Evidence For Character Displacement Research Articles

Observing character displacement from process to pattern in a novel vertebrate community.

Ecological character displacement, whereby shifts in resource use in the presence of competing species leads to adaptive evolutionary divergence, is widely considered an important process in community assembly and adaptive radiation. However, most evidence for character displacement has been inferred from macro-scale geographic or phylogenetic patterns; direct tests of the underlying hypothesis of divergent natural selection driving character displacement in the wild are rare. Here, we document character displacement between two ecologically similar lizards (Anolis sagrei and A. cristatellus) experiencing novel contact. We identify directional selection during the incipient stages of sympatry in a new community that corresponds to repeated trait divergence across multiple established sympatric communities. By identifying the role of natural selection as character displacement unfolds, we connect how natural selection operating at short timescales may drive broader patterns of trait distributions at larger spatial and temporal scales.

Changes in the acoustic structure of Australian bird communities along a habitat complexity gradient

Abstract Avian vocalizations have evolved in response to a variety of abiotic and biotic selective pressures. While there is some support for signal convergence in similar habitats that are attributed to adaptation to the acoustic properties of the environment (the “acoustic adaptation hypothesis,” AAH), there is also evidence for character displacement as a result of competition for signal space among coexisting species (the “acoustic niche partitioning hypothesis”). We explored the acoustic space of avian assemblages distributed along six different habitat types (from herbaceous habitats to warm rainforests) in southeastern Queensland, Australia. We employed three acoustic diversity indices (acoustic richness, evenness, and divergence) to characterize the signal space. In addition, we quantified the phylogenetic and morphological structure (in terms of both body mass and beak size) of each community. Acoustic parameters showed a moderately low phylogenetic signal, indicating labile evolution. Although we did not find meaningful differences in acoustic diversity indices among habitat categories, there was a significant relationship between the regularity component (evenness) and vegetation height, indicating that acoustic signals are more evenly distributed in dense habitats. After accounting for differences in species richness, the volume of acoustic space (i.e., acoustic richness) decreased as the level of phylogenetic and morphological resemblance among species in a given community increased. Additionally, we found a significantly negative relationship between acoustic divergence and divergence in body mass indicating that the less different species are in their body mass, the more different their songs are likely to be. This implies the existence of acoustic niche partitioning at a community level. Overall, while we found mixed support for the AAH, our results suggest that community-level effects may play a role in structuring acoustic signals within avian communities in this region. This study shows that signal diversity estimated by diversity metrics of community ecology based on basic acoustic parameters can provide additional insight into the structure of animal vocalizations.

Evidence of character displacement in microhabitat use between two tropical sympatric Holcosus lizard species (Reptilia, Teiidae)

Interspecific competition between sympatric related species leading to character displacement is critical for species coexistence, especially in tropical habitats. We examined microhabitat use of two sympatric species of tropical lizards of the genus Holcosus in relationship to the microhabitats available in two ecosystems. The species H. festivus lives exclusively in the forest and uses microhabitats in proportion to their availability; while the other, H. quadrilineatus, lives both in forest and on the beach and selects microhabitats with specific characteristics. In the ecosystem where these two lizards live in sympatry (forest), we observed a differential microhabitat use between the two species. However, these differences indicated changes in habitat use by H. quadrilineatus (the smaller species) concerning its patterns of habitat selection in the ecosystem (beach) where only this species occurs. The age of the lizards did not affect the patterns of selection of microhabitats of either species. Shifts in microhabitat use may allow coexistence in sympatry of both species, which might result from the competitive exclusion of the smaller species by the larger species. Key words: Interspecific competition, Holcosus, Lizards, Microhabitat use, Tropical habitats

Genetic, phenotypic and ecological differentiation suggests incipient speciation in two Charadrius plovers along the Chinese coast

BackgroundSpeciation with gene flow is an alternative to the nascence of new taxa in strict allopatric separation. Indeed, many taxa have parapatric distributions at present. It is often unclear if these are secondary contacts, e.g. caused by past glaciation cycles or the manifestation of speciation with gene flow, which hampers our understanding of how different forces drive diversification. Here we studied genetic, phenotypic and ecological aspects of divergence in a pair of incipient shorebird species, the Kentish (Charadrius alexandrinus) and the White-faced Plovers (C. dealbatus), shorebirds with parapatric breeding ranges along the Chinese coast. We assessed divergence based on molecular markers with different modes of inheritance and quantified phenotypic and ecological divergence in aspects of morphometric, dietary and climatic niches.ResultsOur integrative analyses revealed small to moderate levels of genetic and phenotypic distinctiveness with symmetric gene flow across the contact area at the Chinese coast. The two species diverged approximately half a million years ago in dynamic isolation with secondary contact occurring due to cycling sea level changes between the Eastern and Southern China Sea in the mid-late Pleistocene. We found evidence of character displacement and ecological niche differentiation between the two species, invoking the role of selection in facilitating divergence despite gene flow.ConclusionThese findings imply that ecology can indeed counter gene flow through divergent selection and thus contributes to incipient speciation in these plovers. Furthermore, our study highlights the importance of using integrative datasets to reveal the evolutionary history and assist the inference of mechanisms of speciation.

Sperm divergence in a passerine contact zone: Indication of reinforcement at the gametic level.

Postcopulatory sexual selection may promote evolutionary diversification in sperm form, but the contribution of between-species divergence in sperm morphology to the origin of reproductive isolation and speciation remains little understood. To assess the possible role of sperm diversification in reproductive isolation, we studied sperm morphology in two closely related bird species, the common nightingale (Luscinia megarhynchos) and the thrush nightingale (Luscinia luscinia), that hybridize in a secondary contact zone spanning Central and Eastern Europe. We found: (1) striking divergence between the species in total sperm length, accompanied by a difference in the length of the mitochondrial sperm component; (2) greater divergence between species in sperm morphology in sympatry than in allopatry, with evidence for character displacement in sperm head length detected in L. megarhynchos; (3) interspecific hybrids showing sperm with a length intermediate between the parental species, but no evidence for decreased sperm quality (the proportion of abnormal spermatozoa in ejaculates). Our results demonstrate that divergence in sperm morphology between the two nightingale species does not result in intrinsic postzygotic isolation, but may contribute to postcopulatory prezygotic isolation. This isolation could be strengthened in sympatry by reinforcement.

The case for character displacement in plants.

The evidence for character displacement as a widespread response to competition is now building. This progress is largely the result of the establishment of rigorous criteria for demonstrating character displacement in the animal literature. There are, however, relatively few well-supported examples of character displacement in plants. This review explores the potential for character displacement in plants by addressing the following questions: (1) Why aren't examples of character displacement in plants more common? (2) What are the requirements for character displacement to occur and how do plant populations meet those requirements? (3) What are the criteria for testing the pattern and process of character displacement and what methods can and have been used to address these criteria in the plant literature? (4) What are some additional approaches for studying character displacement in plants? While more research is needed, the few plant systems in which character displacement hypotheses have been rigorously tested suggest that character displacement may play a role in shaping plant communities. Plants are especially amenable to character displacement studies because of the experimental ease with which they can be used in common gardens, selection analyses, and breeding designs. A deeper investigation of character displacement in plants is critical for a more complete understanding of the ecological and evolutionary processes that permit the coexistence of plant species.

Multiple routes to reduced interspecific territorial fighting in Hetaerina damselflies

Multiple routes to reduced interspecific territorial fighting in Hetaerina damselflies

Character displacement of song and morphology in African tinkerbirds

Divergence in acoustic signals between populations of animals can lead to species recognition failure, reproductive isolation, and speciation. Character displacement may facilitate coexistence of species in natural communities, yet evidence for character displacement in acoustic signals is scant. Here, we find evidence of character displacement in song as well as body size and bill size of 2 related African tinkerbirds. Playback experiments indicate that related species' songs are perceived differently in sympatry than in allopatry. We suggest character displacement occurs in phenotypic traits facilitating species recognition, which has important implications for understanding the processes that lead to speciation and diversification. Because many of the sites where the 2 species coexist are areas where pristine rainforest has been degraded, results also suggest that anthropogenic pressures resulting from deforestation may be a contributing cause of character displacement in these species.

Pitch-related cues in the songs of sympatric mountain and black-capped chickadees

Acoustic frequency (pitch) cues are known to be important in the recognition of conspecific song in a number of songbird species. Mountain chickadees (Poecile gambeli) and black-capped chickadees (Poecile atricapillus) are sympatric over parts of their ranges and their species-typical songs share many features. I examined the acoustic characteristics of song of these two congeners in a region of sympatry in southern Alberta, Canada. As reported for other populations in allopatry, black-capped chickadees emphasized relative frequency cues in song production. In particular, variation in the ratios between note frequencies was significantly less than variation in the note frequencies themselves. In contrast, songs of mountain chickadees did not have constant frequency ratios and contained an introductory acoustic element absent in black-capped chickadee song. Both species may rely on song note frequency or the presence of this introductory acoustic element when differentiating between conspecific song and heterospecific song. Song measures for chickadees in sympatry were similar to measures in allopatry, providing little evidence for character displacement in song production.

Morphometric study of two species of wood miceApodemus sylvaticus andA. flavicollis (Rodentia: Muridae): traditional and geometric morphometric approach

Skull shape and size were investigated in 311 specimens ofApodemus sylvaticus (Linnaeus, 1758) andA. flavicollis (Melchior, 1834), using traditional and geometric morphometric approach. We searched for potential morphometric diagnostic criteria between these morphologically similar mice and we an alyzed the interpopulation morphometric variation. Specific status of all individuals was identified by scoring four diagnostic allozyme markers. Discriminant analysis based on 16 cranial measure ments as well as that based on land mark data for ventral side of the skull discriminated in dividuals with 99.7% accuracy. The classification functions useful for practical discrimination of unknown specimens are given. Our data showed neither clear intraspecific geographical pattern among studied populations nor evidence for character displacement in syntopic populations ofA. sylvaticus andA. flavicollis.

Ocellus variation and possible functions in the genus Neobythites (Teleostei: Ophidiidae)

Based on the examination of almost 1200 specimens representing 50 species of the secondary deep-sea fish genus Neobythites (family Ophidiidae), this study provides an overview of body coloration and a detailed comparison among species that have typical ocelli or eyespots consisting of a dark spot surrounded by a contrasting pale ring on their dorsal fins. Special interest concerns the possible functions of ocelli as antipredator and social signaling devices and the hypothesis that species differences in ocellus size and position are more pronounced in areas of overlap. Color patterns were found in 78% and ocelli in 44% of the Neobythites species. The 22 ocellus-bearing species occur at shallower depths than those without. Ocellus number varied between one and four ocelli with single-ocellus-bearing species reaching shallower minimum depths than those with multiples. Variation in both ocellus size and position was found among co-occurring species with a single ocellus. For instance, the Northern Indian Ocean population of N. stefanovi differs in ocellus position from the co-occurring N. steatiticus, while the allopatric Red Sea population of N. stefanovi does not. This evidence of character displacement is also supported by the marked difference in ocellus position and size between two specimens of N. meteori that were collected widely separated from each other and co-occurring with two other single-ocellus-bearing species in the Pacific and Indian Ocean, respectively. Ocelli in Neobythites may therefore serve antipredator as well as social communication functions.

A Randomized Nearest-Neighbor Approach for Assessment of Character Displacement: the Vulture Guild as a Model

Character displacement has been analysed in the past but not with an approach that considers nearest-neighbor distances between members of a guild. Vultures provide a model system in which to test a new analytical approach presented here to assess character displacement. Vultures are represented by two geographically isolated and taxonomically distinct groups: the Old World accipitrids and New World vultures. These two groups provide an excellent case of convergent evolution in which functional similarities can be compared among obligate carrion-feeding birds. The vulture guild was analysed from several geographic regions where species occur in a high diversity: East Africa, South Africa, the Indian subcontinent, Amazonia, and the pleistocene deposits from Rancho La Brea, California. A three-dimensional morphospace was constructed derived from features of the skull, beak, and mandible to assess feeding capabilities. Species packing and their distribution within the morphospace were compared using a nearest-neighbor approach through Monte Carlo simulations. Vultures seem to exhibit a similar array of ecomorphological types wherever they occur in a high diversity, even though there are phylogenetic differences among some regions. Phylogenetic effects appear to have little influence on the distribution of functional types in each region and evidence for character displacement was found only at Rancho La Brea where both Old and New World vultures were present.

Geographic variation in size and sexual dimorphism of North American weasels

Geographic variation in size (skull length) and sexual dimorphism in Mustela erminea, Mustela frenata and Mustela nivalis in North America is described and analysed in relation to latitude, longitude, climatic variables, and sympatry or allopatry of these species. Only erminea increases in size with latitude; it does so regardless of the presence or absence of frenata or nivalis. Latitude is a better predictor of size in erminea than available measures of climate, seasonality or prey size. There is no evidence for character displacement between any pair of species. The sexes covary in size in frenata and erminea, and probably in nivalis, although geographic variation in sexual dimorphism occurs in frenata and erminea. The principal cause of sexual dimorphism appears to be sexual selection for large size in males rather than the high energetic requirements resulting from an elongate body shape. However, prey size may constrain female size (and possibly also male size). Regional differences in the abundance of prey during the growth of young weasels may affect adult size much more in males than in females and contribute to geographic variation in sexual dimorphism.

TESTS OF COMMUNITY-WIDE CHARACTER DISPLACEMENT AGAINST NULL HYPOTHESES.

Ecological character displacement is unusual dissimilarity among sympatric species in features such as body size or trophic morphology (Grant, 1972), which allows coexistence by causing species to use the environment in fashions so different that competitive exclusion is avoided (Brown and Wilson, 1956). Character displacement could arise either as a result of divergent natural selection of coexisting species, or by selective survival of immigrating species as a function of distinctness from species already present (Grant, 1969, 1970). But what dissimilarity among species is unusual? What sympatric differences are character displacement? Most evidence for the phenomenon has been taken from species pairs with morphological size ratios that are greater between sympatric than between allopatric populations. The classic example treated a pair of rock nuthatch species (Vaurie, 1951; Brown and Wilson, 1956), populations of which appeared more different in sympatry than in allopatry. However, Grant (1975) has pointed out that bill length varies over geographic clines in both species, and that the zone of sympatry has no discernible influence upon clinal variation of either. A literature review led Grant (1972) to conclude that most examples of ecological character displacement are equivocal. In only two instances do known morphological differences really conform to the pattern, one between two congeneric skinks (Huey and Pianka, 1974; Huey et al., 1974), and another between two species of snails (Fenchel, 1975). The role of competition in these differences is not well established. Community-wide morphological patterns among species have also been taken as evidence of character displacement. The reasoning is that the degree of size difference required to allow coexistence is the same among all contiguous pairs of species within communities. Among species ranked by size, constant ratios of 1.14, 1.2, 1.4, and 2 have been used in this vein (Hutchinson, 1959; Schoener, 1965; Emlen, 1966; Wilson, 1975; Terborgh et al., 1978). However, Hespenheide (1971, 1973) has shown that arbitrary ratios among sympatric species are not good evidence for character displacement, because relationships between the size of trophic structure (or body size) and food size vary substantially within and among taxa. A ratio that provides food separation for one set of species, for one circumstance, will not do so for another. Horn and May (1977) have also commented upon some of the pitfalls of deducing species interaction from constant ratios. Yet, the enthusiasm for character displacement, and for inferring avoidance of interspecific competition from interspecific size ratios, continues. The notion is described as a community principle in recent textbooks of ecology (Odum, 1971; Colinvaux, 1973; Collier et al., 1973; Ricklefs, 1973; Smith, 1974; Pianka, 1978), and is not infrequently given as research motivation or as a theorem in the current primary literature (although under various names [fide Hespenheide, 1973]). Our interest is with the logic of empirical studies of character displacement that involve large fractions of faunas (e.g., Lack, 1947; Hutchinson, 1959; Schoener, 1965; Grant, 1968; Abbott et al., 1977), rather than with studies of particular species pairs. These community-wide studies have uniformly concluded that there is a tendency for coexisting species to be unusually dis-

Character Displacement in Tachysphex tertninatus and T. similis (Hymenoptera: Sphecidae, Larrinae)

Evidence for character displacement between Tachysphex terminatus (Smith) and T. similis Rohwer is demonstrated for several size-related characters. Comparisons of weights of prey taken by each species in allopatry and in sympatry indicate that the condition is probably not related to competition for prey.

Convergence, Polymorphism or Introgressive Hybridization? An Analysis of Interaction between Crotaphytus collaris and C. reticulatus (Sauria: Iguanidae)

Crotaphytus collaris and C. reticulatus are apparently parapatric in northeastern Coahuila, Mexico. Lizards from the zone of potential contact (actual sympatry not yet discovered) resemble C. collaris, but exhibit melanic, subquadrate dorsal spots reminiscent of the markings on C. reticulatus. Convergence, polymorphism by mutation, or introgressive hybridization could explain the origin of the black-spotted Crotaphytus. The black-spotted populations occur in areas environmentally intermediate between those occupied by C. collaris and C. reticulatus, or similar to the latter, and they possess some morphological character states which are intermediate between those of the two species. Productmoment correlations do not indicate a causal relationship between the climatic parameters and the morphological characters. Hence, convergence is discounted. Coexistence between the black-spotted lizards and C. collaris at some localities, differences in dorsal pattern ontogeny, and the occurrence of one or more localized, melanic-spotted populations of C. collaris in the Great Plains region, are taken as evidence for polymorphism. However, additional data do not support this simple explanation. Discriminant Function Analysis yields generally intermediate scores for the Mexican black-spotted Crotaphytus with respect to C. collaris and C. reticulatus. Morphological and ecological evidence of character displacement in the black-spotted lizards imply a history of genetic and competitive interaction with C. reticulatus. Most of the black-spotted populations are distributed along the interface between the Chihuahuan and Tamaulipan biotic provinces in habitat which is considered ecologically marginal for C. collaris and C. reticulatus. Moreover, the geographically annectant black-spotted lizards and C. reticulatus share an albumin variant, suggesting introgression. The tentative conclusion is reached that the black-spotted Crotaphytus are of hybrid origin. Coefficients of morphological variability and electrophoretic patterns of lactate dehydrogenase indicate these lizards are not F1 or early backcross hybrids. Post-Wisconsin events leading to hybridization, and a long history of backcrossing with C. collaris are postulated.

Convergent and divergent character displacement

Consideration of the possibilities and difficulties of detecting character displacement leads to a re-definition of the phenomenon; character displacement is the process by which a morphological character state of a species changes under Natural Selection arising from the presence, in the same environment, of one or more species similar to it ecologically and/or reproductively. This incorporates the principal ideas in the original definition given by Brown & Wilson (1956), but eliminates the restriction of making comparisons of the character states of a species in sympatry and allopatry. The evidence for the ecological (competitive) aspect of character displacement is assessed by analyzing in detail the best documented and well publicized examples in the literature. Some of the examples either do not exhibit displaced characters or, if they do, the “displacement” can be interpreted in other and perhaps simpler ways; this applies to the so-called classical case of character displacement, Sitta tephronota and S. neumayer in Iran. Other examples, involving lizards and birds, constitute better evidence for character displacement, but in no single study is it entirely satisfactory. It is concluded that the evidence for the ecological aspect of character displacement is weak.

Bill Size and the Question of Competition in Allopatric and Sympatric Populations of Dusky and Gray Fly catchers

The Dusky Flycatcher (Empidonax oberholseri Phillips) and the Gray Flycatcher (Empidonax wrightii Baird), sibling species which usually breed in allopatry in the western United States because of altitudinal separation of preferred habitats, occur in local sympatry along the western edge of the Great Basin. Here extensive populations of each species defend mutually exclusive territories where the vegetation forms a mosaic of habitats suitable to both. An analysis of bill size and shape of 774 specimens, comparing allopatric and sympatric populations, revealed no evidence for character displacement, implying that interspecific competition for food is lacking. Sexual dimorphism in the bill, varying geographically in extent, indicates intraspecific competition, probably between the members of a pair on their territory. Differential niche utilization, within and between the two species, is also suggested by differences in wing shape. Apparently these two species evolved distinct enough habitats and foraging spheres when in allopatry so as to preclude competition when in sympatry. Gene flow from extensive regions of allopatry may suppress divergence in bill dimensions of the sympatric individuals.