Reproductive character displacement is not the only possible outcome of reinforcement

We study the form of the clines in a female mating preference and male display trait using simulations of a hybrid zone. Allopatric populations of two species are connected by demes in a stepping stone arrangement. Results show that reproductive character displacement (a pattern of increased prezygotic isolation in sympatry compared with allopatry) may or may not result when there is reinforcement (defined here as the strengthening of prezygotic isolation as a result of selection against hybrids, relative to the amount of prezygotic isolation present when hybrids are not selected against). Further, reproductive character displacement of the preference may or may not occur when it occurs in the male display. We conclude that the absence of reproductive character displacement is not evidence against the operation of reinforcement.     

Song evolution,speciation, and vocal learning in passerine birds

Phenotypic divergence can promote reproductive isolation and speciation, suggesting a possible link between rates of phenotypic evolution and the tempo of speciation at multiple evolutionary scales. To date, most macroevolutionary studies of diversification have focused on morphological traits, whereas behavioral traits─including vocal signals─are rarely considered. Thus, although behavioral traits often mediate mate choice and gene flow, we have a limited understanding of how behavioral evolution contributes to diversification. Furthermore, the developmental mode by which behavioral traits are acquired may affect rates of behavioral evolution, although this hypothesis is seldom tested in a phylogenetic framework. Here, we examine evidence for rate shifts in vocal evolution and speciation across two major radiations of codistributed passerines: one oscine clade with learned songs (Thraupidae) and one suboscine clade with innate songs (Furnariidae). We find that evolutionary bursts in rates of speciation and song evolution are coincident in both thraupids and furnariids. Further, overall rates of vocal evolution are higher among taxa with learned rather than innate songs. Taken together, these findings suggest an association between macroevolutionary bursts in speciation and vocal evolution, and that the tempo of behavioral evolution can be influenced by variation in developmental modes among lineages.     

Variation in sperm morphology among Afrotropical sunbirds

Birds show considerable variation in sperm morphology. Closely related species and subspecies can show diagnostic differences in sperm size. There is also variation in sperm size among males within a population, and recent evidence from passerine birds suggests that the coefficient of inter‐male variation in sperm length is negatively associated with the level of sperm competition. Here we examined patterns of inter‐ and intra‐specific variation in sperm length in 12 species of sunbird (Nectariniidae) from Nigeria and Cameroon, a group for which such information is extremely limited. We found significant variation among species in sperm total length, with mean values ranging from 74 μm to 116 μm, placing these species within the short to medium sperm length range for passerine birds. Most of this variation was explained by the length of the midpiece, which contains the fused mitochondria and is an important structure for sperm energetics. Relative midpiece length was negatively correlated with the coefficient of inter‐male variation in sperm total length across species, suggesting that sperm competition may have selected for greater midpiece length in this group. We also mapped sperm lengths onto a time‐calibrated phylogeny and found support for a phylogenetic signal in all sperm length components, except head length. A test of various evolutionary or tree transformation models gave strongest support for the Brownian motion model for all sperm components, i.e. divergences were best predicted by the phylogenetic distance between lineages. The coefficients of inter‐male variation in sperm total length indicate that sperm competition is high but variable among sunbird species, as is the case with passerine birds at large.     

Spatial variation in gene flow across a hybrid zone reveals causes of reproductive isolation and asymmetric introgression in wall lizards*

Hybrid zones provide insights into the evolution of reproductive isolation. Sexual selection can contribute to the evolution of reproductive barriers, but it remains poorly understood how sexual traits impact gene flow in secondary contact. Here, we show that a recently evolved suite of sexual traits that function in male-male competition mediates gene flow between two lineages of wall lizards (Podarcis muralis). Gene flow was relatively low and asymmetric in the presence of exaggerated male morphology and coloration compared to when the lineages share the ancestral phenotype. Putative barrier loci were enriched in genomic regions that were highly differentiated between the two lineages and showed low concordance between the transects. The exception was a consistently low genetic exchange around ATXN1, a gene that modulates social behavior. We suggest that this gene may contribute to the male mate preferences that are known to cause lineage-assortative mating in this species. Although female choice modulates the degree of reproductive isolation in a variety of taxa, wall lizards demonstrate that both male-male competition and male mate choice can contribute to the extent of gene flow between lineages.     

Song similarity predicts hybridization in flycatchers

Given that population divergence in sexual signals is an important prerequisite for reproductive isolation, a key prediction is that cases of signal convergence should lead to hybridization. However, empirical studies that quantitatively demonstrate links between phenotypic characters of individuals and their likelihood to hybridize are rare. Here we show that song convergence between sympatric pied (Ficedula hypoleuca) and collared flycatchers (F. albicollis) influence social and sexual interactions between the two species. In sympatry, the majority of male pied flycatchers (65%) include various parts of collared flycatcher song in their song repertoire (but not vice versa). Playback experiments on male interactions demonstrate that male collared flycatchers respond similarly to this 'mixed' song as to conspecific song. Long-term data on pairing patterns show that males singing a converged song attract females of the other species: female collared flycatchers only pair with male pied flycatchers if the males sing the mixed song type. From the perspective of a male pied flycatcher, singing a mixed song type is associated with 30% likelihood of hybridization. This result, combined with our estimates of the frequency of mixed singers, accurately predicts the observed occurrence of hybridization among male pied flycatchers in our study populations (20.45% of 484 pairs; predicted 19.5%). Our results support the suggestion that song functions as the most important prezygotic isolation mechanism in many birds.     

Ecological gradient of sexual selection: elevation and song elaboration in finches

Ecological gradients in natural and sexual selection often result in evolutionary diversification of morphological, life history, and behavioral traits. In particular, elevational changes in habitat structure and climate not only covary with intensity of sexual selection in many taxa, but may also influence evolution of mating signals. Here we examined variation in courtship song in relation to elevation of breeding across cardueline finches-a subfamily of birds that occupies the widest elevational range of extant birds and shows extensive variation in life histories and sexual selection along this range. We predicted that decrease in sexual selection intensity with elevation of breeding documented in this clade would result in a corresponding evolutionary reduction in elaboration of courtship songs. We controlled for the effects of phylogeny, morphology, and habitat structure to uncover a predicted elevational decline in courtship song elaboration; species breeding at lower elevations sang more elaborated and louder songs compared to their sister species breeding at higher elevations. In addition, lower elevation species had longer songs with more notes, whereas frequency components of song did not vary with elevation. We suggest that changes in sexual selection account for the observed patterns of song variation and discuss how elevational gradient in sexual selection may facilitate divergence in mating signals potentially reinforcing or promoting speciation.     

Sexual selection on phenotypic traits in a hybrid zone of Littorina saxatilis (Olivi)

Step clinal transitions in inherited character(s) between genetically distinct populations are usually referred to as hybrid zones. An example is found in the population of the intertidal snail Littorina saxatilis in Galicia (NW Spain). We studied the shape of the overall fitness surface for sexual selection in this hybrid zone, and the position of hybrids and pure morphs on this surface. We found that sexual divergent selection acted on a combination of phenotypic traits separating the pure morphs, and therefore that sexual selection contributed to morph differentiation. The average fitness of hybrids as a group was not significantly different from that of the pure morphs, but they did show divergent sexual selection in some traits. These results are in agreement with a model of divergent selection favouring both the pure morph as well as those hybrids most resembling each morph. The finding of divergent selection is remarkable because quadratic selection gradients are usually weak in nature.     

Vireo song repertoires and migratory distance: three sexual selection hypotheses fail to explain the correlation

Previous studies have found a relationship between migrationand the degree of elaboration of sexually selected traits,but investigators have differed in the mechanisms they proposedto account for this association. We examined the relationshipbetween song repertoire size and distance migrated among birdsin the genus Vireo. There is a strong positive relationshipbetween migratory distance and repertoire size in this genus,but our data do not support the specific predictions of anyof the three proposed mechanisms (the "rapid pairing," "goodmigrations," and "territory lottery" hypotheses). Migrationdistance is presumably correlated with other life-history characteristicsthat influence the development of sexually selected traits.     

Variation in complex courtship traits across a hybrid zone between grasshopper species of the Chorthippus albomarginatus group

Closely related grasshopper species of the Chorthippus albomarginatus group are notable for their extremely complex courtship songs, accompanied by a visual display. Two species of this group, Ch. albomarginatus and Ch. oschei, were previously shown to hybridize in a wide mosaic hybrid zone in Ukraine and Moldova. In this paper, variation in five courtship song characters, one character of visual display and the number of stridulatory pegs were analysed across the hybrid zone to estimate selection against hybrids and strength of assortative mating. Comparison of cline width and position across the hybrid zone showed concordant and coincident clines in four traits, such as three song characters and one morphological character, and discordant and non‐coincident clines in two other song characters and the character of visual display. Concordance of clines in different characters suggests an equal strength of selection acting on underlying loci. Increase of variance and covariance between phenotypic traits at the cline centre could more likely result from assortative mating than from selection against hybrids. Most pairwise cases showed the highest covariance for the oschei‐like, than for the albomarginatus‐like hybrid populations. This indicates that introgression of the oschei genes into the albomarginatus genome is stronger than vice versa, and may be evidence of the movement of the hybrid zone in favour of Ch. albomarginatus. Analysis of associations between phenotype and local vegetation showed that mosaic structure of the hybrid zone is explained to a great extent by habitat–phenotype associations. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 102 , 275–291.     

Hybridization between multiple fence lizard lineages in an ecotone: locally discordant variation in mitochondrial DNA, chromosomes, and morphology

We investigated a hybrid zone between two major lineages of fence lizards (Sceloporus cowlesi and Sceloporus tristichus) in the Sceloporus undulatus species complex in eastern Arizona. This zone occurs in an ecotone between Great Basin Grassland and Conifer Woodland habitats. We analysed spatial variation in mtDNA (N=401; 969 bp), chromosomes (N=217), and morphology (N=312; 11 characters) to characterize the hybrid zone and assess species limits. A fine-scale population level phylogenetic analysis refined the boundaries between these species and indicated that four nonsister mtDNA clades (three belonging to S. tristichus and one to S. cowlesi) are sympatric at the centre of the zone. Estimates of cytonuclear disequilibria in the population closest to the centre of the hybrid zone suggest that the S. tristichus clades are randomly mating, but that the S. cowlesi haplotype has a significant nonrandom association with nuclear alleles. Maximum-likelihood cline-fitting analyses suggest that the karyotype, morphology, and dorsal colour pattern clines are all coincident, but the mtDNA cline is skewed significantly to the south. A temporal comparison of cline centres utilizing karyotype data collected in the early 1970s and in 2002 suggests that the cline may have shifted by approximately 1.5 km to the north over a 30-year period. The recent northward expansion of juniper trees into the Little Colorado River Basin resulting from intense cattle overgrazing provides a plausible mechanism for a shifting hybrid zone and the introgression of the mtDNA haplotypes, which appear to be selectively neutral. It is clear that complex interactions are operating simultaneously in this contact zone, including the formation of hybrids between populations within S. tristichus having diagnostic mtDNA, morphology, karyotypes, and dorsal colour patterns, and secondary contact between these and a distantly related yet morphologically cryptic mtDNA lineage (S. cowlesi).     

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.     

Morphological adaptation influences the evolution of a mating signal

Theory predicts that forces of natural selection can reduce the intensity of sexually selected traits. In this study, I investigate how morphological adaptation to feeding ecology influences a mating signal. In birds, changes in feeding ecology can cause rapid divergence in bill morphology. Because bills are also important for song production, feeding ecology may influence song divergence. During song, birds can rapidly change vocal tract resonance using bill movement, yet are constrained in rate and magnitude of bill movements resulting in a trade-off between trill rate and frequency bandwidth. Male swamp sparrows vary in their ability to produce rapid, broad-band trills and females prefer more physically demanding songs. Populations of swamp sparrows adapted to the feeding ecology of tidal marshes have larger bills than inland populations. Larger bills should increase the constraints of producing rapid, broad-band trills allowing for a test of how changes in feeding ecology affect a feature of song used in mate choice. I found significant differences in acoustic features of song consistent with the hypothesis that coastal males are less able to meet the physical demands of song production because of the constraints of having larger bills. As possible compensation for decreases in song performance, coastal populations exhibit an increase in song complexity. These changes support the current model of how motor constraints influence song production and suggest a mechanism by which feeding ecology can influence signal evolution.     

Sexual dimorphism in lizard body shape: the roles of sexual selection and fecundity selection

Sexual dimorphism is widespread in lizards, with the most consistently dimorphic traits being head size (males have larger heads) and trunk length (the distance between the front and hind legs is greater in females). These dimorphisms have generally been interpreted as follows: (1) large heads in males evolve through male-male rivalry (sexual selection); and (2) larger interlimb lengths in females provide space for more eggs (fecundity selection). In an Australian lizard (the snow skink, Niveoscincus microlepidotus), we found no evidence for ongoing selection on head size. Trunk length, however, was under positive fecundity selection in females and under negative sexual selection in males. Thus, fecundity selection and sexual selection work in concert to drive the evolution of sexual dimorphism in trunk length in snow skinks.     

Conservation of Afrotropical antelopes: consequences and efficiency of using different site selection methods and diversity criteria

The consequences of different measures of biotic diversity for the selection of priority sites for conservation were investigated using a dataset on Afrotropical antelopes. Site networks were selected using species richness, taxonomic diversity and restricted-range diversity as selection criteria. Restricted-range diversity was the most efficient criterion at representing all the species in the dataset. However when only a few sites could be conserved (insufficient to include all species) restricted-range diversity was relatively poor at representing absolute numbers of species and also taxonomic diversity. Use of unweighted species richness rather than a taxonomically weighted score did not significantly reduce the amount of taxonomic diversity represented. As expected an iterative selection of sites was considerably more efficient at representing all aspects of diversity than selection of the top-scoring sites. However the efficiency of an iterative selection procedure was reduced when some areas were already part of the reserve network. Since none of the criteria for selecting reserves maximizes all aspects of biodiversity under all circumstances, it is necessary to be clear about the objectives of a reserve network when deciding on a method for site selection.     

Spatially varying natural selection in a fish hybrid zone

A capture-mark-recapture (CMR) study in a hybrid zone between two species of barbel, Barbus barbus and Barbus meridionalis , in the Lergue River (southern France) failed to reject the null hypothesis of no difference in survival among phenotypes. Power calculations indicated that very intensive fieldwork should be carried out if this hypothesis was to be investigated again. The study demonstrated, however, that there were marked differences in survival between upstream and downstream sites. It is suggested that there is an environmental gradient of selection along the hybrid zone (extrinsic component of natural selection).     

Variable sexual ornaments in scarlet-tufted malachite sunbirds (Nectarinia johnstoni) on Mount Kenya

In order to be elaborated by sexual selection, sexual ornaments must vary perceptibly and genetically among individuals in natural populations. Rather little is known about ornament variation in monogamous species, in which sexual selection should act more weakly than in polygynous species. We report phenotypic variation in feather ornament size (elongated tails and pectoral tufts) and body size in the scarlet-tufted malachite sunbird Nectarinia johnstoni , a monogamous, sexually dimorphic nectarivore of East African alpine zones. Fully-expressed male ornaments are highly significantly more variable (CVs = 12–29%) than are skeletal and wing measures primarily affected by natural selection (CVs = 2 4%). Female sunbirds have pectoral tufts which are significantly (22–25%) smaller than those of adult males, but more variable (CV_s= 21–22%, CV_s= 12–15%), and more variable than body size. Among males with fully-grown ornaments, those with longer tails tend to have longer wings and wider tufts. The high variation in fully-grown ornaments in malachite sunbirds is consistent with the view that the ornaments are condition-dependent sexual signals. Finally, we review studies of feather ornament variation to date, and show that ornaments are much more variable in monogamous than non-monogamous species, apparently due to the relatively weak pressure of sexual selection.     

Temporal and spatial dynamics of competitive parapatry in chewing lice

We synthesize observations from 1979 to 2016 of a contact zone involving two subspecies of pocket gophers (Thomomys bottae connectens and T. b. opulentus) and their respective chewing lice (Geomydoecus aurei and G. centralis) along the Río Grande Valley in New Mexico, U.S.A., to test predictions about the dynamics of the zone. Historically, the natural flood cycle of the Rio Grande prevented contact between the two subspecies of pocket gophers. Flood control measures completed in the 1930s permitted contact, thus establishing the hybrid zone between the pocket gophers and the contact zone between their lice (without hybridization). Since that time, the pocket gopher hybrid zone has stabilized, whereas the northern chewing louse species has replaced the southern louse species at a consistent rate of ~150 m/year. The 0.2–0.8 width of the replacement zone has remained constant, reflecting the constant rate of chewing louse species turnover on a single gopher and within a local pocket gopher population. In contrast, the full width of the replacement zone (northernmost G. centralis to southernmost G. aurei) has increased annually. By employing a variety of metrics of the species replacement zone, we are better able to understand the dynamics of interactions between and among the chewing lice and their pocket gopher hosts. This research provides an opportunity to observe active species replacement and resulting distributional shifts in a parasitic organism in its natural setting.