Phenotypic and genetic introgression across a moving woodpecker hybrid zone

In hybrid zones in which two divergent taxa come into secondary contact and interbreed, selection can maintain phenotypic diversity despite widespread genetic introgression. Red‐breasted (Sphyrapicus ruber) and red‐naped (S. nuchalis) sapsuckers meet and hybridize along a narrow contact zone that stretches from northern California to southern British Columbia. We found strong evidence for changes in the structure of this hybrid zone across time, with significant temporal shifts in allele frequencies and in the proportions of parental phenotypes across the landscape. In addition to these shifts, we found that differences in plumage predict genetic differences (R² = 0.80), suggesting that plumage is a useful proxy for assessing ancestry. We also found a significant bimodal distribution of hybrids across the contact zone, suggesting that premating barriers may be driving reproductive isolation, perhaps as a result of assortative mating based on plumage differences. However, despite evidence of selection and strong patterns of population structure between parental samples, we found only weak patterns of genetic divergence. Using museum specimens and genomic data, this study of sapsuckers provides insight into the ways in which phenotypic and genetic structure have changed over a 40‐year period, as well as insight into the mechanisms that may contribute to the maintenance of the hybrid zone over time.     

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.     

INCIPIENT SPECIATION DESPITE LITTLE ASSORTATIVE MATING: THE YELLOW‐RUMPED WARBLER HYBRID ZONE

Hybrid zones between recently diverged taxa are natural laboratories for speciation research, allowing us to determine whether there is reproductive isolation between divergent forms and the causes of that isolation. We present a study of a classic avian hybrid zone in North America between two subspecies of the yellow‐rumped warbler (Dendroica coronata). Although previous work has shown very little differentiation in mitochondrial DNA across this hybrid zone, we identified two nuclear loci (one sex‐linked and one autosomal) that show fixed differences across the hybrid zone, in a close concordance with patterns of plumage variation. Temporal stability and limited width of the hybrid zone, along with substantial linkage disequilibrium between these two diagnostic markers in the center of the zone, indicate that there is moderate reproductive isolation between these populations, with an estimated strength of selection maintaining the zone of 18%. Pairing data indicate that assortative mating is either very weak or absent, suggesting that this reproductive isolation is largely due to postmating barriers. Thus, despite extensive hybridization the two forms are distinct evolutionary groups carrying genes for divergent adaptive peaks, and this situation appears relatively stable.     

Cytoplasmic and nuclear markers reveal contrasting patterns of spatial genetic structure in a natural Ipomopsis hybrid zone

Spatial variation in natural selection may play an important role in determining the genetic structure of hybridizing populations. Previous studies have found that F1 hybrids between naturally hybridizing Ipomopsis aggregata and Ipomopsis tenuituba in central Colorado differ in fitness depending on both genotype and environment: hybrids had higher survival when I. aggregata was the maternal parent, except in the centre of the hybrid zone where both hybrid types had high survival. Here, we developed both maternally (cpDNA PCR-RFLP) and biparentally inherited (nuclear AFLP) species-diagnostic markers to characterize the spatial genetic structure of the natural Ipomopsis hybrid zone, and tested the prediction that the majority of natural hybrids have I. aggregata cytoplasm, except in areas near the centre of the hybrid zone. Analyses of 352 individuals from across the hybrid zone indicate that cytoplasmic gene flow is bidirectional, but contrary to expectation, most plants in the hybrid zone have I. tenuituba cytoplasm. This cytotype distribution is consistent with a hybrid zone in historical transition, with I. aggregata nuclear genes advancing into the contact zone. Further, nuclear data show a much more gradual cline than cpDNA markers that is consistent with morphological patterns across the hybrid populations. A mixture of environment- and pollinator-mediated selection may contribute to the current genetic structure of this hybrid system.     

Cytonuclear genetic architecture in mosquitofish populations and the possible roles of introgressive hybridization

Spatial genetic structure in populations of mosquitofish ( Gambusia ) sampled throughout the south-eastern United States was characterized using mitochondrial (mt) DNA and allozyme markers. Both sets of data revealed a pronounced genetic discontinuity (along a broad path extending from south-eastern Mississippi to north-eastern Georgia) that corresponds to a recently recognized distinction between the nominal forms G. affinis to the west and G. holbrooki to the east. However, several populations from the general contact region exhibited unusual allelic associations in high frequency, suggestive of evolutionary processes within a zone of introgressive hybridization. These involve: (i) cytonuclear profiles representing combinations of nuclear and mitochondrial genotypes that tended to be more nearly species-specific and concordant elsewhere; and (ii) significant nuclear gametic disequilibria, perhaps attributable to positive assortative mating and/or differential fitnesses of homospecific vs. recombinant genotypes. However, outside this suspected hybrid region, 'heterospecific' genetic markers also appeared in low frequency, thus complicating interpretations. These discordant alleles on a broader geographic scale may reflect: (a) the retention of polymorphisms from an ancestral gene pool; (b) occasional evolutionary convergence (especially with respect to electrophoretic mobility of allozyme alleles); (c) the 'footprints' of a moving hybrid zone; or (d) differential introgressive penetrance across the current hybrid region.     

Steep clines within a highly permeable genome across a hybrid zone between two subspecies of the European rabbit

Maintenance of genetic distinction in the face of gene flow is an important aspect of the speciation process. Here, we provide a detailed spatial and genetic characterization of a hybrid zone between two subspecies of the European rabbit. We examined patterns of allele frequency change for 22 markers located on the autosomes, X‐chromosome, Y‐chromosome and mtDNA in 1078 individuals sampled across the hybrid zone. While some loci revealed extremely wide clines (w ≥ 300 km) relative to an estimated dispersal of 1.95–4.22 km/generation, others showed abrupt transitions (w ≈ 10 km), indicating localized genomic regions of strong selection against introgression. The subset of loci showing steep clines had largely coincident centers and stepped changes in allele frequency that did not co‐localize with any physical barrier or ecotone, suggesting that the rabbit hybrid zone is a tension zone. The steepest clines were for X‐ and Y‐chromosome markers. Our results are consistent with previous inference based on DNA sequence variation of individuals sampled in allopatry in suggesting that a large proportion of each genome has escaped the overall barrier to gene flow in the middle of the hybrid zone. These results imply an old history of hybridization and high effective gene flow and anticipate that isolation factors should often localize to small genomic regions.     

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.     

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.     

PATTERNS OF VARIATION AND LINKAGE DISEQUILIBRIUM IN A FIELD CRICKET HYBRID ZONE

The distribution of multilocus genotypes found within a natural hybrid zone is determined by the sample of genotypes present when the hybrid zone first formed, by subsequent patterns of genetic exchange between the hybridizing taxa, and by drift and selection within each of the hybrid zone populations. We have used anonymous nuclear DNA restriction fragment polymorphisms (RFLPs) to characterize the array of multilocus genotypes present within a well-studied hybrid zone between two eastern North American field crickets, Gryllus pennsylvanicus and Gryllus firmus. These crickets hybridize along a zone of contact that extends from New England to Virginia. Previous studies have shown that both premating and postmating barriers exist between the two cricket species, but the absence of diagnostic morphological and allozyme markers has made it difficult to assess the consequences of these barriers for genetic exchange. Analyses based on four diagnostic anonymous nuclear markers indicate that hybrid zone populations in Connecticut contain few F₁ hybrids, and that nonrandom associations persist among nuclear gene markers, between nuclear and cytoplasmic markers, and between molecular markers and morphology. Field cricket populations within the hybrid zone are not “hybrid swarms” but consist primarily of crickets that are very much like one or the other of the parental species. Despite ample opportunity for genetic exchange and evidence for introgression at some loci, the two species remain quite distinct. Such a pattern appears to be characteristic of many natural hybrid zones.     

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.     

Good species despite massive hybridization: genetic research on the contact zone between the watersnakes Nerodia sipedon and N. fasciata in the Carolinas, USA

Genomic markers generated with the amplified fragment length polymorphism method revealed extensive, panmictic-like hybridization along the narrow contact zone between the water snakes Nerodia sipedon and Nerodia fasciata in the Carolinas, USA. However, asymmetric distributions of diagnostic markers between both species and low frequencies of backcrossed hybrids with a high value of interspecific mixture infer selection against certain genotypes. This is consistent with a pronounced genetic and morphological preponderance of N. fasciata characters in the hybrid zone. Despite massive hybridization within the contact zone, the existence of nearly fixed genetic markers and the potential inferiority of certain hybrid genotypes support the species status of the two taxa and corroborate known, but nondiagnostic differences in morphology and ecology. This study stretches the applicability of species concepts to cases, where the genetic compatibility between two closely related species is very high, yet, they still evolve and persist as independent entities.     

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.     

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.     

Movement of a Heliconius hybrid zone over 30 years: A Bayesian approach

Hybrid zones have long been of interest to biologists as natural laboratories where we can gain insight into the processes of adaptation and speciation. Repeated sampling of individual hybrid zones has been particularly useful in elucidating the dynamic balance between selection and dispersal that maintains most hybrid zones. Here, we revisit a hybrid zone between Heliconius erato butterflies in Panamá for a third time over more than 30 years. We combine a novel Bayesian extension of stepped‐cline hybrid zone models with environmental data to understand the genetic and environmental causes of cline dynamics in this species. The cline has continued to move west, likely due to dominance drive, but has slowed and broadened. Environmental analyses suggest that widespread deforestation in Panamá could be leading to decreased avian predation and relaxed selection, causing the observed changes in cline dynamics.     

Strong reproductive isolation and narrow genomic tracts of differentiation among three woodpecker species in secondary contact

Hybrid zones allow the measurement of gene flow across the genome, producing insight into the genomic architecture of speciation. Such analysis is particularly powerful when applied to multiple pairs of hybridizing species, as patterns of genomic differentiation can then be related to age of the hybridizing species, providing a view into the build‐up of differentiation over time. We examined 33 809 single nucleotide polymorphisms (SNPs) in three hybridizing woodpecker species: Red‐breasted, Red‐naped and Yellow‐bellied sapsuckers (Sphyrapicus ruber, Sphyrapicus nuchalis and Sphyrapicus varius), two of which (ruber and nuchalis) are much more closely related than each is to the third (varius). To identify positions of SNPs on chromosomes, we developed a localization method based on comparative genomics. We found narrow clines, bimodal distributions of hybrid indices and genomic regions with decreased rates of introgression. These results suggest moderately strong reproductive isolation among species and selection against specific hybrid genotypes. We found 19 small regions of strong differentiation between species, partly shared among species pairs, but no large regions of differentiation. An association analysis revealed a single strong‐effect candidate locus associated with plumage, possibly explaining mismatch among the three species in genomic relatedness and plumage similarity. Our comparative analysis of species pairs of different age and their hybrid zones showed that moderately strong reproductive isolation can occur with little genomic differentiation, but that reproductive isolation is incomplete even with much greater genomic differentiation, implying there are long periods of time when hybridization is possible if diverging populations are in geographic contact.     

Mammalian hybrid zones: a review

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Changing climate mediates sapsucker (Aves: Sphyrapicus) hybrid zone movement

Hybrid zones, where two divergent taxa meet and interbreed, offer unique opportunities to investigate how climate contributes to reproductive isolation between closely related taxa and how these taxa may respond to climatic changes. Red‐naped (Sphyrapicus nuchalis) and Red‐breasted (Sphyrapicus ruber) sapsuckers (Aves: Picidae) hybridize along a narrow contact zone that stretches from northern California to British Columbia. The hybrid zone between these species has been studied extensively for more than 100 years and represents an excellent system for investigations of the evolution of reproductive isolation. Shifts in the proportions of phenotypes at hybrid localities since 1910 that were inferred using specimens from museum collections were confirmed using species distribution models. We predicted the historical, current, and future distributions of parental and hybrid sapsuckers using Random Forests models to quantify how climate change is affecting hybrid zone movement in the Pacific Northwest. We found observed distribution shifts of parental sapsuckers were likely the result of climate change over the past 100 years, with these shifts predicted to continue for both sapsuckers over the next 80 years. We found Red‐breasted Sapsuckers are predicted to continue to expand, while Red‐naped Sapsuckers are predicted to contract substantially under future climate scenarios. As a result of the predicted changes, the amount of overlap in the distribution of these sapsuckers may decrease. Using hybrid phenotypes, we found the climate niche occupied by the hybrid zone is predicted to disappear under future conditions. The disappearance of this climate niche where the two parental species come into contact and hybridize may lead to a substantial reduction in genetic introgression. Understanding the impacts of global climate change on hybrid zones may help us to better understand how speciation has been shaped by climate in the past, as well as how evolution may respond to climate change in the future.     

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.