STRONG GENETIC DIVERGENCE AMONG POPULATIONS OF A MARINE FISH WITH LIMITED DISPERSAL, ACANTHOCHROMIS POLYACANTHUS, WITHIN THE GREAT BARRIER REEF AND THE CORAL SEA

Abstract Acanthochromis polyacanthus is an unusual tropical marine damselfish that uniquely lacks pelagic larvae and has lost the capacity for broad‐scale dispersal among coral reefs. On the modern Great Barrier Reef (GBR), three color morphs meet and hydridize at two zones of secondary contact. Allozyme electrophoreses revealed strong differences between morphs from the southern zone but few differences between morphs from the northern counterpart, thus suggesting different contact histories. We explore the phylogeography of Acanthochromis polyacanthus with mitochondrial cytochrome b region sequences (alignment of 565 positions) obtained from 126 individuals representing seven to 12 fish from 13 sites distributed over 12 reefs of the GBR and the Coral Sea. The samples revealed three major clades: (1) black fish collected from the southern GBR; (2) bicolored fish collected from the GBR and one reef (Osprey) from the northern Coral Sea; (3) black and white monomorphs collected from six reefs in the Coral Sea. All three clades were well supported (72–100%) by bootstrap analyses. Sequence divergences were very high between the major clades (mean = 7.6%) as well as within them (2.0–3.6%). Within clades, most reefs segregated as monophyletic assemblages. This was revealed both by phylogenetic analyses and AMOVAs that showed that 72–90% of the variance originated from differences among groups, whereas only 5–13% originated within populations. These patterns are discussed in relation to the known geological history of coral reefs of the GBR and the Coral Sea. Finally, we ask whether the monospecific status of Acanthochromis should be revisited because the sequence divergences found among our samples is substantially greater than those recorded among well‐recognized species in other reef fishes.     

A 2000 km genetic wake yields evidence for northern glacial refugia and hybrid zone movement in a pair of songbirds

Hybrid zones are natural experiments that expose the forces maintaining species differences. But for cases where a trait of one of the hybridizing pair appears shifted into the range of the other, the underlying mechanism can be difficult to infer. For example, hybridization between hermit warbler (Dendroica occidentalis) and Townsend's warbler (Dendroica townsendi) is restricted to narrow hybrid zones in Washington and Oregon, yet hermit mtDNA can be found in phenotypically pure Townsend's populations up to 2000 km north along the Pacific coast. This could reflect introgression of selectively favoured hermit mitochondria north across the hybrid zones, or a neutral genetic wake left behind following southern zone movement. Hermit mitochondrial haplotypes in populations of coastal Townsend's exhibit relatively high genetic diversity and significant divergence from those found in populations of hermit warblers. This contradicts the predictions of selective introgression, but is consistent with a northern population of hermits diverging in a glacial refugium before being replaced by Townsend's via aggressive hybridization. Previous field studies showing Townsend's males to be competitively superior to hermit males support this scenario, and suggest that the extreme hybrid zone movement evidenced by the hermit mitochondrial wake represents an extinction in progress.     

Inferences from a rapidly moving hybrid zone

Anartia fatima and Anartia amathea (Lepidoptera: Nymphalidae) are sister taxa whose ranges abut in a narrow hybrid zone in eastern Panama. At the center of the zone, hybrids are abundant, although deviations from Hardy-Weinberg and linkage disequilibria are strong, due in part to assortative mating. We measured differences across the zone in four wing color-pattern characters, three allozyme loci, and mitochondrial haplotype. Wing pattern, allozyme, and mitochondrial clines were coincident (i.e., had the same positions) and concordant (i.e.. all markers had similar cline shapes, about 28 km wide). Repeated samples demonstrated that the hybrid zone has been moving eastwards at an average rate of 2.5 km/year over the past 20 years, accompanied by an equivalent movement of the mtDNA cline. No introgression of mtDNA haplotypes were found in the 'wake" of the moving cline, as might be expected for a neutral marker. The concordance of morphological and mtDNA clines between 1994 and 2000, in spite of hybrid zone movement, suggests strong epistasis between the mitochondrial genome and nuclear loci. Cline movement is achieved mainly by pure fatima immigrating into amathea populations; hybrids had little effect, and were presumably outcompeted by fitter pure fatima genotypes. This movement can be explained if random dispersal of 7-19 km x gen(1/2) is coupled with a competitive advantage to A. fatima genomes of 2-5%. Hybrid zone motion is equivalent to Phase III of Wright's shifting balance. Hybrid zone movement has rarely been considered likely in the past, but our results show that it may be more important in biogeography and evolution than generally realized.     

Introgression between invasive and native blue mussels (genus Mytilus) in the central California hybrid zone

The ecological and genetic factors determining the extent of introgression between species in secondary contact zones remain poorly understood. Here, we investigate the relative importance of isolating barriers and the demographic expansion of invasive Mytilus galloprovincialis on the magnitude and the direction of introgression with the native Mytilus trossulus in a hybrid zone in central California. We use double‐digest restriction‐site‐associated DNA sequencing (ddRADseq) to genotype 1337 randomly selected single nucleotide polymorphisms and accurately distinguish early and advanced generation hybrids for the first time in the central California Mytilus spp. hybrid zone. Weak levels of introgression were observed in both directions but were slightly more prevalent from the native M. trossulus into the invasive M. galloprovincialis. Few early and advanced backcrossed individuals were observed across the hybrid zone confirming the presence of strong barriers to interbreeding. Heterogeneous patterns of admixture across the zone of contact were consistent with the colonization history of M. galloprovincialis with more extensive introgression in northern localities furthest away from the putative site of introduction in southern California. These observations reinforce the importance of dynamic spatial and demographic expansions in determining patterns of introgression between close congeners, even in those with high dispersal potential and well‐developed reproductive barriers. Our results suggest that the threat posed by invasive M. galloprovincialis is more ecological than genetic as it has displaced, and continues to displace the native M. trossulus from much of central and southern California.     

Gene‐flow across the European crow hybrid zone – a spatial simulation

In hybrid zones populations that are otherwise allopatric meet and produce hybrids. One of the most well‐known hybrid zones occurs throughout much of Europe between the hooded and carrion crows. Even though these species (or subspecies) of crows look very different, genetic differentiation is weak, and introgression seems to be common. In a spatial simulation that was based on empirically estimated values, we investigated how introgressing alleles that would confer some fitness advantage would flow across the zone. The advantage was assumed to be some unknown factor that enhanced survival for carriers relative to non‐carriers. We varied the yearly survival advantage between 0 to 10% and recorded how this would affect zone shape and position. In the simulation we cycled ‘yearly’ events such as reproduction, mortality and juvenile dispersal. We started the simulation by equipping all individuals of one crow type outside the zone with a homozygotic set of the beneficial allele. At all levels of advantage the allele first rapidly became fixed in the donator crow type, then transgressed into the receiving type more slowly and finally became fixed in all individuals. The time until fixation varied from around 176 yr for a 10% advantage to around 20 000 yr for 0% advantage. An exciting discovery was that the position of the zone would move during the introgression but then stop. The reason is that the beneficial allele would be common in the donating type but not in the receiving type during the introgression event. At large the crow hybrid zone appears to have been stable for a long time but there have been small recent northward movements of carrion crows in both Scotland and Denmark. We suggest that introgression by itself is a factor that should be considered as an explanation for why hybrid zones move temporarily.     

Patterns of introgression across an expanding hybrid zone: analysing historical patterns of gene flow using nonequilibrium approaches

Previous studies suggest that the hybrid zone between two taxa in the Piriqueta caroliniana complex in central Florida (south-eastern North America) has recently expanded with hybrids replacing parental genotypes across a broad region of the Florida peninsula. Here I use patterns of genetic disequilibria and levels of differentiation among populations to infer historical patterns of introgression across this broad hybrid zone. There were strong positive associations among taxon-specific alleles at the southern extent of hybridization, but disequilibria values were close to zero across the central and northern portions of the hybrid zone. Levels of among-population differentiation in the central portion of the hybrid zone were relatively low, and increased towards the northern extent of hybridization. The high levels of disequilibria are coincident with the sharp clines at the southern end of the hybrid zone, suggesting that there is a tension zone in this region that is maintained by selection against hybrids and dispersal from parental regions. The levels of disequilibria within populations and patterns of differentiation among populations are consistent with historical introgression and northward expansion of this hybrid zone, which may have slowed or ceased in recent generations.     

Comparing clines on molecular and phenotypic traits in hybrid zones: a window on tension zone models

The study of zones of secondary contact provides insight into the maintenance of reproductive isolation. Tension zone theory supplies powerful tools for assessing how dispersal and selection shape hybrid zones. We present a multimodal analysis of phenotypic clines in conjunction with clines at molecular markers in a hybrid zone between Larus glaucescens and Larus occidentalis. We developed a new method to analyze simultaneously clines of quantitative traits and molecular data. Low linkage disequilibrium and the lack of coincidence between clines at six microsatellites, a mitochondrial DNA region, and two phenotypic traits indicated introgression. However, the hypothesis of neutral diffusion was rejected based on evidence that all of the clines were concordant and narrower than expected for neutral clines, indicating some indirect selection. The analysis of phenotypic variance gave evidence of restricted phenotypic introgression and together with the bimodal distribution of phenotypes suggested that disruptive selection is acting across the hybrid zone, especially on the coloration of bare parts. Multimodal analysis of phenotypic clines also highlighted a shift between the peak of intermediates and the cline center, left behind by hybrid zone motion. High-resolution analysis of phenotypes distribution thus proved useful for detecting hybrid zone movement even without temporal data.     

Genetic structure of hybrid zones between Silene latifolia and Silene dioica (Caryophyllaceae): evidence for introgressive hybridization

Natural hybrid zones provide a valuable tool to study introgressive hybridization, because they can contain a wide variety of genotypes that result from many generations of recombination. Here we used molecular markers and morphological variation to describe the structure of two natural hybrid zones between Silene latifolia and Silene dioica in the Swiss Alps. Populations in both hybrid zones consisted of few intermediate hybrids and were dominated by backcross hybrids. The latter were also found in the parental populations at the margins of the hybrid zones. Out of 209 amplified fragment length polymorphism (AFLP) markers scored in 390 individuals, only 7 (3.3%) were species specific. These results indicate that introgression between S. dioica and S. latifolia is extensive, and that hybrid zones act as bridges to gene flow between these two species. Analysis of linkage disequilibrium identified few populations in which hybridization is ongoing, whereas in most populations linkage disequilibrium has eroded. Where hybridization is ongoing, strong changes in species-specific marker frequencies and morphological traits were observed. Plastid introgression into the hybrid zone was found to be bidirectional, but only the S. latifolia plastid haplotype was found in a nuclear S. dioica background. This unidirectional plastid introgression from S. latifolia into S. dioica is most likely due to pollen-flow from S. dioica onto S. latifolia, and results in plastid capture. Comparisons between the molecular and the morphological hybrid indices revealed that morphology in this study system is useful for identifying hybrids, but not for detailed analysis of hybrid zone structure.     

Plumage and mitochondrial DNA haplotype variation across a moving hybrid zone

We analyze variation in phenotypes and mitochondrial DNA (mtDNA) haplotypes over the breeding ranges of hermit and Townsend's warblers and across two of their three hybrid zones. Within these two hybrid zones, we demonstrate that the placement, shape, and width of transitions in seven plumage characters are remarkably similar, suggesting that a balance between dispersal and sexual selection keeps these hybrid zones narrow. A consistent asymmetry in these character transition curves suggests that Townsend's warblers have a selective advantage over hermit warblers, which is presumably due to the aggressive superiority of Townsend's over hermit males (Pearson and Rohwer 2000). An association between plumage and mtDNA haplotypes shows that pure Townsend's warblers, but not pure hermit warblers, immigrate into these hybrid zones, further supporting the competitive superiority of Townsend's warblers over hermit warblers. The mitochondrial haplotype transitions across these hybrid zones are much wider than the phenotypic transitions and provide no indication that the mtDNA haplotypes representing these two warblers are selectively maintained. More importantly, the phenotypically pure populations of Townsend's warblers throughout a 2,000-km coastal strip north of the Washington hybrid zones contain a preponderance of hermit warbler mtDNA haplotypes. This result suggests massive movement of the hybrid zone between these warblers during the 5,000 years since their most recent interglacial contact. We develop a model to explain the phenotypic and genetic divergence between these warblers and the evolution of their dramatic differences in aggressiveness; we also show how differences in male aggression, in combination with biased pairing patterns, can explain the haplotype footprint recording the historical movement of this hybrid zone.     

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.     

REPRODUCTIVE ISOLATION AND INTROGRESSION BETWEEN NOTROPIS CORNUTUS AND NOTROPIS CHRYSOCEPHALUS (FAMILY CYPRINIDAE): COMPARISON OF MORPHOLOGY,ALLOZYMES, AND MITOCHONDRIAL DNA

Hybrid zones in fluvial fishes may be heterogeneous from drainage to drainage. The comparison of data from morphology, allozymes, and mitochondrial DNA (mtDNA) indicates variability in the causes and degree of restriction of gene flow between Notropis cornutus and Notropis chrysocephalus. Allozyme marker loci show frequency-dependent introgression; i.e., the rarer species, whichever it is at a particular locality, tends to exhibit a higher proportion of introgressed alleles. Unlike allozymes, introgression of mtDNA haplotypes varies geographically. In westward-flowing Michigan drainages, N. cornutus mtDNA haplotypes are more common in F₁ hybrids and backcrosses, independent of parental frequencies. In eastward-flowing Michigan drainages, N. chrysocephalus mtDNA is more common in F₁ hybrids and backcrosses; this pattern may be due to local ecological effects or frequency-dependent introgression. Morphological data alone are not sufficient to distinguish all classes of hybrids. The lack of concordance of morphological, allozymic, and mtDNA introgression patterns implies operation of one or two factors: 1) geographically variable patterns of selection against different hybrid and backcross combinations or 2) genetic differences between Michigan populations inhabiting eastward- and westward-flowing drainage systems accumulated during historical 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.     

Biogeography of a southern hemisphere freshwater fish: how important is marine dispersal?

Galaxias maculatus is one of the world's most widely distributed freshwater fish. This species has a marine-tolerant juvenile phase, and a geographical range extending through much of the southern hemisphere. We conducted phylogeographic analyses of 163 control region haplotypes of G. maculatus, including samples from New Zealand (five locations), Tasmania (one location) and Chile (one location). A lack of genetic structure among New Zealand samples suggests that marine dispersal facilitates considerable gene flow on an intra-continental scale. The discovery of a Tasmanian-like haplotype in one of 144 New Zealand samples indicates that inter-continental marine dispersal occurs but is insufficient to prevent mitochondrial DNA differentiation among continents. The sister relationship of Tasmanian and New Zealand clades implies that marine dispersal is an important biogeographical mechanism for this species. However, a vicariant role in the divergence of eastern and western Pacific G. maculatus cannot be rejected.     

Molecular patterns of introgression in a classic hybrid zone between the Australian tree frogs,Litoria ewingii and L. paraewingi: evidence of a tension zone

Hybrid zones provide a rare opportunity to explore the processes involved in reproductive isolation and speciation. The southern hybrid zone between the southeastern Australian tree frogs Litoria ewingii and L. paraewingi has been comprehensively studied over the last 40 years, primarily using reproductive compatibility experiments and male advertisement calls. We used mitochondrial DNA (mtDNA) and eight nuclear microsatellite markers to characterize this hybrid zone along a historically studied transect and to test various dispersal‐dependent and dispersal‐independent hybrid zone models. The species are genetically distinct and the level of hybridization within the contact zone is low, with the majority of admixed individuals representing later‐generation hybrids. Based on previous experimental genetic compatibility studies, we predicted that hybrids with L. paraewingi mtDNA would be more frequent than hybrids with L. ewingii mtDNA. Surprisingly, a greater proportion of the identified hybrids had L. ewingii mtDNA. Geographical cline analyses showed a sharp transition in allele frequencies across the transect, and both the mtDNA and microsatellite data showed concordant cline centres, but were best supported by a model that allowed width to vary. Overall, the L. ewingii–L. paraewingi hybrid zone is best characterized as a tension zone, due to the narrow cline width, concordant genetic clines and low levels of hybridization.     

Within-population structure highlighted by differential introgression across semipermeable barriers to gene flow in Anguilla marmorata

In the marine environment, differential gene exchange between partially reproductively isolated taxa can result in introgression that extends over long distances due to high larval dispersal potential. However, the degree to which this process contributes to interlocus variance of genetic differentiation within introgressed populations remains unclear. Using a genome-scan approach in the Indo-Pacific eel Anguilla marmorata, we investigated the degree of interpopulation genetic differentiation, the rate of introgression, and within-population genetic patterns at 858 AFLP markers genotyped in 1117 individuals. Three divergent populations were identified based on clustering analysis. Genetic assignments of individuals revealed the existence of different types of hybrids that tended to co-occur with parental genotypes in three population contact zones. Highly variable levels of genetic differentiation were found between populations across the AFLP markers, and reduced rates of introgression were shown at some highly differentiated loci. Gene flow across semipermeable genetic barriers was shown to generate spatial introgression patterns at some loci which define within-population structure over long distances. These results suggest that differential introgression in subdivided populations may be relevant when interpreting spatial variation patterns displayed by outlying loci in other marine fish populations.     

Structure of a mosaic hybrid zone between the field crickets Gryllus firmus and G. pennsylvanicus

Hybrid zones provide insight into the nature of species boundaries and the evolution of barriers to gene exchange. Characterizing multiple regions within hybrid zones is essential for understanding both their history and current dynamics. Here, we describe a previously uncharacterized region of a well‐studied hybrid zone between two species of field crickets, Gryllus pennsylvanicus and G. firmus. We use a combination of mitochondrial DNA sequencing, morphological data, and modeling of environmental variables to identify the ecological factors structuring the hybrid zone and define patterns of hybridization and introgression. We find an association between species distribution and natural habitat; Gryllus pennsylvanicus occupies natural habitat along forest edges and natural clearings, whereas G. firmus occupies more disturbed areas in agricultural and suburban environments. Hybridization and introgression occur across patch boundaries; there is evidence of substantial admixture both in morphological characters and mtDNA, over a broad geographic area. Nonetheless, the distribution of morphological types is bimodal. Given that F₁ hybrids are viable and fertile in the lab, this suggests that strong pre‐zygotic barriers are operating in this portion of the hybrid zone.     

Expanding hybrid zone between Solea aegyptiaca and Solea senegalensis: genetic evidence over two decades

Several marine hybrid zones have been described and studied during the last years. Assessing the movements of extending hybrid zones is central to improve our understanding of evolutionary processes. We have re-examined the hybrid zone between Solea aegyptiaca and Solea senegalensis that was first described 22 years ago in northern Tunisia when introgressed S. senegalensis individuals were found in the Gulf of Tunis, whereas locally caught S. aegyptiaca were genetically pure. Six population samples harvested both inside and outside the area where the two fish species coexist were genotyped for allozymes and exon-primed intron length polymorphism. Both types of markers were congruent and revealed that introgression takes place indeed in both directions. A high introgression level (36.4%) in the Bizerta lagoon and much less outside indicate that this is the main area where hybridization occurs while introgression clines towards the south in S. aegyptiaca and towards the north in S. senegalensis plead in favour of the existence of a unimodal hybrid zone. The higher introgression level calculated in the current study (when compared to 16% reported formerly) and the newly found introgressed S. aegyptiaca in Bizerta lagoon seem to indicate that the genetic exchanges occurring between the two taxa are evolving and not stabilized yet.     

Characterization of a contemporaneous hybrid zone between two darter species (Etheostoma bison and E. caeruleum) in the Buffalo River System

Hybrid zones have long intrigued evolutionary biologists and provide a natural laboratory to explore the evolution of reproductive isolation (speciation). Molecular characterization of hybrid zone dynamics can provide insight into the strength of reproductive isolation as well as the underlying evolutionary processes shaping gene flow. Approximately one-third of darter species naturally hybridize making this species-rich North American freshwater teleost fish clade an ideal system to investigate the extent and direction of hybridization. The objective of this study was to use diagnostic microsatellite markers to calculate genetic hybrid index scores of two syntopic, but distantly related darter species, Etheostoma bison and Etheostoma caeruleum. A combination of hybrid index scores, assignment tests, and mitochondrial haplotype profiles uncovered mixed ancestry in approximately 6 % of sampled adult individuals, supporting contemporaneous hybridization that was previously undocumented in E. bison. Moreover, hybrids were not limited to the F₁ generation, but encompassed the entire suite of hybrid categories (F₁, F₂ and backcross hybrids). The low number of hybrids assigned to each hybrid category represents a bimodal hybrid zone, suggesting reproductive isolation is strong (but incomplete) and also advocates for the ability of hybrids to produce second-generation hybrids and backcross into both parental species, mediating introgression across species boundaries. To this end, cytonuclear profiles of the sampled parental species and hybrids were consistent with bidirectional gene flow, although there was an overall trend of asymmetric hybridization between E. caeruleum females and E. bison males. The spatiotemporal variation in hybridization rates and resulting cytonuclear patterns expanded on in this study provide a comparative genetic framework on which future studies can begin to elucidate the underlying processes that not only generate a mosaic hybrid zone, but maintain the distinctness of species in the face of gene flow.     

Hybridization and population genetics of two macaque species in Sulawesi, Indonesia

This study investigates hybridization and population genetics of two species of macaque monkey in Sulawesi, Indonesia, using molecular markers from mitochondrial, autosomal, and Y-chromosome DNA. Hybridization is the interbreeding of individuals from different parental taxa that are distinguishable by one or more heritable characteristics. Because hybridization can affect population structure of the parental taxa, it is an important consideration for conservation management. On the Indonesian island of Sulawesi an explosive diversification of macaques has occurred; seven of 19 species in the genus Macaca live on this island. The contact zone of the subjects of this study, M. maura and M. tonkeana, is located at the base of the southwestern peninsula of Sulawesi. Land conversion in Sulawesi is occurring at an alarming pace; currently two species of Sulawesi macaque, one of which is M. maura, are classified as endangered species. Results of this study indicate that hybridization among M. maura and M. tonkeana has led to different distributions of molecular variation in mitochondrial DNA and nuclear DNA in the contact zone; mitochondrial DNA shows a sharp transition from M. maura to M. tonkeana haplotypes, but nuclear DNA from the parental taxa is homogenized in a narrow hybrid zone. Similarly, within M. maura divergent mitochondrial DNA haplotypes are geographically structured but population subdivision in the nuclear genome is low or absent. In M. tonkeana, mitochondrial DNA haplotypes are geographically structured and a high level of nuclear DNA population subdivision is present in this species. These results are largely consistent with a macaque behavioral paradigm of female philopatry and obligate male dispersal, suggest that introgression between M. maura and M. tonkeana is restricted to the hybrid zone, and delineate one conservation management unit in M. maura and at least two in M. tonkeana.     

Evidence for strong assortative mating,limited gene flow,and strong differentiation across the blue‐footed/Peruvian booby hybrid zone in northern Peru

Hybrid zones represent natural laboratories in which the processes of divergence and genetic isolation can be examined. The generation and maintenance of a hybrid zone requires mispairing and successful reproduction between organisms that differ in one or more heritable traits. Understanding the dynamics of hybridization between two species requires an understanding of the extent to which they have diverged genetically, the frequency of misparing and hybrid production, and the extent of introgression. Three hundred and twenty one blue‐footed Sula nebouxii and Peruvian S. variegata boobies from the eastern tropical Pacific Ocean were analyzed using 19 putatively neutral genetic markers to evaluate interspecific differentiation, to classify morphological hybrids using Bayesian assignments, and to characterize hybridization using cline theory and Bayesian assignments. The species were well differentiated at mitochondrial and nuclear microsatellites, the hybrid zone was bimodal (contained a high frequency of each parental species but a low frequency of hybrids), and morphologically intermediate individuals were most likely F1 hybrids resulting from mating between female Peruvian boobies and male blue‐footed boobies. Clines in allele frequency could be constrained to share a common geographic centre but could not be constrained to share a common width. Peruvian and blue‐footed boobies hybridize infrequently, potentially due to strong premating reproductive isolation; however, backcrossing appears to facilitate introgression from blue‐footed to Peruvian boobies in this hybrid system.