Cline coupling and uncoupling in a stickleback hybrid zone

Strong ecological selection on a genetic locus can maintain allele frequency differences between populations in different environments, even in the face of hybridization. When alleles at divergent loci come into tight linkage disequilibrium, selection acts on them as a unit and can significantly reduce gene flow. For populations interbreeding across a hybrid zone, linkage disequilibria between loci can force clines to share the same slopes and centers. However, strong ecological selection on a locus can also pull its cline away from the others, reducing linkage disequilibrium and weakening the barrier to gene flow. We looked for this “cline uncoupling” effect in a hybrid zone between stream resident and anadromous sticklebacks at two genes known to be under divergent natural selection (Eda and ATP1a1) and five morphological traits that repeatedly evolve in freshwater stickleback. These clines were all steep and located together at the top of the estuary, such that we found no evidence for cline uncoupling. However, we did not observe the stepped shape normally associated with steep concordant clines. It thus remains possible that these clines cluster together because their individual selection regimes are identical, but this would be very surprising given their diverse roles in osmoregulation, body armor, and swimming performance.     

Inferences of selection and migration in the Danish house mouse hybrid zone

We analysed the patterns of allele frequency change for ten diagnostic autosomal allozyme loci in the hybrid zone between the house mouse subspecies Mus musculus domesticus and M. m. musculus in central Jutland. After determining the general orientation of the clines of allele frequencies, we analysed the cline shapes along the direction of maximum gradient. Eight of the ten clines are best described by steep central steps with coincident positions and an average width of 8.9 km (support limits 7.6–12.4) flanked by tails of introgression, indicating the existence of a barrier to gene flow and only weak selection on the loci studied. We derived estimates of migration from linkage disequilibrium in the centre of the zone, and by applying isolation by distance methods to microsatellite data from some of these populations. These give concordant estimates of σ =  0.5–0.8 km generation     . The barrier to gene flow is of the order of 20 km (support limits 14–28), and could be explained by selection of a few per cent at 43–120 underdominant loci that reduces the mean fitness in the central populations to 0.45. Some of the clines appear symmetrical, whereas others are strongly asymmetrical, and two loci appear to have escaped the central barrier to gene flow, reflecting the differential action of selection on different parts of the genome. Asymmetry is always in the direction of more introgression into musculus , indicating either a general progression of domesticus into the musculus territory, possibly mediated by differential behaviour, or past movement of the hybrid zone in the opposite direction, impeded by potential geographical barriers to migration in domesticus territory.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 84 , 593–616.     

Preliminary analysis of a hybrid zone between two subspecies of Zootermopsis nevadensis

For species largely allopatric in distribution, zones of contact provide an opportunity for hybridization, testing grounds for species boundaries, and may result in the formation of a new species. Thus, hybrid zones have the potential to provide important insights into speciation. In this study, we performed a preliminary analysis of a hybrid zone between two subspecies of the dampwood termite, Zootermopsis nevadensis (Z. n. nuttingi and Z. n. nevadensis) near Bartle, CA, using 12 microsatellite loci and a mitochondrial gene. Fifty-seven colonies collected in 36 locations were analyzed. The analysis of genetic markers revealed a large hybrid zone approximately 104 km in width. Although stepped clines best explained the data, we are unable to rule out the existence of a mosaic hybrid zone. We inferred weak selection (s* < 0.05%) against hybrids, but the data also suggested the existence of a barrier to gene flow from Z. n. nevadensis to Z. n. nuttingi, but not in the other direction. Given the large zone of contact, extensive sampling is needed to obtain a more complete characterization of the hybrid zone. However, the results of this study suggest that despite the accumulation of phenotypic and genetic differences and time since divergence (~2 million years), Z. n. nuttingi and Z. n. nevadensis are capable of extensive hybridization.     

GENETIC ANALYSIS OF A HYBRID ZONE BETWEEN THE FIRE-BELLIED TOADS,BOMBINA BOMBINA AND B. VARIEGATA,NEAR CRACOW IN SOUTHERN POLAND

The fire-bellied toads Bombina bombina and B. variegata differ extensively in biochemistry, morphology, and behavior. We use a survey of five diagnostic enzyme loci across the hybrid zone near Cracow in Southern Poland to estimate the dispersal rate, selection pressures, and numbers of loci which maintain this zone. The enzyme clines coincide closely with each other and with morphological and mitochondrial DNA clines. Although the zone lies on a broad transition between environments suitable for bombina and variegata, the close concordance of diverse characters, together with increased aberrations and mortality in hybrids, suggest that the zone is maintained largely by selection against hybrids. There are strong “linkage disequilibria” between each pair of (unlinked) enzyme loci (R? = 0.129 [2-unit support limits: 0.119–0.139]). These are probably caused by gene flow into the zone, and they give an estimate of dispersal (σ = 890 [790–940] m gen^?½). The clines are sharply stepped, with most of the change occurring within 6.15 (5.45–6.45) km, but with long tails of introgression on either side. This implies that the effective selection pressure on each enzyme marker (due largely to disequilibrium with other loci) is s* = 0.17 (0.159–0.181) at the center but that the selection acting directly on the enzyme loci is weak or zero (s_e < 0.0038). The stepped pattern implies a barrier to gene flow of 220 (48–415) km. This would substantially delay neutral introgression but would have little effect on advantageous alleles; the two taxa need not evolve independently. Strong selection is needed to maintain such a barrier: hybrid populations must have their mean fitness reduced by a factor of 0.65 (0.60–0.77). This selection must be spread over a large number of loci to account for the concordant patterns and the observed cline widths (N = 300 [80–2,000]).     

Differential introgression across newt hybrid zones: Evidence from replicated transects

Genomic heterogeneity of divergence between hybridizing species may reflect heterogeneity of introgression, but also processes unrelated to hybridization. Heterogeneous introgression and its repeatability can be directly tested in natural hybrid zones by examining multiple transects. Here, we studied hybrid zones between the European newts Lissotriton montandoni and two lineages of Lissotriton vulgaris, with replicate transects within each zone. Over 1,000 nuclear genes located on a linkage map and mitochondrial DNA were investigated using geographical and genomic clines. Overall, the five transects were all similar, showing hallmarks of strong reproductive isolation: bimodal distribution of genotypes in central populations and narrow allele frequency clines. However, the extent of introgression differed between the zones, possibly as a consequence of their different ages, as suggested by the analysis of heterozygosity runs in diagnostic markers. In three transects genomic signatures of small‐scale (~2 km) zone movements were detected. We found limited overlap of cline outliers between transects, and only weak evidence of stronger differentiation of introgression between zones than between transects within zones. Introgression was heterogeneous across linkage groups, with patterns of heterogeneity similar between transects and zones. Predefined candidates for increased or reduced introgression exhibited only a subtle tendency in the expected direction, suggesting that interspecific differentiation is not a reliable indicator for the strength of introgression. These hierarchically sampled hybrid zones of apparently different ages show how introgression unfolds with time and offer an excellent opportunity to dissect the dynamics of hybridization and architecture of reproductive isolation at advanced stages of speciation.     

Genetic and phenotypic variation across a hybrid zone between ecologically divergent tree squirrels (Tamiasciurus)

A hybrid zone along an environmental gradient should contain a clinal pattern of genetic and phenotypic variation. This occurs because divergent selection in the two parental habitats is typically strong enough to overcome the homogenizing effects of gene flow across the environmental transition. We studied hybridization between two parapatric tree squirrels (Tamiasciurus spp.) across a forest gradient over which the two species vary in coloration, cranial morphology and body size. We sampled 397 individuals at 29 locations across a 600‐km transect to seek genetic evidence for hybridization; upon confirming hybridization, we examined levels of genetic admixture in relation to maintenance of phenotypic divergence despite potentially homogenizing gene flow. Applying population assignment analyses to microsatellite data, we found that Tamiasciurus douglasii and T. hudsonicus form two distinct genetic clusters but also hybridize, mostly within transitional forest habitat. Overall, based on this nuclear analysis, 48% of the specimens were characterized as T. douglasii, 9% as hybrids and 43% as T. hudsonicus. Hybrids appeared to be reproductively viable, as evidenced by the presence of later‐generation hybrid genotypes. Observed clines in ecologically important phenotypic traits—fur coloration and cranial morphology—were sharper than the cline of putatively neutral mtDNA, which suggests that divergent selection may maintain phenotypic distinctiveness. The relatively recent divergence of these two species (probably late Pleistocene), apparent lack of prezygotic isolating mechanisms and geographic coincidence of cline centres for both genetic and phenotypic variation suggest that environmental factors play a large role in maintaining the distinctiveness of these two species across the hybrid zone.     

Structure and dynamics of hybrid zones at different stages of speciation in the common vole (Microtus arvalis)

The genetic structure and dynamics of hybrid zones provide crucial information for understanding the processes and mechanisms of evolutionary divergence and speciation. In general, higher levels of evolutionary divergence between taxa are more likely to be associated with reproductive isolation and may result in suppressed or strongly restricted hybridization. In this study, we examined two secondary contact zones between three deep evolutionary lineages in the common vole (Microtus arvalis). Differences in divergence times between the lineages can shed light on different stages of reproductive isolation and thus provide information on the ongoing speciation process in M. arvalis. We examined more than 800 individuals for mitochondrial (mtDNA), Y‐chromosome and autosomal markers and used assignment and cline analysis methods to characterize the extent and direction of gene flow in the contact zones. Introgression of both autosomal and mtDNA markers in a relatively broad area of admixture indicates selectively neutral hybridization between the least‐divergent lineages (Central and Eastern) without evidence for partial reproductive isolation. In contrast, a very narrow area of hybridization, shifts in marker clines and the quasi‐absence of Y‐chromosome introgression support a moving hybrid zone and unidirectional selection against male hybrids between the lineages with older divergence (Central and Western). Data from a replicate transect further support non‐neutral processes in this hybrid zone and also suggest a role for landscape history in the movement and shaping of geneflow profiles.     

Distinct sources of gene flow produce contrasting population genetic dynamics at different range boundaries of a Choristoneura budworm

Populations are often exposed to multiple sources of gene flow, but accounts are lacking of the population genetic dynamics that result from these interactions or their effects on local evolution. Using a genomic clines framework applied to 1,195 single nucleotide polymorphisms, we documented genomewide, locus‐specific patterns of introgression between Choristoneura occidentalis biennis spruce budworms and two ecologically divergent relatives, C. o. occidentalis and Choristoneura fumiferana, that it interacts with at alternate boundaries of its range. We observe contrasting hybrid indexes between the two hybrid zones, no overlap in “gene‐flow outliers” (clines showing relatively extreme extents or rates of locus‐specific introgression) and variable linkage disequilibrium among those outliers. At the same time, correlated genomewide rates of introgression between zones suggest the presence of processes common to both boundaries. These findings highlight the contrasting population genetic dynamics that can occur at separate frontiers of a single population, while also suggesting that shared patterns may frequently accompany cases of divergence‐with‐gene‐flow that involve a lineage in common. Our results point to potentially complex evolutionary outcomes for populations experiencing multiple sources of gene flow.     

Discordant patterns of introgression across a narrow hybrid zone between two cryptic lineages of an Iberian endemic newt

The study of natural hybrid zones can illuminate aspects of lineage divergence and speciation in morphologically cryptic taxa. We studied a hybrid zone between two highly divergent but morphologically similar lineages (south‐western and south‐eastern) of the Iberian endemic Bosca's newt (Lissotriton boscai) in SW Iberia with a multilocus dataset (microsatellites, nuclear and mitochondrial genes). STRUCTURE and NEWHYBRIDS analyses retrieved few admixed individuals, which classified as backcrosses involving parental individuals of the south‐western lineage. Our results show asymmetric introgression of mtDNA beyond the contact from this lineage into the south‐eastern lineage. Analysis of nongeographic introgression patterns revealed asymmetries in the direction of introgression, but except for mtDNA, we did not find evidence for nonconcordant introgression patterns across nuclear loci. Analysis of a 150‐km transect across the hybrid zone showed broadly coincident cline widths (ca. 3.2–27.9 km), and concordant cline centres across all markers, except for mtDNA that is displaced ca. 60 km northward. Results from ecological niche modelling show that the hybrid zone is in a climatically homogenous area with suitable habitat for the species, suggesting that contact between the two lineages is unlikely to occur further south as their distributions are currently separated by an extensive area of unfavourable habitat. Taken together, our findings suggest the genetic structure of this hybrid zone results from the interplay of historical (biogeographic) and population‐level processes. The narrowness and coincidence of genetic clines can be explained by weak selection against hybrids and reflect a degree of reproductive isolation that is consistent with cryptic speciation.     

Clinal variation in a brown lemur (Eulemur spp.) hybrid zone: Combining morphological,genetic and climatic data to examine stability

Studies of hybrid zones can inform our understanding of reproductive isolation and speciation. Two species of brown lemur (Eulemur rufifrons and E. cinereiceps) form an apparently stable hybrid zone in the Andringitra region of south‐eastern Madagascar. The aim of this study was to identify factors that contribute to this stability. We sampled animals at 11 sites along a 90‐km transect through the hybrid zone and examined variation in 26 microsatellites, the D‐loop region of mitochondrial DNA, six pelage and nine morphological traits; we also included samples collected in more distant allopatric sites. Clines in these traits were noncoincident, and there was no increase in either inbreeding coefficients or linkage disequilibrium at the centre of the zone. These results could suggest that the hybrid zone is maintained by weak selection against hybrids, conforming to either the tension zone or geographical selection‐gradient model. However, a closer examination of clines in pelage and microsatellites indicates that these clines are not sigmoid or stepped in shape but instead plateau at their centre. Sites within the hybrid zone also occur in a distinct habitat, characterized by greater seasonality in precipitation and lower seasonality in temperature. Together, these findings suggest that the hybrid zone may follow the bounded superiority model, with exogenous selection favouring hybrids within the transitional zone. These findings are noteworthy, as examples supporting the bounded superiority model are rare and may indicate a process of ecologically driven speciation without geographical isolation.     

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.     

THE PONTIA DAPLIDICE-ED USA HYBRID ZONE IN NORTHWESTERN ITALY

The pierid butterflies Pontia daplidice and P. edusa, parapatrically distributed in southern Europe, have very similar morphologies and life histories, but show fixed differences at four allozyme markers. We sampled these allozymes in a 28-population transect north of Genoa in Italy, through the hybrid zone where these taxa meet. We used the numerical techniques developed for hybrid zone analysis to study the patterns of genetic differentiation and their underlying evolutionary causes. The hybrid zone is characterized by a very short and steep central region, flanked by broad tails of introgression extended up to 100 km in either direction. From mean two-locus disequilibium of D = 0.148 (maximum-likelihood two-unit support limits 0.139-0.153), and after accounting for minor differences in the center locations of the single-locus clines, which act to bias the dispersal estimate, we estimated a dispersal rate of σ = 4.4 (3.7-5.5) km/gen^1/2. The effective selection needed to maintain the steep central portion is strong, 0.47 < s? < 0.64, when combined over potential intrinsic (genetic background) and extrinsic (ecological) sources of selection. The clines in allozyme loci showed variation that was significantly different between the most divergent shapes, and the differences are attributable to different degrees of introgression on the edusa side of the zone. The average selection acting on individual allozyme loci was high at s_???e  1.5%, but because of the narrowness of the central region of the cline, we suspect that this estimate is somewhat biased by selection on loci closely linked to the allozyme markers. A common question for taxa that show fixed allozyme differences in parapatry is whether or not they are genetically isolated. A fairly general measure of genetic isolation across hybrid zones is the time, T, that it takes a neutral allele to cross the hybrid zone and recombine into the opposite genetic background, given by T = (β/σ)², where β is the barrier strength of the hybrid zone. Genetic isolation in the Pontia zone is weak, with T  25 generations for most allozyme markers. By this measure, populations of daplidice and edusa on opposite sides of the hybrid zone share more identical-by-descent alleles than do populations of phenotypically pure daplidice in, say, France and Morocco. Accordingly, we think it best for systematists to consider edusa as a well-marked subspecies of P. daplidice.     

Formation of a recent hybrid zone offers insight into the geographic puzzle and maintenance of species boundaries in musk turtles

Speciation is the result of an accumulation of reproductive barriers between populations, but pinpointing the factors involved is often difficult. However, hybrid zones can form when these barriers are not complete, especially when lineages come into contact in intermediate or modified habitats. We examine a hybrid zone between two closely related riverine turtle species, Sternotherus depressus and S. peltifer, and use dual‐digest RAD sequencing to understand how this hybrid zone formed and elucidate genomic patterns of reproductive isolation. First, the geographical extent and timing of formation of the hybrid zone is established to provide context for understanding the role of extrinsic and intrinsic reproductive isolating mechanisms in this system. The strength of selection on taxon‐specific contributions to maintenance of the hybrid zone is then inferred using a Bayesian genomic cline model. These analyses identify a role for selection inhibiting introgression in some genomic regions at one end of the hybrid zone and promoting introgression in many loci at the other. When selective pressures necessary to generate outliers to the genomic cline are considered with the geographical and temporal context of this hybrid zone, we conclude that habitat‐specific selection probably limits introgression from S. depressus to S. peltifer in the direction of river flow. However, selection is mediating rapid, unidirectional introgression from S. peltifer to S. depressus, which is probably facilitated by anthropogenic habitat alteration. These findings indicate a potentially imminent threat of population‐level genomic extinction for an already imperiled species due to ongoing human‐caused habitat alteration.     

Evidence for concerted movement of nuclear and mitochondrial clines in a lizard hybrid zone

Moving hybrid zones provide compelling examples of evolution in action, yet long‐term studies that test the assumptions of hybrid zone stability are rare. Using replicated transect samples collected over a 10‐year interval from 2002 to 2012, we find evidence for concerted movement of genetic clines in a plateau fence lizard hybrid zone (Sceloporus tristichus) in Arizona. Cline‐fitting analyses of SNP and mtDNA data both provide evidence that the hybrid zone shifted northward by approximately 2 km during the 10‐year interval. For each sampling period, the mtDNA cline centre is displaced from the SNP cline centre and maintaining an introgression distance of approximately 3 km. The northward expansion of juniper trees into the Little Colorado River Basin in the early 1900s provides a plausible mechanism for hybrid zone formation and movement, and a broadscale quantification of recent land cover change provides support for increased woody species encroachment at the southern end of the hybrid zone. However, population processes can also contribute to hybrid zone movement, and the current stability of the ecotone habitats in the centre of the hybrid zone suggests that movement could decelerate in the future.     

A CONTACT ZONE WITH NONCOINCIDENT CLINES FOR SEX-SPECIFIC MARKERS IN THE FIELD VOLE (MICROTUS AGRESTIS)

A field vole (Microtus agrestis) population characterized by unique mitochondrial DNA (mtDNA) and Y chromosome markers occurs in southwest Sweden. A contact zone between this Lund (Lu) population and a standard (St) population was examined with two sex-specific genetic markers. The field vole mtDNA and Y chromosome clines display a remarkable lack of coincidence, rarely observed in contact zones. The cline width for both markers is about 50 km, but the two clines are displaced from each other: the mtDNA cline is found in the central part of the study area, whereas the cline for the Y chromosome is located in the eastern part. Thus, the absolute width of the Lu-St zone spans about 95–110 km. As a result of the cline shift, all male hybrids carry the Lu-Y chromosome and St-type mtDNA. The other possible male hybrid class is lacking. The distinct noncoincidence of the mtDNA and Y chromosome clines is most likely explained by selection against male hybrids with the St-Y chromosome and Lu-mtDNA. It is possible that incompatibilities between the maternal genome of Lu-type animals and the paternal genome of St-type individuals exist. However, alternative explanations based on neutrality or selective advantage cannot be totally dismissed.     

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.     

Variation in complex mating signals in an “island” hybrid zone between Stenobothrus grasshopper species

Two grasshopper species Stenobothrus rubicundus and S. clavatus were previously shown to meet in a narrow hybrid zone on Mount Tomaros in northern Greece. The species are remarkable for their complex courtship songs accompanied by conspicuous movements of antennae and wings. We analyzed variations in forewing morphology, antenna shape, and courtship song across the hybrid zone using a geographic information system, and we documented three contact zones on Mount Tomaros. All male traits and female wings show abrupt transitions across the contact zones, suggesting that these traits are driven by selection rather than by drift. Male clines in antennae are displaced toward S. clavatus, whereas all clines in wings are displaced toward S. rubicundus. We explain cline discordance as depending on sexual selection via female choice. The high covariance between wings and antennae found in the centers of all contact zones results from high levels of linkage disequilibria among the underlying loci, which in turn more likely results from assortative mating than from selection against hybrids. The covariance is found to be higher in clavatus‐like than rubicundus‐like populations, which implies asymmetric assortative mating in parental‐like sites of the hybrid zone and a movement of the hybrid zone in favor of S. clavatus.     

Cross‐decades stability of an avian hybrid zone

Hybrid zones are particularly valuable for understanding the evolution of partial reproductive isolation between differentiated populations. An increasing number of hybrid zones have been inferred to move over time, but in most such cases zone movement has not been tested with long‐term genomic data. The hybrid zone between Townsend's Warblers (Setophaga townsendi) and Hermit Warblers (S. occidentalis) in the Washington Cascades was previously inferred to be moving from northern S. townsendi southwards towards S. occidentalis, based on plumage and behavioural patterns as well as a 2000‐km genetic wake of hermit mitochondrial DNA (mtDNA) in coastal Townsend's Warblers. We directly tested whether hybrid zone position has changed over 2–3 decades by tracking plumage, mtDNA and nuclear genomic variation across the hybrid zone over two sampling periods (1987–94 and 2015–16). Surprisingly, there was no significant movement in genomic or plumage cline centres between the two time periods. Plumage cline widths were narrower than expected by neutral diffusion, consistent with a ‘tension zone’ model, in which selection against hybrids is balanced by movement of parental forms into the zone. Our results indicate that this hybrid zone is either stable in its location or moving at a rate that is not detectable over 2–3 decades. Despite considerable gene flow, the stable clines in multiple phenotypic and genotypic characters over decades suggest evolutionary stability of this young pair of sister species, allowing divergence to continue. We propose a novel biogeographic scenario to explain these patterns: rather than the hybrid zone having moved thousands of kilometres to its current position, inland Townsend's met coastal Hermit Warbler populations along a broad front of the British Columbia and Alaska coast and hybridization led to replacement of the Hermit Warbler plumage with Townsend's Warbler plumage patterns along this coastline. Hence, hybrid zones along British Columbia and Alaska moved only a short distance from the inland to the coast, whereas the Hermit Warbler phenotype appears stable in Washington and further south. This case provides an example of the complex biogeographic processes that have led to the distribution of current phenotypes within and among closely related species.     

A New Barrier to Dispersal Trapped Old Genetic Clines That Escaped the Easter Microplate Tension Zone of the Pacific Vent Mussels

Comparative phylogeography of deep-sea hydrothermal vent species has uncovered several genetic breaks between populations inhabiting northern and southern latitudes of the East Pacific Rise. However, the geographic width and position of genetic clines are variable among species. In this report, we further characterize the position and strength of barriers to gene flow between populations of the deep-sea vent mussel Bathymodiolus thermophilus. Eight allozyme loci and DNA sequences of four nuclear genes were added to previously published sequences of the cytochrome c oxidase subunit I gene. Our data confirm the presence of two barriers to gene flow, one located at the Easter Microplate (between 21°33′S and 31°S) recently described as a hybrid zone, and the second positioned between 7°25′S and 14°S with each affecting different loci. Coalescence analysis indicates a single vicariant event at the origin of divergence between clades for all nuclear loci, although the clines are now spatially discordant. We thus hypothesize that the Easter Microplate barrier has recently been relaxed after a long period of isolation and that some genetic clines have escaped the barrier and moved northward where they have subsequently been trapped by a reinforcing barrier to gene flow between 7°25′S and 14°S.