus to B. bufo and is in support of southward movement of the hybrid zone. Conversely, a peak of admixture linkage disequilibrium north of the hybrid zone suggests northward movement. These contrasting results can be explained by reproductive isolation of the B. spinosus and B. bufo gene pools at the southern (B. spinosus) side of the hybrid zone. The joint occurrence of asymmetric introgression and admixture linkage disequilibrium can also be explained by the combination of low dispersal and random genetic drift due to low effective population sizes.     

AN EMPIRICAL TEST OF PREDICTIONS OF TWO COMPETING MODELS FOR THE MAINTENANCE AND FATE OF HYBRID ZONES: BOTH MODELS ARE SUPPORTED IN A HARD-CLAM HYBRID ZONE

Two models developed to discern the mode of selection in hybrid zones differ in some predictions. The tension-zone model predicts that selection acts against hybrids and independently of the environment (endogenous selection) and that selection is invariant throughout the hybrid zone. The ecological selection-gradient, or ecotone, model maintains that fitness of different genotypes varies in response to environmental variation (exogenous selection) and thus, that in a region of the zone, fitness of hybrids is at least equal to that of the parental species. Therefore, to assess the predominant mode of selection operating in a hybrid zone, it is fundamental to evaluate whether selection is acting specifically against hybrid individuals, that is, whether hybridity alone is the basis for deficiencies of hybrids, and to evaluate whether the relative fitness of hybrids versus that of pure species varies across the zone. In a hardclam (genus Mercenaria) hybrid zone located in a polyhaline lagoon in east-central Florida, we used age-specific and location-specific analyses to determine that a hybrid deficit occurrs, that the deficit seems to be due to selection against hybrids, and that selection varies across the zone. Various measures of deviation from Hardy-Weinberg equilibrium, linkage disequilibrium analyses, and shifts in allele frequencies at semidiagnostic loci support the idea that selection is strongest in the northern region of the lagoon, the zone of sympatry and hybridization. Southward, into the range of M. mercenaria (the numerically predominant species), the percentage of hybrids remains relatively high and selection against hybrids decreases. For some genetic linkage groups, selection for M. mercenaria alleles seems to be occurring, but selection seems to be acting principally against alleles characteristic of M. mercenaria and, to a lesser degree, for alleles characteristic of M. campechiensis (the rarer species). These findings and others from previous analyses we have done on this hybrid zone demonstrate that selection in the zone is complex, and that characteristics of both the tension-zone and ecotone models are present. Supporting the tension-zone model, selection against hybrids per se clearly occurs, but specific genotypes seem to be at a selective disadvantage, whereas others have a selective advantage, and selection operates differentially on the two parental species within the zone. Supporting the ecotone model, the strength of overall selection varies throughout the zone, and environmentally mediated selection in which each species and hybrids have an advantage in specific habitats occurs, but some selection against hybrids is invariant throughout the zone. Thus, the structure and genetic architecture of this hybrid zone appear to be products of a complicated interaction between both types of selective forces cited in the two competing models.