Strong selection on male plumage in a hybrid zone between a hybrid bird species and one of its parents

Homoploid hybrid speciation (HHS) requires reproductive barriers between hybrid and parent species, despite incomplete reproductive isolation (RI) between the parents. Novel secondary sexual trait values in hybrids may cause prezygotic isolation from both parents, whereas signals inherited by the hybrid from one parent species may cause prezygotic isolation with the other. Here we investigate whether differences in male plumage function as a premating barrier between the hybrid Italian sparrow and one of its parent species, the house sparrow, in a narrow Alpine hybrid zone. Italian sparrow male plumage is a composite mosaic of the parental traits, with its head plumage most similar to its other parent, the Spanish sparrow. We use geographical cline analysis to examine selection on three plumage traits, 75 nuclear single nucleotide polymorphisms (SNPs) and hybrid indices based on these SNPs. Several SNPs showed evidence of restricted introgression in the Alps, supporting earlier findings. Crown colour exhibited the narrowest plumage cline, representing a 37% (range 4–65%) drop in fitness. The cline was too narrow to be due to neutral introgression. Only crown colour was significantly bimodal in the hybrid zone. Bimodality may be due to RI or a major QTL, although fitness estimates suggest that selection contributes to the pattern. We discuss the implications with respect to HHS and the species status of the Italian sparrow.     

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.     

THE MAINTENANCE OF A CLINE IN THE MARINE SNAIL LITTORINA SAXATILIS: THE ROLE OF HOME SITE ADVANTAGE AND HYBRID FITNESS

Steep clinal transitions in one or several inherited characters between genetically distinct populations are usually referred to as hybrid zones. Essentially two different mechanisms may maintain steep genetic clines. Either selection acts against hybrids that are unfit over the entire zone due to their mixed genetic origin (endogenous selection), or hybrids and parental types attain different fitness values in different parts of the cline (exogenous selection). Survival rate estimates of hybrids and parental forms in different regions of the cline may be used to distinguish between these models to assess how the cline is maintained. We used reciprocal transplants to test the relative survival rates of two parental ecotypes and their hybrids over microscale hybrid zones in the direct-developing marine snail Littorina saxatilis (Olivi) on the rocky shores of Galicia, Spain. One of the parental forms occupies upper and the other lower shores, and the hybrids are found at various proportions (1–38%) along with both parental forms in a midshore zone a few meters wide. The survival rate over one month was 39-52% of the native ecotype on upper shores, but only 2-8% for the lower-shore ecotype. In contrast, only 4-8% of the upper-shore ecotype but 53% of large (> 6 mm) and 8% of small (3-6 mm) native lower-shore ecotype survived in the lower shores. In the midshores, both the two parental ecotypes and the hybrids survived about equally well. Thus there is a considerable advantage for the native ecotypes in the upper and lower shores, while in the hybrid zone none of the morphs, hybrids included, are favored. This indicates that the dimorphism of L. saxatilis is maintained by steep cross-shore selection gradients, thus supporting the selection-gradient model of hybrid zones. We performed field and laboratory experiments that suggest physical factors and predation as important selective agents. Earlier studies indicate assortative mating between the two ecotypes in the midshore. This is unexpected in a hybrid zone maintained by selection gradients, and it seems as if the reproductive barrier compresses the hybrid zone considerably.     

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.     

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

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

EVOLUTIONARY IMPLICATIONS OF DIVERGENT CLINES IN AN AVIAN (MANACUS: AVES) HYBRID ZONE

Abstract A previous study of the hybrid zone in western Panama between white‐collared (Manacus candei) and golden‐collared manakins (M. vitellinus) documented the unidirectional introgression of vitellinus male secondary sexual traits across the zone. Here, we examine the hybrid zone in greater genetic and morphological detail. Statistical comparisons of clines are performed using maximum‐likelihood and nonparametric bootstrap methods. Our results demonstrate that an array of six molecular and two morphometric markers agree in cline position and width. Clines for male collar and belly color are similar in width to the first eight clines, but are shifted in position by at least five cline widths. The result is that birds in intervening populations are genetically and morphometrically very like parental candei, but males have the plumage color of parental vitellinus. Neither neutral diffusion nor nonlinearity of color scales appear to be viable explanations for the large cline shifts. Genetic dominance of vitellinus plumage traits is another potential explanation that will require breeding experiments to test. Sexual selection remains a plausible explanation for the observed introgression of vitellinus color traits in these highly dimorphic, polygynous, lek‐mating birds. Two other clines, including a nondiagnostic isozyme locus, are similar in position to the main cluster of clines, but are broader in width. Thus, introgression at some loci is greater than that detected with diagnostic markers. Assuming that narrow clines are maintained by selection, variation in cline width indicates that selection is not uniform throughout the genome and that diagnostic markers are under more intense selective pressure. The traditional focus on diagnostic markers in studies of hybrid zones may therefore lead to underestimates of average introgression. This effect may be more pronounced in organisms with low levels of genetic divergence between hybridizing taxa.     

Flycatcher song in allopatry and sympatry--convergence, divergence and reinforcement

The theory of reinforcement predicts that natural selection against the production of unfit hybrids favours traits that increase assortative mating. Whether culturally inherited traits, such as bird song, can increase assortative mating by reinforcement is largely unknown. We compared songs of pied (Ficedula hypoleuca) and collared flycatchers (F. albicollis) from two hybrid zones of different ages with songs from allopatric populations. Previously, a character divergence in male plumage traits has been shown to reinforce premating isolation in sympatric flycatchers. In contrast, we find that the song of the pied flycatcher has converged towards that of the collared flycatcher (mixed singing). However, a corresponding divergence in the collared flycatcher shows that the species differences in song characters are maintained in sympatry. Genetic analyses suggest that mixed song is not caused by introgression from the collared flycatcher, but rather due to heterospecific copying. Circumstantial evidence suggests that mixed song may increase the rate of maladaptive hybridization. In the oldest hybrid zone where reinforcement on plumage traits is most pronounced, the frequency of mixed singing and hybridization is also lowest. Thus, we suggest that reinforcement has reduced the frequency of mixed singing in the pied flycatcher and caused a divergence in the song of the collared flycatcher. Whether a culturally inherited trait promotes or opposes speciation in sympatry may depend on its plasticity. The degree of plasticity may be genetically determined and accordingly under selection by reinforcement.     

UNIDIRECTIONAL INTROGREESION OF A SEXUALLY SELECTED TRAIT ACROSS AN AVIAN HYBRID ZONE: A ROLE FOR FEMALE CHOICE?

Hybridization can be an evolutionary creative force by forming new polyploid species, creating novel genetic variation or acting as conduits of potentially advantageous traits between hybridizing forms. Evidence for the latter is often difficult to find because alleles under positive selection can spread rapidly across a hybrid zone and sweep to fixation. In Western Panama, an avian hybrid zone between two species of manakins in the genus Manacus exists where the unidirectional introgression of bright, yellow plumage into a white population provides evidence for the importance of hybrid zones as conduits of advantageous traits. Several lines of indirect evidence suggest that sexual selection favoring yellow plumage drives this asymmetrical spread, but more direct evidence is lacking. Along the edge of the hybrid zone, both yellow- and white-collared manakins are found in the same mating arenas or leks and compete for the same females ("mixed leks"), providing us with a unique opportunity to understand the dynamics of yellow plumage introgression. We studied these mixed leks to determine whether yellow males have a mating advantage over white males and, if so, whether the mating advantage is driven by male-male interactions, female choice, or both. We found that yellow males mated more than white males, suggesting that sexual selection favoring yellow males can, indeed, explain the spread of yellow plumage. However, we found that this advantage occurred only in mixed leks where the frequency of yellow males is greater than white males. This suggests that the advantage of yellow males may depend on the presence of other yellow males, which may slow the rate of introgression in leks where yellow frequency is low such as in areas where yellow males are beginning to colonize the white population. This, along with the geographic barrier posed by major rivers in the hybrid zone, may initially limit or slow the spread of yellow plumage. Finally, we found that yellow and white males were similar in aggression and body size, and held comparable positions within leks. Because these traits or factors are often important in or dictated by aggressive male-male interactions, these comparisons indicate that male-male interaction is not the primary mechanism for the spread of yellow plumage. However, white and yellow males received similar numbers of courtship visits from females but differed in the number of matings, suggesting that females actively rejected white in favor of yellow males. Our results indicate that sexual selection by female choice has driven the unidirectional introgression of yellow plumage into the white population, providing a mechanism for how hybrid zones act as conduits of novel and advantageous traits.     

Experimental evidence that extra-pair mating drives asymmetrical introgression of a sexual trait

Theory suggests that traits under positive selection may introgress asymmetrically across a hybrid zone, potentially driven by sexual selection. Two subspecies of the red-backed fairy-wren (Malurus melanocephalus) differ primarily in a sexual signal used in mate choice—red versus orange male back plumage colour—but phylogeographic analyses suggest asymmetrical introgression of red plumage into the genetic background of the orange subspecies. We hypothesized that this asymmetrical introgression may be facilitated by sexual selection if red males have a mating advantage over orange males. We tested this hypothesis with correlational data and a plumage manipulation experiment where we reddened the back plumage of a subset of orange males to mimic males of the red subspecies. There was no correlational evidence of a mating advantage to naturally redder males in this population. Experimentally reddened males sired a similar amount of within-pair young and lost paternity at the same rate as orange males, but they sired significantly more extra-pair young, leading to substantially higher total reproductive success. Thus, we conclude that sexual selection via extra-pair mating is a likely mechanism responsible for the asymmetrical introgression of plumage colour in this system, and is potentially driven by a sensory bias for the red plumage signal.     

ECOLOGICAL SEGREGATION IN A SMALL MAMMAL HYBRID ZONE: HABITAT‐SPECIFIC MATING OPPORTUNITIES AND SELECTION AGAINST HYBRIDS RESTRICT GENE FLOW ON A FINE SPATIAL SCALE

The degree to which closely related species interbreed is determined by a complex interaction of ecological, behavioral, and genetic factors. We examine the degree of interbreeding between two woodrat species, Neotoma bryanti and N. lepida, at a sharp ecological transition. We identify the ecological association of each genotypic class, assess the opportunity for mating between these groups, and test whether they have similar patterns of year‐to‐year persistence on our study site. We find that 13% of individuals have a hybrid signature but that the two parental populations and backcrosses are highly segregated by habitat type and use. Also, we find that adult hybrids are comparable to parental types in terms of year‐to‐year persistence on our site but that, among juveniles, significantly fewer hybrids reach adulthood on site compared to their purebred counterparts. Our analyses show that this hybrid zone is maintained by occasional nonassortative mating coupled with hybrid fertility, but that these factors are balanced by lower apparent survival of juvenile hybrids and habitat‐based preference or selection that limits heterospecific mating while promoting backcrossing to habitat‐specific genotypes. This system presents a novel example of the role that sharp resource gradients play in reproductive isolation and the potential for genetic introgression.     

Variation in contact zone dynamics between two species of topminnows, Fundulus notatus and F. olivaceus, across isolated drainage systems

Spatially variable selection pressure within heterogeneous environments can result in the evolution of specialist phenotypes that facilitate co-occurrence of closely related species and limit genetic exchange. If divergent selection pressures maintain reproductive isolation, hybridization is expected to correlate with the strength of underlying ecological gradients and the traits shaped by adaptive processes. We sampled ten replicate topminnow (Fundulus olivaceus and Fundulus notatus) hybrid zones in isolated drainages throughout central and southern North America. In all drainages, species were distributed in an upstream–downstream manner with contact zones localized at confluences featuring abrupt shifts from tributary to river habitat. In two drainages, the typical up and downstream positions of species were reversed. Phenotype differences between the species reflect predicted selection differences along stream gradients. Downstream populations (lower food availability and greater predator pressure) generally showed larger investment in reproduction (higher gonadal somatic index), smaller body size and lower somatic condition compared to upstream populations. Phenotypic differences between the species in the two reversed drainages were consistent with convergence of life history traits in the respective habitats. Phenotypes of individuals of hybrid origin (F₁ hybrids or backcrosses) were not significantly different from the average of the two parental forms, though there were trends towards reduced fitness. The prevalence of hybridization among drainages ranged from no hybrids in two drainages to near random mating. The strongest correlates of hybridization rate among replicate hybrid zones were similarity in body shape and the homogeneity of habitat through tributary-river confluences. The two reversed orientation hybrid zones also exhibited high prevalence of hybrids suggesting that phenotypic convergence could lead to increased hybridization.     

REPRODUCTIVE BEHAVIOR OF FEMALE BUNTINGS: ISOLATING MECHANISMS IN A HYBRIDIZING PAIR OF SPECIES

The plumage characteristics of male Indigo and Lazuli Buntings are distinct, but the two species can learn each other's songs. Populations comprising Indigo, Lazuli and hybrid individuals occur in the Great Plains of North America, and assortative mating has been inferred from morphometric data. We devised a laboratory assay for determining female preferences for visual and vocal characteristics of conspecific and heterospecific males and for mixtures of these characteristics, such as might be encountered in an overlap population. Females of both species gave more copulation-solicitation displays when exposed to conspecific plumage and vocalizations than when exposed to heterospecific plumage and vocalizations. Females gave intermediate and similar responses to the combinations of conspecific plumage with heterospecific vocalizations and heterospecific plumage with conspecific vocalizations. Thus, in the absence of other potentially important variables, female reproductive behavior is consistent with the hypothesis of assortative mating, based upon both vocal and visual traits of the males and caused by female choice in this semispecies pair.     

Sperm length divergence as a potential prezygotic barrier in a passerine hybrid zone

The saltmarsh sparrow Ammospiza caudacuta and Nelson''s sparrow A. nelsoni differ in ecological niche, mating behavior, and plumage, but they hybridize where their breeding distributions overlap. In this advanced hybrid zone, past interbreeding and current backcrossing result in substantial genomic introgression in both directions, although few hybrids are currently produced in most locations. However, because both species are nonterritorial and have only brief male–female interactions, it is difficult to determine to what extent assortative mating explains the low frequency of hybrid offspring. Since females often copulate with multiple males, a role of sperm as a postcopulatory prezygotic barrier appears plausible. Here, we show that sperm length differs between the two species in the hybrid zone, with low among‐male variation consistent with strong postcopulatory sexual selection on sperm cells. We hypothesize that divergence in sperm length may constitute a reproductive barrier between species, as sperm length co‐evolves with the size of specialized female sperm storage tubules. Sperm does not appear to act as a postzygotic barrier, as sperm from hybrids was unexceptional.     

Effects of assortative mate choice on the genomic and morphological structure of a hybrid zone between two bird subspecies

Phenotypic differentiation plays an important role in the formation and maintenance of reproductive barriers. In some cases, variation in a few key aspects of phenotype can promote and maintain divergence; hence, the identification of these traits and their associations with patterns of genomic divergence is crucial for understanding the patterns and processes of population differentiation. We studied hybridization between the alba and personata subspecies of the white wagtail (Motacilla alba), and quantified divergence and introgression of multiple morphological traits and 19,437 SNP loci on a 3,000 km transect. Our goal was to identify traits that may contribute to reproductive barriers and to assess how variation in these traits corresponds to patterns of genome‐wide divergence. Variation in only one trait—head plumage patterning—was consistent with reproductive isolation. Transitions in head plumage were steep and occurred over otherwise morphologically and genetically homogeneous populations, whereas cline centres for other traits and genomic ancestry were displaced over 100 km from the head cline. Field observational data show that social pairs mated assortatively by head plumage, suggesting that these phenotypes are maintained by divergent mating preferences. In contrast, variation in all other traits and genetic markers could be explained by neutral diffusion, although weak ecological selection cannot be ruled out. Our results emphasize that assortative mating may maintain phenotypic differences independent of other processes shaping genome‐wide variation, consistent with other recent findings that raise questions about the relative importance of mate choice, ecological selection and selectively neutral processes for divergent evolution.     

PLUMAGE EVOLUTION IN THE OROPENDOLAS AND CACIQUES: DIFFERENT DIVERGENCE RATES IN POLYGYNOUS AND MONOGAMOUS TAXA

Avian plumage colors are frequently used in studies of sexual selection, yet surprisingly little is known about how these traits evolve under different mating systems. We compared historical rates of divergence in male color patterns among the oropendolas and caciques (genera Cacicus , Gymnostinops, Ocyalus , and Psarocolius ), a group with both polygynous and monogamous representatives. Reconstructing the evolution of individual color patches on a molecular phylogeny showed that overall color patterns have changed much more rapidly in oropendolas, which comprise two groups that evolved polygyny independently, than in caciques, which are predominantly monogamous. None of these taxa are notably sexually dichromatic, however, suggesting that higher rates of plumage evolution occurred in both sexes rather than just males. Despite high rates of change, color patterns show few examples of convergence among taxa, similar to the lack of homoplasy in male song among oropendolas but in a stark contrast to the repeated convergence in both plumage and song patterns found in a closely related, monogamous clade, the New World orioles ( Icterus ). Our results support previous suggestions that display traits evolve more rapidly and with less homoplasy in polygynous mating systems, and we provide surprising evidence that these patterns may occur in both sexes.     

Habitat preferences and positive assortative mating in an avian hybrid zone

The relatively narrow hybrid zone between hooded and carrion crow is characterised by a high proportion of breeding hybrids, no known intrinsic postzygotic costs, and most likely, high gene flow between populations. It is therefore of interest to identify factors preventing a breakdown of the zone. In this study, we investigate the possibility that habitat choice could create assortative mating in the German–Danish part of the zone. In a GIS analysis, we used two approaches to investigate the role of habitat selection in the maintenance of this zone. We tested if there are 1) any correlations between habitat and the shape of the hybrid zone, and 2) any differences in habitat preferences between territorial individuals of the crow phenotypes. The most extensive habitat, non‐irrigated arable land (Habitat 1), which covers almost 50% of the area in the zone was preferred by all three phenotypes (carrion, hooded and hybrid crows), but showed no change in frequency across the zone. Pastures (Habitat 2) and agricultural areas mixed with natural vegetation (Habitat 4) correlated with the shape of the zone, but only H4 differed in preference between the crow phenotypes. H4 was preferred by hybrids and hooded crows, but not by carrion crows. This habitat is present only in 17% of the hooded crow territories, and thus, it is not likely to have a strong influence on the maintenance of the hybrid zone. There was evidence of assortative mating in pure phenotypes, whereas hybrids showed no consistent mating pattern. Because our GIS analyses suggest that species specific habitat preferences are not involved in shaping the assortativeness, we discuss other possible mechanisms.     

Influence of Environment and Mitochondrial Heritage on the Ecological Characteristics of Fish in a Hybrid Zone

Background

Ecological characteristics (growth, morphology, reproduction) arise from the interaction between environmental factors and genetics. Genetic analysis of individuals'' life history traits might be used to improve our understanding of mechanisms that form and maintain a hybrid zone.

Methodology/Principal Findings

A fish hybrid zone was used to characterize the process of natural selection. Data were collected during two reproductive periods (2001 and 2002) and 1117 individuals (nase, Chondrostama nasus nasus, sofie C. toxostoma toxostoma and hybrids) were sampled. Reproductive dates of the two parental species overlapped at sympatric sites. The nase had an earlier reproductive period than the sofie; males had longer reproductive periods for both species. Hybridisation between female nase and male sofie was the most likely. Hybrids had a reproductive period similar to the inherited parental mitochondrial type. Growth and reproductive information from different environments has been synthesised following a bayesian approach of the von Bertalanffy model. Hybrid life history traits appear to link with maternal heritage. Hybrid size from the age of two and size at first maturity appeared to be closer to the size of the maternal origin species (nase or sofie).Median growth rates for hybrids were similar and intermediate between those of the parental species. We observed variable life history traits for hybrids and pure forms in the different parts of the hybrid zone. Geometrical analysis of the hybrid fish shape gave evidence of two main morphologies with a link to maternal heritage.

Conclusions/Significance

Selective mating seemed to be the underlying process which, with mitochondrial heritage, could explain the evolution of the studied hybrid zone. More generally, we showed the importance of studies on hybrid zones and specifically the study of individuals'' ecological characteristics, to improve our understanding of speciation.     

Variable visual habitats may influence the spread of colourful plumage across an avian hybrid zone

Several studies have shown that hybridization can be a creative process by acting as a conduit for the spread of adaptive traits between species, but few provide the mechanism that favours this spread. In the hybrid zone between the golden- (Manacus vitellinus) and white-collared (Manacus candei) manakins, sexual selection drives the introgression of golden/yellow plumage into the white species; however, the mechanism for the yellow male's mating advantage and the reasons why yellow plumage has not swept further into the white species remain mostly speculative. We quantified the colour properties of male plumage, the background and the ambient light at the hybrid zone, and allopatric golden and white populations. As measured by the perceived difference in colour between plumage and background, we found that yellow plumage appears more conspicuous than white plumage in the hybrid zone and allopatric golden-collar habitats, whereas white plumage appears more conspicuous than yellow plumage in the allopatric white-collared habitat. These results suggest a mechanism for the unidirectional spread of yellow plumage across the hybrid zone but slowed movement beyond it.     

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.     

NATURAL SELECTION ON QUANTITATIVE TRAITS IN THE BOMBINA HYBRID ZONE

Observations on the means, variances, and covariances of quantitative traits across hybrid zones can give information similar to that from Mendelian markers. In addition, they can identify particular traits through which the cline is maintained. We describe a survey of six traits across the hybrid zone between Bombina bombina and Bombina variegata (Amphibia: Discoglossidae) near Pe??enica in Croatia. We obtained laboratory measuments of the belly pattern, skin thickness, mating call, skeletal form, egg size, and the developmental time of tadpoles. Although offspring from hybrid populations showed no evidence of reduced viability, a third of the F₁ families failed completely, irrespective of the direction of the cross. All traits differed significantly between the taxa. Clines in belly pattern, skin thickness, mating call, and skeletal form were closely concordant with clines in four diagnostic enzyme loci. However, the cline in developmental time was displaced towards bombina, and the cline in egg size was displaced towards variegata. This discordance could be because the traits are not inherited additively or because they are subject to different selection pressures. We favor the latter explanation in the case of developmental time. We show that moderate selection acting directly on a trait suffices to shift its position; rather stronger selection is needed to change its width appreciably. Within hybrid populations, there are significant associations among quantitative traits, and between traits and enzymes. Phenotypic variances also increase in hybrid populations. These observations can be explained by linkage disequilibria among the underlying loci. However, the average magnitude of the covariance between traits is about half that expected from the linkage disequilibria between enzyme loci. The discrepancy is not readily explained by nonadditive gene action. This puzzle is now unresolved and calls for further investigation.