Convergent Character Displacement Research Articles

A test of the mechanistic process behind the convergent agonistic character displacement hypothesis.

In this era of rapid global change, understanding the mechanisms that enable or prevent species from co-occurring has assumed new urgency. The convergent agonistic character displacement (CACD) hypothesis posits that signal similarity enables the co-occurrence of ecological competitors by promoting aggressive interactions that reduce interspecific territory overlap and hence, exploitative competition. In northwestern Switzerland, ca. 10% of Phylloscopus sibilatrix produce songs containing syllables that are typical of their co-occurring sister species, Phylloscopus bonelli ("mixed singers"). To examine whether the consequences of P. sibilatrix mixed singing are consistent with CACD, we combined a playback experiment and an analysis of interspecific territory overlap. Although P. bonelli reacted more aggressively to playback of mixed P. sibilatrix song than to playback of typical P. sibilatrix song, interspecific territory overlap was not reduced for mixed singers. Thus, the CACD hypothesis was not supported, which stresses the importance of distinguishing between interspecific aggressive interactions and their presumed spatial consequences.

Is there evidence of convergent displacement of characters in the release calls of Cuban species of the genus Peltophryne (Anura: Bufonidae)?

Abstract Anuran release calls are produced by females and males in response to mating attempts by other males. The efficiency of the release signal to initiate dismount ensures that amplexus is not prolonged, avoiding exposure to predation and increasing the time available for breeding opportunities. Convergent character displacement of release calls may facilitate interspecific communication. In this contribution, we described and compared the release calls of sexually active males of four endemic species of Cuban toads of the genus Peltophryne in allopatric and sympatric conditions, exploring possible evidence of convergent displacement in areas of sympatry. One of these species, Peltophryne peltocephala, is widely distributed in the Cuban archipelago, with contact areas with two other species of large toads. Convergent character displacement requires that the allopatric character states of the species are significantly different from each other, and the sympatric character states are not significantly different. Although significant differences were detected between pairs of different species, in terms of duration of call sequence, call duration, interval between calls and dominant frequency, we did not find a robust pattern consistent with the expected pattern of convergent displacement of acoustic characters. Almost all call variability of allopatric populations fell inside the variability of sympatric populations.

Birds of a feather speak together: Understanding the different dialects of animals can help to decipher their communication.

As humans, many animal species that communicate via vocalization show a wide range of accents and dialects driven by environmental and social factors.

3D Geometric Morphometrics Reveals Convergent Character Displacement in the Central European Contact Zone between Two Species of Hedgehogs (Genus Erinaceus)

Simple SummaryHedgehogs, being insectivores with slow metabolisms, are quite sensitive to temperature and food availability. As a consequence, their ranges have oscillated in relation to past climate changes. Species that have evolved in different regions, but their ranges have shifted and overlapped subsequently, often represent intense competitors as a result of ecological similarities. The present study focuses on this phenomenon in the contact zone in central Europe and adjacent regions, using genetic determination of species and description of size and shape of skull, the morphological structure mirroring many selection pressures related to ecology. While animals living outside of the contact zone show marked differences between the two species, individuals within the contact zone are more alike with a smaller skull size and a convergent jawbone shape. Changes in skull size can be related to inter-species competition and also facilitated by selection pressure, mediated by overpopulated medium-sized predators such as foxes or badgers. Since the function of the lower jaw is mainly connected to feeding, we hypothesize that this pattern is due to the selection to size and shape related to competition for food resources. The present study helps to describe general patterns related to species formation, as well as species responses to anthropogenic environmental changes.Hedgehogs, as medium-sized plantigrade insectivores with low basal metabolic rates and related defensive anti-predator strategies, are quite sensitive to temperature and ecosystem productivity. Their ranges therefore changed dramatically due to Pleistocene climate oscillations, resulting in allopatric speciation and the subsequent formation of secondary contact zones. Such interactions between closely related species are known to generate strong evolutionary forces responsible for niche differentiation. In this connection, here, we detail the results of research on the phenotypic evolution in the two species of hedgehog present in central Europe, as based on genetics and geometric morphometrics in samples along a longitudinal transect that includes the contact zone between the species. While in allopatry, Erinaceus europaeus is found to have a larger skull than E. roumanicus and distinct cranial and mandibular shapes; the members of the two species in sympatry are smaller and more similar to each other, with a convergent shape of the mandible. The relevant data fail to reveal any major role for either hybridisation or clinal variation. We, therefore, hypothesise that competitive pressure exerted on the studied species does not generate divergent selection sufficient for divergent character displacement to evolve, instead giving rise to convergent selection in the face of resource limitation in the direction of smaller skull size. Considering the multi-factorial constraints present in the relevant adaptive landscape, reduction in size could also be facilitated by predator pressure in ecosystems characterised by mesopredator release and other anthropogenic factors. As the function of the animals’ lower jaw is mainly connected with feeding (in contrast to the cranium whose functions are obviously more complex), we interpret the similarity in shape as reflecting local adaptations to overlapping dietary resources in the two species and hence as convergent character displacement.

Beyond Pairwise Interactions: Multispecies Character Displacement in Mexican Freshwater Fish Communities.

Competition has long been recognized as a central force in shaping evolution, particularly through character displacement. Yet research on character displacement is biased, as it has focused almost exclusively on pairs of interacting species while ignoring multispecies interactions. Communities are seldom so simple that only pairs of species interact, and it is not clear whether inferences from pairwise interactions are sufficient to explain patterns of phenotypes in nature. Here, we test for character displacement in a natural system of freshwater fishes in western Mexico that contains up to four congeneric species of the genus Poeciliopsis. We analyzed body shape differences between populations with different numbers of competitors while accounting for confounding environmental variables. Surprisingly, we found evidence for convergent character displacement in populations of P. prolifica, P. viriosa, and P. latidens. We also found that the convergence in body shape was not consistently in the same direction, meaning that when three or more competitors co-occurred, we did not find more extreme body shapes compared with when there were only two competitors. Instead, when three or more competitors co-occurred, body shape was intermediate between the shape found with a pair of species and the shape found with no competitor present. This intermediate shape suggests that evolution in multispecies communities likely occurs in response to several competitors rather than to simple pairwise interactions. Overall, our results suggest that competition among multiple species is more complex than simple pairwise competitive interactions.

Environmental filtering and convergent evolution determine the ecological specialization of subterranean spiders

Abstract Ecological specialization is an important mechanism enhancing species coexistence within a given community. Yet, unravelling the effect of multiple selective evolutionary and ecological factors leading the process of specialization remains a key challenge in ecology. Subterranean habitats provide highly replicated experimental arenas in which to disentangle the relative contribution of evolutionary history (convergent evolution vs. character displacement) and ecological setting (environmental filtering vs. competitive exclusion) in driving community assembly. We tested alternative hypotheses about the emergence of ecological specialization using the radiation of a lineage of sheet‐weaver cave‐dwelling spiders as model system. We observed that at the local scale, a differential specialization to cave microhabitats generally parallels moderate levels of morphological similarity and close phylogenetic relatedness among species. Conversely, geographic distance contributed little in explaining microhabitat occupation, possibly mirroring a limited role of competitive exclusion. Yet, compared to non‐coexisting species, co‐occurring species adapted to different microhabitats showed lower morphological niche overlap (i.e. higher dissimilarity) and deeper genetic distance. The framework here developed suggests that in the subterranean domain, habitat specialization is primarily driven by environmental filtering, secondarily by convergent evolution, and only marginally by character displacement or competitive exclusion. This pattern results in the establishment of replicated communities across geographical space, composed by ecologically equivalent species. Such process of community assembly well explains the numerous adaptive radiations observed in subterranean habitats, an eco‐evolutionary pattern well documented in oceanic islands or mountain summit communities. A free Plain Language Summary can be found within the Supporting Information of this article.

Range-wide spatial mapping reveals convergent character displacement of bird song.

A long-held view in evolutionary biology is that character displacement generates divergent phenotypes in closely related coexisting species to avoid the costs of hybridization or ecological competition, whereas an alternative possibility is that signals of dominance or aggression may instead converge to facilitate coexistence among ecological competitors. Although this counterintuitive process-termed convergent agonistic character displacement-is supported by recent theoretical and empirical studies, the extent to which it drives spatial patterns of trait evolution at continental scales remains unclear. By modelling the variation in song structure of two ecologically similar species of Hypocnemis antbird across western Amazonia, we show that their territorial signals converge such that trait similarity peaks in the sympatric zone, where intense interspecific territoriality between these taxa has previously been demonstrated. We also use remote sensing data to show that signal convergence is not explained by environmental gradients and is thus unlikely to evolve by sensory drive (i.e. acoustic adaptation to the sound transmission properties of habitats). Our results suggest that agonistic character displacement driven by interspecific competition can generate spatial patterns opposite to those predicted by classic character displacement theory, and highlight the potential role of social selection in shaping geographical variation in signal phenotypes of ecological competitors.

Interspecific territoriality in two songbird species: potential role of song convergence in male aggressive interactions

In animals, interspecific interference competition is often associated with their aggressive behaviour. The intensity of interspecific aggression and the outcomes of interference competition between closely related species might be substantially modified by copying of vocal signals used in territory defence. Here we tested the hypothesis that song convergence observed in a secondary contact zone of two songbird species, the common nightingale. Luscinia megarhynchos, and the thrush nightingale, Luscinia luscinia, might be an adaptive response to interspecific interference competition. These species are morphologically and ecologically very similar. However, the thrush nightingale is slightly larger and several lines of evidence indicate its competitive dominance. In the secondary contact zone most thrush nightingales incorporate common nightingale song types in their repertoires. Using playback experiments, we evaluated the strength of nonvocal aggressive responses of both species to conspecific and heterospecific stimuli. The species did not differ in aggressive responses to a heterospecific stimulus, suggesting that competitive dominance is not associated with higher interspecific aggressiveness in nightingales. Interestingly, while the common nightingale reacted significantly more aggressively to the conspecific than the heterospecific stimulus, the thrush nightingale showed similarly strong responses to both stimuli. We suggest that this similar level of interspecific and conspecific aggression in the thrush nightingale results from mixed singing of this species in sympatry, as males may not distinguish conspecifics from heterospecifics by song alone. Our results are consistent with the concept of convergent agonistic character displacement. According to this theory, vocal convergence might be adaptive in species that overlap broadly in resource use, as it leads to better distinction of territory boundaries between the species and thus reduces the level of interspecific competition.

Continental variation in wing pigmentation inCalopteryxdamselflies is related to the presence of heterospecifics

Wing pigmentation in Calopteryx damselflies, caused by the deposition of melanin, is energetically expensive to produce and enhances predation risk. However, patterns of melanisation are used in species identification, greater pigmentation is an accurate signal of male immune function in at least some species, and there may be a role for pigment in thermoregulation. This study tested two potential hypotheses to explain the presence of, and variation in, this pigmentation based on these three potential benefits using 907 male specimens of Calopteryx maculata collected from 49 sites (34 discrete populations) across the geographical range of the species in North America: (i) pigmentation varies with the presence of the closely related species, Calopteryx aequabilis, and (ii) pigment increases at higher latitudes as would be expected if it enhances thermoregulatory capacity. No gradual latitudinal pattern was observed, as might be expected if pigmentation was involved in thermoregulation. However, strong variation was observed between populations that were sympatric or allopatric with C. aequabilis. This variation was characterised by dark wings through allopatry in the south of the range and then a step change to much lighter wings at the southern border of sympatry. Pigmentation then increased further north into the sympatric zone, finally returning to allopatry levels at the northern range margin. These patterns are qualitatively similar to variation in pigmentation in C. aequabilis, meaning that the data are consistent with what would be expected from convergent character displacement. Overall, the results corroborate recent research that has suggested sexual selection as a primary driver behind the evolution of wing pigmentation in this group.

SIGNAL DESIGN AND PERCEPTION INHYPOCNEMISANTBIRDS: EVIDENCE FOR CONVERGENT EVOLUTION VIA SOCIAL SELECTION

Natural selection is known to produce convergent phenotypes through mimicry or ecological adaptation. It has also been proposed that social selection--i.e., selection exerted by social competition--may drive convergent evolution in signals mediating interspecific communication, yet this idea remains controversial. Here, we use color spectrophotometry, acoustic analyses, and playback experiments to assess the hypothesis of adaptive signal convergence in two competing nonsister taxa, Hypocnemis peruviana and H. subflava (Aves: Thamnophilidae). We show that the structure of territorial songs in males overlaps in sympatry, with some evidence of convergent character displacement. Conversely, nonterritorial vocal and visual signals in males are strikingly diagnostic, in line with 6.8% divergence in mtDNA sequences. The same pattern of variation applies to females. Finally, we show that songs in both sexes elicit strong territorial responses within and between species, whereas songs of a third, allopatric and more closely related species (H. striata) are structurally divergent and elicit weaker responses. Taken together, our results provide compelling evidence that social selection can act across species boundaries to drive convergent or parallel evolution in taxa competing for space and resources.

Investigating opposing patterns of character displacement in release and advertisement vocalizations ofBufo fowleriandBufo americanus(Anura; Bufonidae)

Anuran release vocalizations function to prevent prolonged amplexus between males, whereas advertisement vocalizations potentially act as a premating isolation mechanism. To facilitate rapid release during heterospecific amplexus, selection should favor convergent character displacement in release vocalizations. Conversely, to promote isolation between sympatric congeners, selection should favor divergent character displacement in advertisement vocalizations. I examined properties of release and advertisement vocalizations of Bufo fowleri and Bufo americanus that are important in mate recognition (periodicity and dominant frequency) to determine if the two calls exhibit opposing patterns of character displacement. Release vocalizations showed no pattern of displacement. The periodicity of sympatric advertisement vocalizations differed from that of allopatric vocalizations for B. americanus but not for B. fowleri. However, the results were not consistent with the pattern expected for divergent character displacement because the two species differed in this parameter in allopatry. These results are compared with published reports of char acter displacement in the B. americanus species-group in the context of a possible mosaic hybrid zone for these species.

Evidence of convergent character displacement in release vocalizations of Bufo fowleri and Bufo terrestris (Anura; Bufonidae)

Anuran release vocalizations prevent prolonged amplexus of males by conspecific and heterospecific males, whereas advertisement vocalizations potentially act as a premating isolation mechanism. Selection favouring prompt release during heterospecific amplexus should result in convergent character displacement in release vocalizations. Conversely, selection should produce divergent character displacement in advertisement vocalizations. I examined call properties important to mate recognition from release and advertisement vocalizations of allopatric and sympatric populations of Bufo fowleri andB.terrestris to determine whether the two calls exhibit opposing patterns of character displacement. Results indicated convergent character displacement in the periodicity of release vocalizations (a measure equal to the inverse of pulse rate and an estimate of release vibration rate). The periodicity of advertisement vocalizations was significantly displaced from the allopatric character state resulting in greater divergence in sympatry. However, this parameter was already largely divergent in allopatry and, therefore, results were not consistent with the pattern expected for divergent character displacement. The dominant frequency of sympatric advertisement vocalizations was not significantly displaced from allopatry. Results for advertisement vocalizations provided evidence that convergence of release vocalizations was not the result of hybridization. There was no evidence that clinal variation accounted for displacement of call characteristics. Convergent character displacement in the periodicity of release vocalizations may facilitate interspecific communication during heterospecific amplexus, whereas divergent advertisement vocalizations promote species isolation.

Field data do not support a textbook example of convergent character displacement.

Experimental evidence supporting convergent character displacement is rare; only one example exists and it is in the form of orientation and territory competition experiments performed in the laboratory. However, outcomes of laboratory experiments involving behaviour or competition can be artefacts of unnatural conditions and, therefore, the results of the previous experiments supporting convergent character displacement are equivocal. In this study, we re-examine the evolution of melanic nuptial coloration in male three-spined stickleback (Gasterosteus aculeatus) inhabiting the Chehalis River drainage in Washington State. This novel nuptial coloration has been thought to have evolved in response to competition for nesting territories with the co-distributed Olympic mudminnow (Norzumbra hubbsi), which is also melanic and breeds at the same time. I found that melanic stickleback males did not have an advantage over their red counterparts from typical populations when competing for nesting territories with Olympic mudminnows. Additionally competitive interactions between sticklebacks and mudminnows were rare in both cage experiments and naturally breeding sticklebacks. Finally, melanic coloration in the Chehalis populations did not develop until males were parental, well after the hypothesized territory establishment period. These results refute the only experimental support for convergent character displacement and emphasize the importance of conducting behavioural experiments and observations under natural conditions.

Mixed responses to resource densities and their implications for character displacement

How should a consumer of two resource types adapt to changes in their abundances? This paper shows that many different biological circumstances produce mixed responses; i.e. increasing availability of one resource increases the consumer's efforts to obtain it, while increasing availability of the other resource decreases the consumer's efforts at exploitation. This implies that competition from a second consumer species may cause convergent or divergent character displacement of the first species. The signs and magnitudes of the second derivative of the fitness function are important in determining which outcome occurs. The degree of resource limitation of the consumer species also influences the nature of adaptive shifts in resource use.

EVIDENCE FOR ECOLOGICAL CHARACTER DISPLACEMENT IN WESTERN AMERICAN CATOSTOMID FISHES.

Much of recent ecological theory rests on assumption that primary factor responsible for organization of natural communities, particularly of animals, is interspecific competition (e.g., MacArthur, 1972; Schoener, 1975). The validity of this assumption depends almost exclusively on inference drawn from observational evidence of two types. Morphological changes in form of ecological character displacement (Brown and Wilson, 1959; Grant, 1972) and niche shifts correlated with geographic variation in number or density of closely related species are generally considered strongest available evidence for importance of interspecific competition (MacArthur, 1972; Cody, 1974; Schoener, 1974, 1975). Grant (1972, 1975) defines ecological character displacement to be the process by which a morphological character state of a species changes under natural selection arising from presence, in same environment, of one or more species similar to it ecologically . By this definition Grant would include both character convergence and divergence but would exclude cases of changes in character state due to related process of character release. It seems clear that interspecific interaction implicit in Grant's (1972, 1975) definition of ecological character displacement is competition. Furthermore, this causal relationship must be demonstrated if ecological character displacement is to be used as observational evidence for occurrence of interspecific competition. Therefore, we define competitive character displacement to be a change of character state in a population that is due to natural selection arising as a result of competition with individuals of another population in same environment. The characters may be morphological, ecological, behavioral, or physiological and change may be either divergent or convergent. By divergent competitive character displacement we mean that a character state of a population in ecological sympatry (=syntopy, Rivas, 1964) exhibits greater difference than is case when allopatric populations are contrasted, and that this effect can be attributed to sympatry. If contrast of allopatric and sympatric populations reveals that character states of two populations are more similar in areas of sympatry than in areas of allopatry and if this effect can be attributed to competition, then situation is one of convergent character displacement. Ecological theory predicts those situations in which convergent or divergent displacement would be expected (MacArthur and Levins, 1964, 1967; MacArthur and Wilson, 1967; Schoener, 1969a, 1969b, 1970; MacArthur, 1972). In all of these models selection responsible for character state change or niche shift arises as a result of interspecific competition. Grant (1972) in his critical review of evidence for ecological character displacement, concluded that the evidence for ecological aspect of morphological character displacement is weak and that the detection of character displacement in its ecological aspect, particularly in mainland regions, will require a level of detail that has so far not been produced in a single study. The weakness of available evidence for competitive character displacement lies in difficulty of establishing unambiguously that character state of

CHARACTER DISPLACEMENT FOR SEXUAL ISOLATION BETWEEN DROSOPHILA MOJAVENSIS AND DROSOPHILA ARIZONENSIS.

According to Brown and Wilson (1956) character displacement exists between two closely related species when their allopatric populations are very similar and their sympatric populations are quite distinct in one or more characters. The disparate characters could be either morphological, physiological, ecological, or behavioral. They viewed the phenomenon as a result of species interaction in sympatry, where divergence would decrease competition or reduce hybridization, and thus be at a selective advantage. They gave several instances which have since been widely cited as classical, textbook examples. More recently, Grant (1972, 1975) reexamined the phenomenon of interspecific interaction, including convergent character displacement as well as the divergent character displacement described by Brown and Wilson. Restricting himself to morphological attributes, Grant reviewed the literature and concluded that there really was no good example of character displacement. Where divergent character displacement had been proposed to exist, Grant argued, the data were incomplete or else the reason for the divergence was not a result of interspecific competition but rather other ecological factors. We feel that many of these problems could be avoided if one could demonstrate character displacement for sexual isolation. Although ethological barriers to interspecific mating are expected to be developed in allopatric populations (Muller, 1939, 1942), their reinforcement in sympatric populations can only be attributed to interspecific interaction and not to other correlated factors. Dobzhansky (1940) proposed on theoretical grounds that sexual isolation between closely related species should be greater in sympatric than in allopatric populations. His arguments essentially are that if there is selection against the hybrids, those individuals which are involved in interspecific crosses would be wasting their gametes. Any gene which improved the ability of the species to discriminate would be advantageous and would increase in frequency, or be fixed, in the region of sympatry. Reinforcement of sexual isolation by means of artificial selection against the hybrids can sometimes be accomplished in the laboratory (Koopman, 1950; Thoday and Gibson, 1962; Kessler, 1966; Ehrman, 1971, 1973; Soans et al., 1974). However, despite its theoretical importance and despite the moderate degree of success obtained in the laboratory, evidence for reproductive character displacement in natural populations is poor and controversial (see e.g., Loftus-Hills, 1975 and Jones, 1975). It has only been shown to exist in some anurans (Fouquette, Jr., 1975; Littlejohn and Loftus-Hills, 1968), some grasshoppers (Cohn and Cantrall, 1974), possibly in two species of damselfly (Waage, 1975), and between races of Drosophila paulistorum (Ehrman, 1965). In 1973 we undertook an investigation on the sexual isolation between natural populations of Drosophila mojavensis and Drosophila arizonensis species. The advantages of using Drosophila for this purpose are manifest in the availability of techniques which would not only enable us to detect reinforcement if it exists, but also to measure its strength and perhaps determine its genetic basis. D. mojavensis

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.