Allopatric Sister Species Research Articles

First record of Scotocampa sheljuzhkoi Gyulai & L. Ronkay (Lepidoptera: Noctuidae: Noctuinae) from Kazakhstan

The little-known Noctuidae species Scotocampa sheljuzhkoi Gyulai & L. Ronkay, 2002 is reported from Kazakhstan for the first time. The species was previously considered as endemic to South and Southwest Mongolia and the new distribution data increase its known range by over 750 km to the west. Adults, male genitalia and the habitats of S. sheljuzhkoi and its allopatric sister species Scotocampa indigesta Staudinger, 1888 occurring in the Ili River Valley (SE Kazakhstan) are illustrated. The photograph of the female genitalia of the holotype of Scotocampa crassipuncta (Püngeler, 1905) known from Tarim Basin is illustrated for the first time.

Climatic niche evolution and speciation modes in the transisthmian Alpheus shrimps (Caridea: Alpheidae)

AbstractAimWe studied the niche evolution and diversification modes in transisthmian Alpheus shrimps by examining the interplay between environmental niche divergence and conservatism in allopatric sister species. In a broader perspective, the current study analysed the evolution of climatic niche and the role of the environment in species diversification of Alpheus transisthmian shrimp.LocationAtlantic and Eastern‐Pacific oceans.Taxon Alpheus shrimps (Caridea: Alpheidae).MethodsWe assembled georeferenced occurrences for 33 species of Alpheus (with 24 sister species) from a time‐calibrated molecular phylogeny. We modelled their ecological niches and assessed niche overlap through pairwise comparisons. Additionally, we performed phylogenetic reconstructions of the ancestral environmental niche, for each niche axis.ResultsOur results demonstrate that thermal tolerances, food availability and hydrodynamic forces were relevant environmental axes in evolutionary processes in transisthmian species of Alpheus. Among the 528 paired comparisons, we found that most niches were divergent, including in 12 clades formed by pairs of sister species (in only two of these clades were the niches fully equivalent). Phylogenetic reconstructions of ancestral niches showed an initial niche conservatism in all axes, with divergences intensifying in the last 12 million years.Main ConclusionsWe found evidence that confirms the relevance of the environmental changes that occurred in the West Atlantic and East Pacific for niche evolution in transisthmian Alpheus species, as well as for the emergence of some lineages. Our findings provide evidence for different modes of Alpheus species speciation in a period consistent with the closure of the Isthmus of Panama.

Widespread sympatry in a species-rich clade of marine fishes (Carangoidei).

A universal paradigm describing patterns of speciation across the tree of life has been debated for decades. In marine organisms, inferring patterns of speciation using contemporary and historical patterns of biogeography is challenging due to the deficiency of species-level phylogenies and information on species' distributions, as well as conflicting relationships between species' dispersal, range size and co-occurrence. Most research on global patterns of marine fish speciation and biogeography has focused on coral reef or pelagic species. Carangoidei is an ecologically important clade of marine fishes that use coral reef and pelagic environments. We used sequence capture of 1314 ultraconserved elements (UCEs) from 154 taxa to generate a time-calibrated phylogeny of Carangoidei and its parent clade, Carangiformes. Age-range correlation analyses of the geographical distributions and divergence times of sister species pairs reveal widespread sympatry, with 73% of sister species pairs exhibiting sympatric geographical distributions, regardless of node age. Most species pairs coexist across large portions of their ranges. We also observe greater disparity in body length and maximum depth between sympatric relative to allopatric sister species. These and other ecological or behavioural attributes probably facilitate sympatry among the most closely related carangoids.

The reproductive isolation mechanism of allopatric sister species Iris dichotoma and I. domestica (Iridaceae) under the sympatric condition

In allopatric species, the geographical isolation is generally disregarded as a legitimate cause of isolation. Beside geographical isolation, the other prezygotic isolating mechanisms in allopatric species have not been well proved to cause isolation. In order to explore if two allopatric sister species Iris dichotoma and I. domestica (Iridaceae) have other prezygotic isolating mechanisms, the natural hybrids, the positions of pollen deposited on the honeybee's body, the insect visiting period and frequency, and the daily dynamic changes of pollen quantity were detected; the daily flowering times were compared; the interspecific crossing and the pollen transfer were tested; the flower-visiting insects were categorized and identified. No natural hybrids of the two species were detected under the sympatric condition. Their daily flowering times were significantly different but had 2.6 h overlap, during which they had good cross compatibility. They had common legitimate pollinators, but the visiting time did not overlap and I. domestica’s pollen quantity was down to 0 before I. dichotoma open. The same honeybee could visit both of them as I. domestica had pollen in the net house, but their pollens were deposited on different parts of the honeybee’s body. The temporal isolation caused complete reproductive isolation, and the difference in flower structure also played an important role in reproductive isolation.

Evidence of historical pairing between two cryptic species of Short-tailed Albatross

When secondary contact occurs between allopatric sister species, several evolutionary consequences are expected, such as reinforcement of reproductive isolation, hybrid speciation, de-speciation, introgressive hybridization, or formation of a stable hybrid zone. The Short-tailed Albatross (<em>Phoebastria albatrus</em>) is a vulnerable seabird that breeds mainly in Torishima, the Izu Islands, and two islets in the Senkaku Islands in the western North Pacific. Recent studies revealed that Short-tailed Albatross comprises two cryptic species (Senkaku-type and Torishima-type) that breed sympatrically on Torishima. Ringed (hatched in Torishima) and unringed (probably hatched in the Senkaku Islands) birds mate in a mutually assortative manner at the Hatsunezaki colony (artificially established in 1995) on Torishima. However, observations of some ringed–unringed pairs suggest possible hybridization between the two cryptic species. To clarify the degree of hybridization, we analyzed microsatellite DNA and mitochondrial DNA control region 2 (CR2) sequences of chicks from Hatsunezaki and Tsubamezaki (original colony discovered in 1951) colonies and of unringed birds from Hatsunezaki. In general, both CR2 sequences and microsatellites revealed genetic differentiation between immigrants from the Senkaku Islands (unringed birds) and chicks hatched in Tsubamezaki. These findings support the existence of two cryptic species. Each chick obtained from four ringed–unringed parent pairs at Hatsunezaki displayed a high proportion of alleles from just a single population. In contrast, some chicks in Tsubamezaki had a medium proportion of alleles from both populations. Breeding unringed subadult plumage birds, which were probable immigrants from the Senkaku Islands, were observed in Hatsunezaki but not in Tsubamezaki. Therefore, we propose that interspecific pairing occurred in the past but infrequently in recent generations on Torishima, suggesting historical reinforcement of reproductive isolation. Further microsatellite DNA studies of chicks from Hatsunezaki are required to confirm whether reinforcement of reproductive isolation is achieved. Alternatively, nearly complete pre-mating isolation between the two species was established in the past, but the scarcity of Senkaku-type birds in Torishima has facilitated hybridization.

Ecological niche overlap of two allopatric karst-adapted tiger geckos (Goniurosaurus) from northern Vietnam: microhabitat use and implications for conservation

ABSTRACT Anthropogenic pressures have caused a substantial decline of global biodiversity and have been further reported to strongly affect the ecological performance of species in their habitat, especially reptiles. Understanding the ecology of species and how species respond to habitat alterations is basic knowledge needed to develop conservation programmes and address issues of biodiversity loss. All five species of tiger geckos (Goniurosaurus) in Vietnam are known to be threatened by extinction due to anthropogenic impacts such as habitat degradation and harvesting for the international pet trade. However, conservation actions have only been initiated for the better-studied species. This study provides detailed basic data on microhabitat use of two allopatric sister species, namely Goniurosaurus huuliensis and G. luii. In total, 145 geckos (including 59 records of G. huuliensis and 86 records of G. luii) were observed during field surveys. All Goniurosaurus individuals were mostly recorded in the forest on karst formations, covered with evergreen broad-leaved woody trees, intermixed with ferns, shrubs and vines. Microhabitats of the two species were relatively similar in other traits, such as high vegetation coverage, high humidity, stable ambient temperature and dry-rock substrates. A multiple factor analysis supported that the ecological niche spaces of the two species highly overlap, even though their distribution ranges are geographically separated. We further found no intraspecific niche segregation in both species. The present data provide baseline knowledge for both in situ and ex situ conservation measures to protect species in the genus Goniurosaurus.

Centres of endemism of Noctuidae (Lepidoptera) in the Palaearctic arid mountains: biogeographical and phylogenetic implications

The oreal fauna is connected with orographically limited non-arboreal habitats. Its chorological centres can be recognised by the high species-diversity of numerous typical genera, and by the accumulated occurrence of endemic species and/or subspecies of disjunct species. The oreal fauna is partitioned to the alpine type, as the faunal type of humid high-mountains with strong connections to the tundral zonobiome, and the xeromontane type, as the faunal type of arid high-mountains with close connections to the eremic zonobiomes. As the results of revisions of several Noctuinae genera, species groups and/or sister species were recognised and their distributions were mapped. The restricted areas of allopatric sister species, often described by us as new for science, fulfil the criteria of the “areas of endemism”. Core areas of the Palaearctic xeromontane Noctuidae, outlined by the distribution of endemic species, have been proven by the occurrence of allopatric subspecies of polytypic species, and/or by the presence of allopatric sister species. In the revised genera of Noctuidae several types of allopatric speciation have been identified based on the analysis of the areas of endemism and of vicariance patterns. As a result of these analyses, it is proved that allopatric sister species, as elementary monophyletic supraspecific units, are suitable for phylogenetic biogeographical surveys. Although the major part of the xeromontane fauna appears to be range-restricted, a considerable fraction of the species could have expanded into the steppic zonobiome due to adaptive changes of their life cycles. High diversity of cold-adapted species originated from the Sino-Himalayan mountains by passing two main filtercorridors. One track of this bifurcation was directed across the “Rhododendron-corridor” to the Holarctic taiga zone while the other one, across the “Xeromontane filter-corridor” to the mountain systems of Central and Inner Asia. This bifurcation becomes apparent from the taxonomic division of the genera, composing both of these main faunal types. Supposedly, the faunal movements of the xeromontanean species in the West Palaearctic had been shaped by the Messinian salinity crisis and, additionally, significantly influenced by the Mid-Pleistocene climatic transition which deeply transformed the zonality of the vegetation by cooling and aridisation of vast areas.

Centres of endemism of Noctuidae (Lepidoptera) in the Palaearctic arid mountains: biogeographical and phylogenetic implications

The oreal fauna is connected with orographically limited non-arboreal habitats. Its chorological centres can be recognised by the high species-diversity of numerous typical genera, and by the accumulated occurrence of endemic species and/or subspecies of disjunct species. The oreal fauna is partitioned to the alpine type, as the faunal type of humid high-mountains with strong connections to the tundral zonobiome, and the xeromontane type, as the faunal type of arid high-mountains with close connections to the eremic zonobiomes. As the results of revisions of several Noctuinae genera, species groups and/or sister species were recognised and their distributions were mapped. The restricted areas of allopatric sister species, often described by us as new for science, fulfil the criteria of the “areas of endemism”. Core areas of the Palaearctic xeromontane Noctuidae, outlined by the distribution of endemic species, have been proven by the occurrence of allopatric subspecies of polytypic species, and/or by the presence of allopatric sister species. In the revised genera of Noctuidae several types of allopatric speciation have been identified based on the analysis of the areas of endemism and of vicariance patterns. As a result of these analyses, it is proved that allopatric sister species, as elementary monophyletic supraspecific units, are suitable for phylogenetic biogeographical surveys. Although the major part of the xeromontane fauna appears to be range-restricted, a considerable fraction of the species could have expanded into the steppic zonobiome due to adaptive changes of their life cycles. High diversity of cold-adapted species originated from the Sino-Himalayan mountains by passing two main filtercorridors. One track of this bifurcation was directed across the “Rhododendron-corridor” to the Holarctic taiga zone while the other one, across the “Xeromontane filter-corridor” to the mountain systems of Central and Inner Asia. This bifurcation becomes apparent from the taxonomic division of the genera, composing both of these main faunal types. Supposedly, the faunal movements of the xeromontanean species in the West Palaearctic had been shaped by the Messinian salinity crisis and, additionally, significantly influenced by the Mid-Pleistocene climatic transition which deeply transformed the zonality of the vegetation by cooling and aridisation of vast areas.

Ecological niche differentiation in Chiroxiphia and Antilophia manakins (Aves: Pipridae).

Species distribution models are useful for identifying the ecological characteristics that may limit a species’ geographic range and for inferring patterns of speciation. Here, we test a hypothesis of niche conservatism across evolutionary time in a group of manakins (Aves: Pipridae), with a focus on Chiroxiphia boliviana, and examine the degree of ecological differentiation with other Chiroxiphia and Antilophia manakins. We tested whether allopatric sister species were more or less similar in environmental space than expected given their phylogenetic distances, which would suggest, respectively, ecological niche conservatism over time or ecologically mediated selection (i.e. niche divergence). We modeled the distribution of nine manakin taxa (C. boliviana, C. caudata, C. lanceolata, C. linearis, C. p. pareola, C. p. regina, C. p. napensis, Antilophia galeata and A. bokermanni) using Maxent. We first performed models for each taxon and compared them. To test our hypothesis we followed three approaches: (1) we tested whether C. boliviana could predict the distribution of the other manakin taxa and vice versa; (2) we compared the ecological niches by using metrics of niche overlap, niche equivalency and niche similarity; and (3) lastly, we tested whether niche differentiation corresponded to phylogenetic distances calculated from two recent phylogenies. All models had high training and test AUC values. Mean AUC ratios were high (>0.8) for most taxa, indicating performance better than random. Results suggested niche conservatism, and high niche overlap and equivalency between C. boliviana and C. caudata, but we found very low values between C. boliviana and the rest of the taxa. We found a negative, but not significant, relationship between niche overlap and phylogenetic distance, suggesting an increase in ecological differentiation and niche divergence over evolutionary time. Overall, we give some insights into the evolution of C. boliviana, proposing that ecological selection may have influenced its speciation.

A new species of Okanagana from the Walker Lane region of Nevada and California (Hemiptera: Auchenorrhyncha: Cicadidae).

Okanagana boweni sp. n. is described from the western margin of the Great Basin of North America. The new species is diagnosed from allopatric O. simulata Davis and sympatric O. utahensis Davis using morphological, bioacoustical, and molecular characters. The distribution of this new species coincides with the Walker Lane region that lies along the border of California and Nevada, USA. Based on geography, bioacoustics, morphology, and molecular phylogenetics, we hypothesize that O. boweni sp. n. is the allopatric sister species of O. simulata.

Hidden or unnoticed? Multiple lines of evidence support the recognition of a new species of Pseudocorynopoma (Characidae: Corynopomini).

Species delimitation is a permanent issue in systematics. The increasing recognition of geographically isolated populations as independent lineages allowed by new methods of analysis has inflated the species-populations dilemma, which involves deciding whether to consider separate lineages as different species or structured genetic populations. This is commonly observed between fishes of adjacent river basins, with some lineages being considered allopatric sister species and others considered isolated populations or variants of the same species. Pseudocorynopoma doriae is a characid diagnosed from its single congener by the number of anal-fin rays and sexually dimorphic characters of males, including distinct fin colouration. The authors found variation in the colour pattern between isolated populations previously identified as P. doriae but no variation in scale or fin-ray counts. They analysed molecular evidence at the population level and morphological differences related to life history (e.g., colour dimorphism related to inseminating behaviour). The results provide compelling evidence for the recognition of a new species of Pseudocorynopoma despite the lack of discrete differences in meristic data. The recognition of the new species is consistent with biogeographical evidence for the long-term isolation of the respective river drainages and with differences between the ichthyofaunal communities of these rivers.

Climatic niche evolution in the viviparous Sceloporus torquatus group (Squamata: Phrynosomatidae).

The cold-climate hypothesis maintains that viviparity arose as a means to prevent increased egg mortality in nests owing to low temperatures, and this hypothesis represents the primary and most strongly supported explanation for the evolution of viviparity in reptiles. In this regard, certain authors have stated that viviparous species will exhibit speciation via climatic niche conservatism, with similar climatic niches being observed in allopatric sister species. However, this prediction remains to be tested with bioclimatic variables relevant to each viviparous group. In the present study, we examined climatic niche evolution in a group of North American viviparous lizards to determine whether their diversification is linked to phylogenetic niche conservatism (PNC). We evaluated the phylogenetic signal and trait evolution of individual bioclimatic variables and principal component (PC) scores of a PC analysis, along with reconstructions of ancestral climate tolerances. The results suggest that diversification of the Sceloporus torquatus group species is associated with both niche differentiation and PNC. Furthermore, we did not observe PNC across nearly all bioclimatic variables and in PC2 and PC3. However, in Precipitation Seasonality (Bio15), in Precipitation of Coldest Quarter (Bio19) and in PC1 (weakly associated with variability of temperature), we did observe PNC. Additionally, variation of the scores along the phylogeny and Pagel’s delta (δ) >1 of PC3 suggests a fast, recent evolution to dry conditions in the clade that sustains S. serrifer.

Are assortative mating and genital divergence driven by reinforcement?

The evolution of assortative mating is a key part of the speciation process. Stronger assortment, or greater divergence in mating traits, between species pairs with overlapping ranges is commonly observed, but possible causes of this pattern of reproductive character displacement are difficult to distinguish. We use a multidisciplinary approach to provide a rare example where it is possible to distinguish among hypotheses concerning the evolution of reproductive character displacement. We build on an earlier comparative analysis that illustrated a strong pattern of greater divergence in penis form between pairs of sister species with overlapping ranges than between allopatric sister‐species pairs, in a large clade of marine gastropods (Littorinidae). We investigate both assortative mating and divergence in male genitalia in one of the sister‐species pairs, discriminating among three contrasting processes each of which can generate a pattern of reproductive character displacement: reinforcement, reproductive interference and the Templeton effect. We demonstrate reproductive character displacement in assortative mating, but not in genital form between this pair of sister species and use demographic models to distinguish among the different processes. Our results support a model with no gene flow since secondary contact and thus favor reproductive interference as the cause of reproductive character displacement for mate choice, rather than reinforcement. High gene flow within species argues against the Templeton effect. Secondary contact appears to have had little impact on genital divergence.

Testing the role of climate in speciation: New methods and applications to squamate reptiles (lizards and snakes).

Climate may play important roles in speciation, such as causing the range fragmentation that underlies allopatric speciation (through niche conservatism) or driving divergence of parapatric populations along climatic gradients (through niche divergence). Here, we developed new methods to test the frequency of climate niche conservatism and divergence in speciation, and applied it to species pairs of squamate reptiles (lizards and snakes). We used a large-scale phylogeny to identify 242 sister species pairs for analysis. From these, we selected all terrestrial allopatric pairs with sufficient occurrence records (n=49 pairs) and inferred whether each originated via climatic niche conservatism or climatic niche divergence. Among the 242 pairs, allopatric pairs were most common (41.3%), rather than parapatric (19.4%), partially sympatric (17.7%), or fully sympatric species pairs (21.5%). Among the 49 selected allopatric pairs, most appeared to have originated via climatic niche divergence (61-76%, depending on the details of the methods). Surprisingly, we found greater climatic niche divergence between allopatric sister species than between parapatric pairs, even after correcting for geographic distance. We also found that niche divergence did not increase with time, further implicating niche divergence in driving lineage splitting. Overall, our results suggest that climatic niche divergence may often play an important role in allopatric speciation, and the methodology developed here can be used to address the generality of these findings in other organisms.

The role of niche divergence and geographic arrangement in the speciation of Eared Pheasants (Crossoptilon, Hodgson 1938)

One of the most contentious theories in current ecology is the ecological niche conservatism, which is defined as conservatism among closely related species; however, the ecological niche can also be shifted, as documented in several cases. Genetic drift and ecological divergent selection may cause ecological niche divergence. The current study aims to test whether the ecological niche is conserved or divergent and to determine the main factor that drives ecological niche divergence or conservation. We analyzed the phylogenetic relationship, ecological niche model (ENM) and demographic history of Eared Pheasants in the genus Crossoptilon (Galliformes: Phasianidae) to test niche conservatism with respect to different geographically distributed patterns. The phylogenetic relationship was reconstructed using ∗BEAST with mitochondrial cytochrome b (cyt b) and 44 unlinked autosomal exonic loci, and ENMs were reconstructed in MAXENT using an average of 41 occurrence sites in each species and 22 bioclimatic variables. A background similarity test was used to detect whether the ecological niche is conserved. Demographic history was estimated using the isolation with migration (IM) model. We found that there was asymmetric gene flow between the allopatric sister species Crossoptilon mantchuricum and C. auritum and the parapatric sister species C. harmani and C. crossoptilon. We found that ecological niches were divergent, not conserved, between C. mantchuricum and C. auritum, which began to diverge at approximately 0.3 million years ago. However, the ecological niches were conserved between C. crossoptilon and C. harmani, which gradually diverged approximately half a million years ago. Ecological niches can be either conserved or divergent, and ecological divergent selection for local adaptation is probably an important factor that promotes and maintains niche divergence in the face of gene flow. This study provides a better understanding of the role that divergent selection has in the initial speciation process. The platform combined demographic processes and ecological niches to offer new insights into the mechanism of biogeography patterns.

Divergence in male cricket song and female preference functions in three allopatric sister species.

Multivariate female preference functions for male sexual signals have rarely been investigated, especially in a comparative context among sister species. Here we examined male signal and female preference co-variation in three closely related, but allopatric species of Gryllus crickets and quantified male song traits as well as female preferences. We show that males differ conspicuously in either one of two relatively static song traits, carrier frequency or pulse rate; female preference functions for these traits also differed, and would in combination enhance species discrimination. In contrast, the relatively dynamic song traits, chirp rate and chirp duty cycle, show minimal divergence among species and relatively greater conservation of female preference functions. Notably, among species we demonstrate similar mechanistic rules for the integration of pulse and chirp time scales, despite divergence in pulse rate preferences. As these are allopatric taxa, selection for species recognition per se is unlikely. More likely sexual selection combined with conserved properties of preference filters enabled divergent coevolution of male song and female preferences.

Species divergence in field crickets: genetics, song, ecomorphology, and pre- and postzygotic isolation

Studies that simultaneously estimate levels of species divergence in genetics, reproductive and ecological traits, and pre- and postzygotic isolation are relatively rare. Here we compare levels of divergence in three allopatric sister species of field crickets. We compare divergence in both nuclear and mitochondrial DNA, male song, female ovipositor length, levels of pre- and postzygotic isolation, and male versus female contributions to prezygotic isolation. Taken together, our data show the accumulation of a multitude of potential reproductive isolating barriers if secondary contact were to become established. Furthermore, ecological and behavioural prezygotic isolation appear significantly more advanced than postzygotic isolation, with prezygotic isolation due to female behaviour exceeding that due to male behaviour.

Competitive Interactions upon Secondary Contact Drive Elevational Divergence in Tropical Birds.

Tropical mountains harbor exceptionally high biodiversity, which is in part due to the marked elevational stratification of tropical biotas. However, the factors that influence the evolution of elevational distributions remain uncertain. I used a database of sister species of tropical montane birds from 41 families and three regions-the Neotropics, the Himalayas, and New Guinea-to test whether patterns of elevational divergence were consistent with (1) a stochastic process, (2) ecological sorting of elevational divergence that occurred in allopatry, or (3) elevational divergence driven by competitive interactions upon secondary contact. The stochastic and ecological sorting hypotheses predict that increased elevational divergence in sympatric sister species is explained by their greater evolutionary age, whereas the competitive interactions hypothesis predicts that elevational divergence is explained by geographical overlap. I found that genetic distances were unrelated to elevational divergence and that allopatric sister species occupied similar elevational distributions regardless of genetic distance in each region. Instead, sympatry was the only significant predictor of elevational divergence; regardless of evolutionary age, sympatric sister species had greater elevational divergence than allopatric sister species in each region, as predicted by the competitive interactions hypothesis. Importantly, this pattern occurred in all three geographic regions, which suggests that competition-driven elevational divergence upon secondary contact is a general process of community assembly in tropical montane avifaunas.

Color patterns of closely related bird species are more divergent at intermediate levels of breeding-range sympatry.

Closely related species of birds often differ markedly in their color patterns. Here we examine the influence of breeding-range overlap (breeding sympatry) on the evolution of color pattern differences in a sample of closely related bird species. We used a sister-lineage method to analyze 73 phylogenetically independent comparisons among 246 species and 39 families of birds worldwide. We found that divergence of color patterns among closely related species was greater between sympatric than between allopatric lineages, but only at intermediate levels of sympatry (50%-80% breeding-range overlap). This pattern suggests that closely related species incur costs at intermediate levels of sympatry if they exhibit similar color patterns-costs that could include hybridization, interspecific aggression, competition for signaling space, or ecological interactions that secondarily influence color patterns. The decline in color pattern divergence with further increase in sympatry suggests either the relaxation of divergent selection, increased impediment of gene flow, or an increased role for counteracting selection at higher levels of sympatry. We also found that the differences in color pattern between sympatric and allopatric sister species were greatest at lower latitudes. The global scale and broad taxonomic coverage in our study suggest that the divergence of color patterns between sympatrically breeding closely related species is widespread in birds.

Incomplete behavioural isolation and asymmetric female preference in darter sister species (Percidae: Etheostoma)

In this study, female preferences and behavioural isolation were estimated in a pair of allopatric sister species, Etheostoma duryi and Etheostoma flavum. Dichotomous mate preference trials were conducted to determine whether females prefer to associate with conspecific over heterospecific males and free-spawning assays were conducted to determine whether those preferences translated into behavioural isolation. Dichotomous mate choice trials revealed asymmetric female preference, as female E. flavum preferred conspecific males, whereas female E. duryi showed no preference. Free-spawning assays indicated that behavioural isolation remains incomplete between E. duryi and E. flavum (IB = 0·19). In addition to female mating preferences, male behaviour also appeared to influence mating outcomes as male E. flavum consistently courted conspecific females more often in free-spawning assays whereas male E. duryi did not. The data therefore suggest that despite marked divergence in male nuptial colour, divergence in female preferences between these species may not be sufficient to maintain species boundaries upon secondary contact. These results contrast with similar work in a sympatric darter species pair and may be explained by considering the contributions of reinforcement and differences in colour pattern as well as colour value.