Interspecific Morphological Variation Research Articles

A practical, step-by-step, guide to taxonomic comparisons using Procrustes geometric morphometrics and user-friendly software (part A): introduction and preliminary analyses

Taxonomy lays the foundations for the study of biodiversity and its conservation. Procrustean geometric morphometrics (GMM) is a most common technique for the taxonomic assessment of phenotypic population differences. To measure biological variation and detect evolutionarily significant units, GMM is often used on its own, although it is much more powerful with an integrative approach, in combination with molecular, ecological and behavioural data, as well as with meristic morphological traits. GMM is particularly effective in taxonomic research, when applied to 2D images, which are fast and low cost to obtain. Yet, taxonomists who may want to explore the usefulness of GMM are rarely experts in multivariate statistical analyses of size and shape differences. In these twin papers, I aim to provide a detailed step-by-step guideline to taxonomic analysis employing Procrustean GMM in user-friendly software (with tips for R users). In the first part (A) of the study, I will focus on preliminary analyses (mainly, measurement error, outliers and statistical power), which are fundamental for accuracy, but often neglected. I will also use this first paper, and its appendix (Appendix A), to informally introduce, and discuss, general topics in GMM and statistics, that are relevant to taxonomic applications. In the second part (B) of the work, I will move on to the main taxonomic analyses. Thus, I will show how to compare size and shape among groups, but I will also explore allometry and briefly examine differences in variance, as a potential clue to population bottlenecks in peripheral isolates. A large sample of North American marmot mandibles provides the example data (available online, for readers to replicate the study and practice with analyses). However, as this sample is larger than in previous studies and mostly unpublished, it also offers a chance to further explore the patterns of interspecific morphological variation in a group, that has been prominent in mammalian sociobiology, and whose evolutionary divergence is complex and only partially understood.

Evolution of compound eye morphology underlies differences in vision between closely related Drosophila species

BackgroundInsects have evolved complex visual systems and display an astonishing range of adaptations for diverse ecological niches. Species of Drosophila melanogaster subgroup exhibit extensive intra- and interspecific differences in compound eye size. These differences provide an excellent opportunity to better understand variation in insect eye structure and the impact on vision. Here we further explored the difference in eye size between D. mauritiana and its sibling species D. simulans.ResultsWe confirmed that D. mauritiana have rapidly evolved larger eyes as a result of more and wider ommatidia than D. simulans since they recently diverged approximately 240,000 years ago. The functional impact of eye size, and specifically ommatidia size, is often only estimated based on the rigid surface morphology of the compound eye. Therefore, we used 3D synchrotron radiation tomography to measure optical parameters in 3D, predict optical capacity, and compare the modelled vision to in vivo optomotor responses. Our optical models predicted higher contrast sensitivity for D. mauritiana, which we verified by presenting sinusoidal gratings to tethered flies in a flight arena. Similarly, we confirmed the higher spatial acuity predicted for Drosophila simulans with smaller ommatidia and found evidence for higher temporal resolution.ConclusionsOur study demonstrates that even subtle differences in ommatidia size between closely related Drosophila species can impact the vision of these insects. Therefore, further comparative studies of intra- and interspecific variation in eye morphology and the consequences for vision among other Drosophila species, other dipterans and other insects are needed to better understand compound eye structure–function and how the diversification of eye size, shape, and function has helped insects to adapt to the vast range of ecological niches.

Comparative morphology of oral glands in snakes of the family Homalopsidae reveals substantial variation and additional independent origins of salt glands within Serpentes.

Using diffusible iodine-based contrast-enhanced computed tomography (diceCT), we examined the morphology of the oral glands of 12 species of the family Homalopsidae. Snakes of this family exhibit substantial interspecific morphological variation in their oral glands. Particular variables are the venom glands, ranging from large (e.g., Subsessor bocourti) to small (e.g., Erpeton tentaculatum). The supra- and infralabial glands are more uniform in morphology, being the second most developed in almost all the sampled species. Premaxillary glands distinct from the supralabial glands were observed in five species (Myron richardsonii, Bitia hydroides, Cantoria violacea, Fordonia leucobalia, and Gerarda prevostiana), in addition to Cerberus rynchops, the only species in which this condition was previously documented associated with the excretion of salt. In the three species of the saltwater group of homalopsids (C. violacea, F. leucobalia, and G. prevostiana), the premaxillary glands also extend posteriorly, occupying a large area above the supralabial gland, a condition not observed in any other species of snake studied thus far. Character evolution analyses indicate that premaxillary glands differentiated from the supralabial gland and evolved independently three or four times in the family, always in lineages that invaded marine habitats. Our results suggest that the differentiated premaxillary glands are likely salt glands, as is the case in C. rynchops. If corroborated, this increases to six or seven the number of independent evolutionary origins of salt glands in snakes that have undergone an evolutionary transition to marine life.

Coevolutionary maintenance of forked tails and song in hirundines (Aves: Hirundininae)

Both conspicuous plumage ornamentation and song are well‐known examples of sexually selected traits, but their interrelationship is not well known, perhaps in part because of confounding factors, including interspecific variation in ecology, habitat, morphology, and type of ornamentation. Here, using a phylogenetic comparative approach and the 69 species with available information (i.e. 96% of all 72 species), I examined the evolutionary relationship between forked tails and the presence/absence of song in hirundines (Aves: Hirundininae). Hirundines have similar ecology (e.g. aerial insectivores, social monogamy, and biparental provisioning), morphology (e.g. syrinx with nearly complete bronchial rings), and plumage ornamentation (i.e. a sexually selected forked tail), which provides a unique opportunity to examine the evolutionary associations between plumage ornamentation and song. In particular, hirundines have repeatedly lost their ornamentation, forked tails, setting up a condition to test their association with the evolutionary gain/loss of their simple song. After controlling for phylogeny and covariates, I demonstrated that song was less likely to be found in species with forkless tails than in species with forked tails. Two correlates of tail shape, sexual dimorphism in the overall plumage characteristics as a well‐known proxy of sexual selection and incubation type as a measure of extrapair mating opportunity, were not positively related to the presence or absence of song, indicating the importance of forked tails, rather than their correlates. The analysis of the correlated evolution of discrete characters further supported the correlated evolution of the two traits, in which forked tails and song are maintained together and less likely to be lost under the presence of each other. The current study provided macroevolutionary support for the integrated use of visual and acoustic courtship traits.

The selective drivers of allometry in sharks (Chondrichthyes: Elasmobranchii)

AbstractIn addition to the selective importance of interspecific morphological variation, ontogenetic morphological variation may reflect different selective regimes to which successive developmental stages are subjected. The typical body form of carcharhiniform sharks is considered relatively conserved, yet sharks exhibit a wide range of body sizes and shapes, representing adaptations to distinct ecological niches. Previous investigations of ontogenetic shifts in shark body form have provided evidence for both isometric and allometric changes, depending on the morphological characters and species investigated. These findings have led to suggestions of a relationship between body size and allometric growth in sharks. In this study we present evidence of ontogenetic allometric shifts in two species of carcharhiniform sharks (Sphyrna lewini and Rhizoprionodon longurio) from novel measurements. Our results are generally consistent with previous suggestions of body form conservatism across shark phylogeny, yet also suggest potential selective factors underlying observed instances of ontogenetic allometric shifts, and highlight where additional studies are required. We propose the ‘allometric niche shift’ hypothesis for interspecific differences in scaling trends, suggesting that long-distance movements and ontogenetic trophic niche shifts represent key drivers of allometry in sharks.

A revision of the genus Malus Mill. (Rosaceae)

A revision of the wild species in the genus Malus Mill. (Rosaceae) is presented based on numerical analyses and specimens from herbaria around the world, while cultivated species such as Malus domestica (Suckow) Borkh. are not included because of their complicated domestication history. Infra- and interspecific morphological variation and species delimitation are clarified based on Principal Component Analyses (PCA) and Cluster Analyses (UPGMA). We found that several morphological characters traditionally used to distinguish species have limited taxonomic value because of high phenotypic variation or plasticity. There is a substantial conflict between traditional morphological and genetic taxonomic concepts, and as a result species lineages are often morphologically indistinguishable. None of the analyses supports the recognition of infraspecific categories in Malus transitoria (Batalin) C.K.Schneid. and interspecific categories between Malus doumeri (Bois) A.Chev. and Malus leiocalyca S.Z.Huang. Based on our analyses, we recognize 26 wild species in the genus, and propose seven new synonymies.

The overlooked underground diversity: physical and chemical edaphic structure predict morphological variation in South American amphisbaenians (Squamata: Amphisbaenidae)

ABSTRACT Morphological patterns may vary spatially in response to adaptations to regional environmental conditions. In fossorial animals, edaphic gradients potentially predict intra and interspecific morphological variation because they can limit dispersal and select morphotypes. We measured morphological traits of South American worm lizards, including three Amphisbaena and two Leposternon species, to test whether inter- and intraspecific morphological variation and morphotype distribution are predicted by edaphic gradients measured at different soil profiles. Interspecifically, we found that the variation in head length is mainly predicted by the chemical edaphic structure, which can be explained by spatial variation in prey availability. The variation in the width of the prefrontal and frontal shields was mainly predicted by clay and sand-soil contents, which can be explained by relationships of force and friction associated with digging. Intraspecifically, we found that the effects of edaphic gradients on morphological variation reflects the level of specialization to dig, since the most adapted species were mainly affected by variables measured in deeper soil profiles. We also show that soil-mediated intraspecific morphological variation shape morphotype distribution across different biomes that form the South American dry diagonal, which are discussed in light of biological, ecological, and biogeographic premises.

Intra and interspecific morphological variations of three species of Capsicum (Solanaceae)

The widely cultivated pepper, Capsicum spp., important as a vegetable and spice crop worldwide, is one of the most diverse crops. Considerable morphological variation, especially in fruit shape, color, and size exist in the genus. The present study analysed the intra and interspecific morphological variations of three species of Capsicum (Solanaceae) using thirty nine morphological characters. Morphological variations with respect to vegetative, floral and fruit characteristics were observed among the members of the same species. Morphological data was used to obtain UPGMA derived dendrogram and the cluster analysis indicated genetic divergence among the three species. Present observations imply a great potential for chilli breeding through a hybridization programme or direct use of the varieties for the successful production of promising cultivars.

Range-wide variation in grey seal (Halichoerus grypus) skull morphology

The large interspecific variation in marine mammal skull and dental morphology reflects ecological specialisations to foraging and communication. At the intraspecific level, the drivers of skull shape variation are less well understood, having implications for identifying putative local foraging adaptations and delineating populations and subspecies for taxonomy, systematics, management and conservation. Here, we assess the range-wide intraspecific variation in 71 grey seal skulls by 3D surface scanning, collection of cranial landmarks and geometric morphometric analysis. We find that skull shape differs slightly between populations in the Northwest Atlantic, Northeast Atlantic and Baltic Sea. However, there was a large shape overlap between populations and variation was substantially larger among animals within populations than between. We hypothesize that this pattern of intraspecific variation in grey seal skull shape results from balancing selection or phenotypic plasticity allowing for a remarkably generalist foraging behaviour. Moreover, the large overlap in skull shape between populations implies that the separate subspecies status of Atlantic and Baltic Sea grey seals is questionable from a morphological point of view.

Northeastern Pacific Octopus Beak Shape for Species-Level Detection

Cephalopods have upper and lower chitinous beaks, and their shape can be used to identify species. However, the morphological attributes of beaks are commonly obtained from sources that contain little information on shape, compromising the determination of taxa. Thus, in this study, we used geometric morphometrics to analyze inter-specific differences in beak shape of six octopus species from the northeastern Pacific. We found evidence of species-level discrimination according to upper and lower beak shape. In addition, we provided a detailed description of these chitinous structures for each species, and we developed a key to identify the species based on upper and lower beak shape. The inter-specific morphological variation of beaks of the octopuses evaluated in our study could be the result of adaptive processes related to habitat and feeding habits. The information generated in our study can be used for identification of six octopus species from the northeastern Pacific based on upper and lower beak shape.

Interspecific morphological variation in Juglandoideae resting bud organization: a winter's tale?

Dormant resting buds are frequently regarded as static units, with protective cataphylls on the outside and embryonic foliage leaves on the inside. How the presence of cataphylls influences the dynamic, cyclical, annually repeating sequence of leaf forms that a resting bud gives rise to has rarely been interrogated. To examine the connection between dormant structure and growing-season development, we compare the complete seasonal heteroblastic sequence of leaf forms of six species of temperate Juglandaceae with distinctly different vegetative resting bud structures. These include buds with cataphylls; buds without cataphylls; and buds with caducous cataphylls that are lost before the onset of winter. In a common garden setting over a 7-month growing season, the dimensions of 2249 individual vegetative metamers were tracked from first exposure to abscission along the shoots of saplings and mature trees. The timing of metamer initiation within terminal buds was investigated using micro-CT scanning. Character state transitions of resting bud types were estimated using a phylogenetic tree of Juglandaceae. The presence of cataphylls within a heteroblastic sequence is associated with a single cohort of foliage leaves that flush and abscise synchronously. This growing pattern is highly determinate, with next year's terminal-bud cataphylls already initiated before spring leaf out. In contrast, in sequences without cataphylls, shorter-lived foliage leaves appear and abscise in a staggered fashion. Despite these differences in leaf demography, all examined heteroblastic sequences produce a series of small, caducous leaf forms that precede terminal bud set. The ubiquity of caducous leaf forms in Juglandoideae may point to the importance of shoot tip protection far beyond the dormant season. In addition, the presence or absence of cataphylls in resting buds is indicative of distinct shoot ontogenetic patterns, and functional strategies, in summer.

Microsatellites, morphological, and alkaloids characterization of Zephyranthes fosteri and Z. alba (Amaryllidaceae): Allopatric populations

Zephyranthes (Amaryllidaceae) is a taxonomically complex genus due to the frequent overlap of interspecific morphological variation. In Mexico, Z. alba and Z. fosteri are herbaceous plants that, when distributed in sympatry, generate individuals with complex patterns of morphological variation, leading to taxonomic confusion. Therefore, it is necessary to first characterize these species in allopatric populations. In this contribution, molecular, morphological, and alkaloid profiles were used to characterize both species in allopatric sites. Our results show that Z. alba and Z. fosteri allopatric populations are two well-defined genetic and morphological groups. Flower-related characters were the ones that best allowed us to distinguish between species. In a similar fashion, the alkaloid profile showed remarkable differences among species: four alkaloids were specific to Z. alba and five to Z. fosteri. Lycorine (43.3-88.8%) and galanthamine (87.7-91.4%) were the most abundant alkaloids for each species, respectively. In conclusion, Z. fosteri and Z. alba exhibit noticeable differences when distributed in allopatry. In addition, Z. fosteri has greater genetic and phenotypic plasticity compared to Z. alba, which could be related to the former's ability to colonize new habitats. Finally, the molecular, genetic and chemical markers developed here will provide a framework to further studies aiming to explore if hybridization among Z. alba and Z. fosteri occurs in sympatric populations.

Phylogenomic Delimitation of Morphologically Cryptic Species in Globetrotting Nylanderia (Hymenoptera: Formicidae) Species Complexes

Abstract The ant genus Nylanderia Emery has a cosmopolitan distribution and includes 150 extant described species and subspecies, with potentially hundreds more undescribed. Global taxonomic revision has long been stalled by strong intra- and interspecific morphological variation, limited numbers of diagnostic characters, and dependence on infrequently collected male specimens for species description and identification. Taxonomy is further complicated by Nylanderia being one of the most frequently intercepted ant genera at ports of entry worldwide, and at least 15 globetrotting species have widespread and expanding ranges, making species-level diagnoses difficult. Three species complexes (‘bourbonica complex’, ‘fulva complex’, and ‘guatemalensis complex’) include globetrotting species. To elucidate the phylogenetic positions of these three complexes and delimit species boundaries within each, we used target enrichment of ultraconserved elements (UCEs) from 165 specimens representing 98 Nylanderia morphospecies worldwide. We also phased the UCEs, effectively doubling sample size and increasing population-level sampling. After recovering strong support for the monophyly of each complex, we extracted COI barcodes and SNPs from the UCE data and tested within-complex morphospecies hypotheses using three molecular delimitation methods (SODA, bPTP, and STACEY). This comparison revealed that most methods tended to over-split taxa, but results from STACEY were most consistent with our morphospecies hypotheses. Using these results, we recommend species boundaries that are conservative and most congruent across all methods. This work emphasizes the importance of integrative taxonomy for invasive species management, as globetrotting occurs independently across at least nine different lineages across Nylanderia.

Overview of the genus Sympodium Ehrenberg, 1834 (Octocorallia, Alcyonacea, Xeniidae), with the description of new species, revealing regional endemism.

The oldest existing type material for any of the xeniid soft corals, Sympodium caeruleum Ehrenberg, 1834, is re-described. An integrated analysis of molecular and morphological characters of Indo-Pacific Xeniidae support the description of seven new species of that genus. The extent of interspecific morphological variation within the genus is extensive; colonies arise from an encrusting membrane of variable thickness that can be either mat-like or may have ribbon-like extensions or irregularly shaped low mounds. The polyps can either arise separately from the membrane or may be arranged into clusters of polyps that bud off at different levels to form small branched groups. The sclerites of all species are uniformly ellipsoid platelets, abundant throughout the colony. The genetic results suggest that Sympodium species demonstrate restricted geographic ranges and regional endemism, with distinct genotypes (molecular operational taxonomic units) each mostly found at a single Indo-Pacific location. The results emphasize the importance of integrating classical taxonomy with a re-examination of original old type material and molecular phylogenetic analyses, in order to delineate species boundaries and to recognize biodiversity patterns.

Variation among 91 stone oak species (Fagaceae, Lithocarpus) in fruit and vegetative morphology in relation to climatic factors

Stone oaks (genus Lithocarpus, Fagaceae) are common canopy trees in the tropical and subtropical forests across China and Southeast Asia, which exhibit both great species diversity and interspecific variation in fruit morphology represented by two major fruit types. Acorn (AC) fruits of Lithocarpus are similar to oak (Quercus) acorns, and enclosed receptacle (ER) fruits generally have larger seeds enclosed by thick lignified fruit husks. It is therefore worth examining the adaptation of stone oaks to a wide range of ecosystems and climatic conditions by interspecific functional differentiation for understanding their diversification. By applying the herbarium-specimen database (20,516 specimens) and records from the Flora of China, we examined the variation in both reproductive (seed, fruit husk and cupule size) and vegetative (leaf length, leaf width and maximum tree height) traits among 91 species, in relation to climatic factors (mean annual temperature and wetness annual precipitation minus annual potential evapotranspiration). We found that even though species representing both fruit types co-occurred over Southern China, they exhibited different correlations with climatic factors: three reproductive components of AC fruits were all positively related to both temperature and wetness, whereas only husk volume of ER fruits representing mechanical protection to seed increased with temperature. In vegetative traits, leaf length and tree height of ER-type species were both negatively related with excessive wetness. Our results suggested that vegetative traits of stone oaks were independent of fruit types and fruit morphological variations. Overall, the distinctive correlation between interspecific reproductive and vegetative traits with climatic factors could be important in the diversification of the stone oaks in the tropical and subtropical evergreen broad-leaved forests.

Intra- and interspecific variability among congeneric Pagellus otoliths

Otolith features are useful tools for studying taxonomy, ecology, paleontology, and fish biology since they represent a permanent record of life history. Nevertheless, the functional morphology of otoliths remains an open research question that is useful to completely understand their eco-morphology. This study aims to deepen the knowledge of intra- and interspecific variation in sagitta morphology in three congeneric seabreams, to understand how such variability could be related to the lifestyles of each species. Therefore, the sagittae (n = 161) of 24 Pagellus bogaraveo, 24 Pagellus acarne, and 37 Pagellus erythrinus specimens, collected from the south Tyrrhenian Sea, were analyzed using scanning electron microscopy and a stereomicroscope to assess morphometric features, variability between otolith pairs and the external crystalline structure the of sulcus acusticus. Statistical analysis demonstrated that, between the species, variability in sagittal otolith rostral length growth and sulcus acusticus features, correlated with increased fish total length and body weight. Moreover, slight differences between otolith pairs were detected in P. acarne and P. erythrinus (P < 0.05). The results confirm changes in otolith morphometry and morphology between different congeneric species and populations of the same species from different habitats.

Revisiting the type of Cespitularia stolonifera/ Gohar, 1938 leads to the description of a new genus and a species of the family Xeniidae (Octocorallia, Alcyonacea).

Because of the problematical identity and status of the type of the xeniid soft coral genus Cespitularia Milne-Edwards Haime, 1850, the species C. stolonifera Gohar, 1938 is revised. Examination of the type colonies has led to the establishment of the new genus Unomia gen. n. which is described and depicted. This genus features a stalk, commonly divided into branches featuring a diffuse polypiferous part consisting of distal clustered polyps and proximal individual ones on the stalk or the basal membranous part of the colonies. The sclerites are ellipsoid platelets composed of dendritic calcite rods whose tips are distinct on the surface of the platelets. Freshly collected material from Venezuelan reefs where the species is invasive was subjected to molecular phylogenetic analysis, the results of which substantiate the taxonomic assignment of the new genus under U. stolonifera comb. n. A new species, U. complanatis, from Japan and Green Island (Taiwan) is described and further illustrates the extent of the interspecific morphological variation within the genus. The results reveal that the biogeographic distribution of Unomia gen. n. includes Pacific Ocean reefs in addition to the previously reported invaded Caribbean reefs.

Hominin lower limb bones from Sterkfontein Caves, South Africa (1998–2003 excavations)

We describe late Pliocene and early Pleistocene hominin fossils from Sterkfontein Caves (South Africa), including two femoral specimens, as well as a partial tibia and a partial fibula. The fossils are likely assignable to Australopithecus africanus and/or Australopithecus prometheus and the morphology of each corroborates previous interpretations of Sterkfontein hominins as at least facultative bipeds.&#x0D; Significance:&#x0D; &#x0D; A recent series of papers by our research team describes the morphology of a hominin skeleton from Sterkfontein Caves (South Africa), nicknamed ‘Little Foot’. Based on its unique skull morphology, R.J. Clarke, the skeleton’s discoverer, places it in the species Australopithecus prometheus, as distinct from the better-known and co-occurring Australopithecus africanus. Here we describe additional hominin thigh and leg fossils from Sterkfontein that, when considered in a comparative context, support the hypothesis that there was significant (probably interspecific) variation in South African hominin postcranial morphology during the late Pliocene and early Pleistocene.&#x0D;

Addressing the Linnean shortfall in a cryptic species complex

AbstractBiodiversity is being lost at an alarming rate, but the rate of this loss is likely to be underestimated as a result of a deficit in taxonomic knowledge (i.e. the Linnean shortfall). This knowledge gap is more extensive for morphologically indistinct taxa. The advancement of molecular techniques and delimitation methods has facilitated the identification of such cryptic species, but a majority of these taxa remain undescribed. To investigate the effects of taxonomic uncertainty on understanding of biodiversity, we applied the general lineage concept of species to an amphipod species complex, the Gammaruslacustris lineage that occupies springs of the northern Chihuahuan Desert, which is emerging in contemporary times. We investigated species boundaries using a validation-based approach and examined genetic structure of the lineage using a suite of microsatellite markers to identify independently evolving metapopulations. Our results show that each spring contains a genetically distinct population that is geographically isolated from other springs, suggesting evolutionary independence and status as separate species. Additionally, we observed subtle interspecific morphological variation among the putative species. We used multiple lines of evidence to formally describe four new species (Gammarus langi sp. nov., G. percalacustris sp. nov., G. colei sp. nov. and G. malpaisensis sp. nov.) endemic to the northern Chihuahuan Desert. Cryptic speciation is likely to be high in other aquatic taxa within these ecosystems, and across arid landscapes throughout North America and elsewhere, suggesting that the magnitude of the Linnean shortfall is currently underestimated in desert springs worldwide.

Intra- and interspecific morphological variation in sympatric and allopatric populations of Mustela putorius and M. eversmanii (Carnivora: Mustelidae) and detection of potential hybrids

European mustelids include the European polecat, Mustela putorius, and the steppe polecat, M. eversmanii. Both occur sympatrically in the Pannonian Basin, where M. eversmanii hungarica represents the westernmost part of the latter species and they allegedly hybridize. We investigated the morphological relationships in sympatric and allopatric populations of these mustelids with representative sampling, taxonomic and geographic coverage. We evaluated inter- and intraspecific patterns of morphological differentiation of 20 cranial measurements and four external traits by distance-based morphometric approaches and multivariate analyses. Our results revealed a considerable heterogeneity in cranial morphology. The two species appeared to be clearly differentiated although sympatric populations were closer to each other and had a slight overlap in the morphometric space. Within M. eversmanii, the subspecies and the nominal taxon only partially overlapped, and M. eversmanii eversmanii was more distant from M. putorius than subspecies hungarica. Although morphometric analyses revealed several intermediate individuals in size in sympatric M. eversmanii and M. putorius populations, only a small fraction of such specimens showed conflict in discrete morphological characters with the diagnostic discriminant function. We interpret these results as an indication of ongoing hybridisation between sympatric populations, but the low number of hybrids identified suggests limited genetic exchange between the species.