Community-wide character displacement in Miocene hyaenas

Recent studies have found regularities in the pattern of distribution of dental parameters such as canine or carnassial length among sympatric carnivores. These regularities are taken to be indicative of community-wide character displacement. This study documents similar patterns in late Miocene and earliest Pliocene hyaenids from several localities in Eurasia and Africa. Statistical tests show ratios of lower carnassial total lengths and blade lengths between species to be suggestively equal among sympatric late Miocene hyaenas. Other measurements do not show this regular pattern. This finding mirrors that regarding modern canids in the Middle East, suggesting that a process leading to community-wide character displacement was in effect among these hyaenid taxa. Their response to this pattern suggests that they occupied a similar ecological role to modern canids. The causal basis for such a process is unknown but is suggested to lie in direct interspecific competition between carnivores rather than being a response to regularly spaced features of the environment. Character displacement, competition, Hyaenidae, Miocene, Eurasia, Africa.     

Community-wide character displacement in New Zealand skinks

Aim  To test for community-wide character displacement in New Zealand skinks.
Location  Four small islands in the New Zealand archipelago.
Methods  (1) We conducted a field experiment on a single island to evaluate whether prey size selection is correlated with lizard body size. We pitfall trapped 69 skinks from three species, measured several aspects of their morphology and presented each animal with a variety of different-sized prey in a food choice experiment. (2) We tested whether the morphological characteristics associated with prey size selection were evenly partitioned in four island skink communities using null models.
Results  Prey size selection was associated with skink morphology; larger skinks consumed larger prey. Null model analyses showed support for evenly displaced body sizes on one island, weak support on one island and no support on two islands.
Main conclusions  Results showed mixed support for community-wide character displacement in New Zealand skinks. Differences in body sizes appear to reflect the use of different-sized prey. Even differentiation in body sizes on one island suggests that species coexistence is facilitated by interspecific differences in prey size selection. However, little support was found on other islands, suggesting that other factors, such as interspecific differences in habitat selection and/or diurnal activity patterns, may interact with differences in prey size selection to promote coexistence among New Zealand skinks.     

Community-wide character displacement in barnacles: a new perspective for past observations

We tested for community‐wide character displacement of feeding leg length and shell morphology in two barnacle communities on the west coast of North America (southern California, USA and Vancouver Island, Canada). Neither community exhibited even displacement in shell morphology. Both barnacle communities, however, exhibited remarkably evenly displaced feeding leg length, despite large differences in geography and species composition (between the orders Pedunculata and Sessilia). Previous experiments suggest that this pattern results from competition, although the competitive mechanism remains unknown. Displacement of leg length may reflect dietary specialization, spatial competition, or both. In some cases the results from two null models differed, illustrating the importance of employing a null model that considers mean and variance, rather than character means alone. Overall, the observed pattern of character displacement provides a new perspective for re‐examining the complex relationship between morphology and interspecific competition among intertidal barnacles.     

Stability of displacement clines arising in allospecies competition

Summary A stability analysis for geographic displacement clines between competing allospecies is presented. The competition model incorporates the effects of annual dispersal and of selective recruitment determined by geographically varying conditions at the breeding sites. It is assumed that a species gains a local competitive advantage wherever it attains sufficient numerical predominance. This assumption is valid when the species crosspair but the hybrids produced are not recruited into the adult population, because the minority species loses proportionately more of its reproductive potential. It is shown that no stable equilibria occur when the competitive balance is independent of location, but that even a slight geographic variation in the competitive balance allows stable equilibria. The greater the length of the combined breeding ranges of the two species, the smaller the geographic shift in competitive balance needed to produce stability.     

Divergent character clines across a recent secondary contact zone in a Hispaniolan lizard

Studies of genetic contact zones provide valuable information regarding the processes of population divergence, adaptation and speciation. In this paper, I examine transitions in morphology, mitochondrial DNA (mtDNA) and nuclear DNA (nDNA) haplotypes across a recent secondary contact zone in a Hispaniolan lizard Ameiva chrysolaema . Maximum likelihood cline fitting analyses suggest non-coincidence of cline centers and that the mtDNA cline is significantly displaced to the west of the remaining clines. nDNA and morphological clines are coincident and tend to be associated with the prevailing environmental gradient. The lack of cytonuclear disequilibrium near the center of the contact zone and the non-coincidence of character clines suggest that this zone does not conform to a tension zone model of hybridization; thus, gene flow across the zone does not seem to be impeded. The extremely narrow width of the dorsal scale size cline and the close association of this cline with the steepness of the environmental (precipitation) gradient suggest that this character may be under environmental selection. Taken together, this contact zone appears to be structured by a combination of mtDNA introgression, possibly associated with eastward movement of the zone, and environmental selection on some characters.     

Ecological and community-wide character displacement: the next generation

Ecological character displacement, mostly seen as increased differences of size in sympatry between closely‐related or similar species, is a focal hypothesis assuming that species too similar to one another could not coexist without diverging, owing to interspecific competition. Thus, ecological character displacement and community‐wide character displacement (overdispersion in size of potential competitors within ecological guilds) were at the heart of the debate regarding the role of competition in structuring ecological communities. The debate has focused on the evidence presented in earlier studies and generated a new generation of rigorous, critical studies of communities. Character displacement research in the past two decades provides sound statistical support for the hypothesis in a wide variety of taxa, albeit with a phylogenetically skewed representation. A growing number of studies are strongly based in functional morphology, and some also demonstrate actual morphologically related resource partitioning. Phylogenetic models and experimental work have added to the scope and depth of earlier research, as have theoretical studies. However, many challenging ecological and evolutionary issues, regarding both selective forces (at the inter‐ and intraspecific level) and resultant patterns, remain to be addressed. Ecological character displacement and community‐wide character displacement are here to stay as the focus of much exciting research.     

Population regulation and character displacement in a seasonal environment

Competition has negative effects on population size and also drives ecological character displacement, that is, evolutionary divergence to utilize different portions of the resource spectrum. Many species undergo an annual cycle composed of a lean season of intense competition for resources and a breeding season. We use a quantitative genetic model to study the effects of differential reproductive output in the summer or breeding season on character displacement in the winter or nonbreeding season. The model is developed with reference to the avian family of Old World leaf warblers (Phylloscopidae), which breed in the temperate regions of Eurasia and winter in tropical and subtropical regions. Empirical evidence implicates strong winter density-dependent regulation driven by food shortage, but paradoxically, the relative abundance of each species appears to be determined by conditions in the summer. We show how population regulation in the two seasons becomes linked, with higher reproductive output by one species in the summer resulting in its evolution to occupy a larger portion of niche space in the winter. We find short-term ecological processes and longer-term evolutionary processes to have comparable effects on a species population size. This modeling approach can also be applied to other differential effects of productivity across seasons.     

Coevolution of competing species: ecological character displacement

Character displacement of competing species is studied. A model, originally developed by MacArthur and Levins (Proc. Natl. Acad. Sci. USA 51 (1964), 1207-1210) and further analyzed by Lawlor and Maynard Smith (Amer. Nat. 110 (1976), 70-99), has been reanalyzed. In the present paper, a more formally correct analysis of the MacArthur-Levins model is provided. A standard population genetics approach to sexually reproducing populations is adopted. The same conclusion as proposed by Lawlor and Maynard Smith emerges; competition can lead only to character divergence. In our analysis we either require that allopatrically evolved consumer populations must be able to coexist at an ecologically stable equilibrium (hence, we require mutual invasibility), or consider the feasibility of allopatric equilibria.     

Asymmetrical reproductive character displacement in the house mouse

Our study addressed reproductive character displacement between two subspecies of the house mouse, Mus musculus musculus and Mus musculus domesticus that hybridize in Europe along a zone where selection against hybridization is known to occur. Based on a multi-population approach, we investigated spatial patterns of divergence of mate preference in the two taxa. Mate preference was significantly higher in the contact zone than in allopatry in both subspecies, suggesting that reproductive character displacement occurs. Moreover, patterns of preference were stronger in M. m. musculus than in M. m. domesticus, indicating an asymmetrical divergence between the two. In the context of selection against hybridization, our results may provide empirical support for the hypothesis of reinforcement in a parapatric hybrid zone. We discuss factors that could explain the asymmetrical pattern of divergence and the possible impact of a unimodal structure on the maintenance of premating divergence between the two subspecies.     

Evaluating the targets of selection during character displacement

Ecological character displacement occurs when competition imposes divergent selection on interacting species, causing divergence in traits associated with resource use. Generally, divergence is assumed to occur when selection acts on the same, continuously varying trait in both species. However, selection might target multiple traits, and even closely related heterospecifics involved in character displacement might differ in selective targets. We investigated the targets of selection in a species of spadefoot toad, Spea multiplicata, during experimentally imposed competition with a congener, S. bombifrons. When examining traits separately, we found significant selection acting on multiple resource-acquisition traits. Yet, controlling for the independent effects of these traits in a multiple regression revealed that direct selection on a single trait might have contributed toward indirect selection on other correlated traits. Moreover, although we found evidence for plasticity in most traits, competition with S. bombifrons imposed selection on morphology and not on plasticity. Additional experiments suggest that the selective targets during character displacement might differ between the two species involved in this one instance of character displacement. Identifying the targets of competitively mediated selection is crucial, because whether and how character displacement ultimately unfolds depends on the nature of these targets and correlations among them.     

Reconstructing asymmetrical reproductive character displacement in a periodical cicada contact zone

Selection against costly reproductive interactions can lead to reproductive character displacement (RCD). We use information from patterns of displacement and inferences about predisplacement character states to investigate causes of RCD in periodical cicadas. The 13-year periodical cicada Magicicada neotredecim exhibits RCD and strong reproductive isolation in sympatry with a closely related 13-year species, Magicicada tredecim. Displacement is asymmetrical, because no corresponding pattern of character displacement exists within M. tredecim. Results from playback and hybridization experiments strongly suggest that sexual interactions between members of these species were possible at initial contact. Given these patterns, we evaluate potential sources of selection for displacement. One possible source is 'acoustical interference', or mate-location inefficiencies caused by the presence of heterospecifics. Acoustical interference combined with the species-specificity of song pitch and preference appears to predict the observed asymmetrical pattern of RCD in Magicicada. However, acoustical interference does not appear to be a complete explanation for displacement in Magicicada, because our experiments suggest a significant potential for direct sexual interactions between these species before displacement. Another possible source of selection for displacement is hybrid failure. We evaluate the attractiveness of inferred hybrid mating signals, and we examine the viability of hybrid eggs. Neither of these shows strong evidence of hybrid inferiority. We conclude by presenting a model of hybrid failure related to life cycle differences in Magicicada.     

Interspecific aggression and character displacement in the damselfly Calopteryx splendens

Problems in species recognition are thought to affect the evolution of secondary sexual characters mainly through avoidance of maladaptive hybridization. Another, but much less studied avenue for the evolution of sexual characters due to species recognition problems is through interspecific aggression. In the damselfly, Calopteryx splendens, males have pigmented wing spots as a sexual character. Large-spotted males resemble males of another species, Calopteryx virgo, causing potential problems in species recognition. In this study, we investigate whether there is character displacement in wing spot size and whether interspecific aggression could cause this pattern. We found first that wing spot size of C. splendens in populations decreased with increasing relative abundance of C. virgo. Secondly, C. virgo males were more aggressive towards large- than small-spotted C. splendens males. Thirdly, in interspecific contests C. virgo males had better territory holding ability than C. splendens males. These results suggest that interspecific aggression may have caused character displacement in wing spot size of C. splendens, because the intensity of aggression towards large-spotted males is likely to increase with relative abundance of C. virgo males. Thus, interspecific aggression may be an evolutionarily significant force that is able to cause divergence in secondary sexual characters.     

Canine carnassials: character displacement in the wolves, jackals and foxes of Israel

Previous research implies that competitive character displacement in felids and mustelids of Israel is expressed by canine size. Anatomy and observed killing behaviour of canids suggest that canines in this group are less adapted for the stylized role they play in felids and mustelids. Thus we hypothesized that character displacement, if it exists in canids, should not be manifested more clearly by canine size than by other traits. Five sympatric and at least partially syntopic canids occupy Israel, while in North Africa the largest (wolf) and smallest (Blanford's fox) are absent. Sexual size dimorphism in Israeli canids is generally less than in felids and mustelids (in which we analysed each sex as a separate ‘morphospecies’), so we used mixed-sex samples to represent each species. The three largest species (wolf, golden jackal and red fox) are also represented by Middle Palaeolithic samples in Israel, and all three had larger carnassial lengths then. Carnassial lengths, canine diameters and skull lengths are all remarkable evenly spaced among the five recent species in Israel. In Egypt, no trait manifests significant equality. Despite regional fluctuations in size, the carnassial length ratios of the three smaller species (foxes) are strikingly constant (1.18–1.21) throughout the region, while the ratios for the three larger species (wolf, jackal and red fox), sympatric only in Israel, are larger (1.33–1.34). Finally, mean carnassial length of jackals is constant across North Africa, while skull length and canine diameter both increase from Algeria through Egypt. All three traits are larger in Egypt than in Israel. We tentatively ascribe the equal ratios in Israel to competitive character displacement, though this hypothesis is speculative because of numerous lacunae in knowledge of diet, killing behaviour, available resources and extent of food limitation. Furthermore, humans have greatly affected range, density and ecology of wolves and jackals in the last century. Larger sizes in the Palaeolithic may well be manifestations of Bergmann's rule. The constancy of carnassial length in North African jackals, notwithstanding a longitudinal cline in CBLs of these populations, and the constant ratio between jackal and red fox carnassial length are both consistent with a hypothesis of character release in the absence of the wolf.     

Genetic accommodation in the wild: evolution of gene expression plasticity during character displacement

Ecological character displacement is considered crucial in promoting diversification, yet relatively little is known of its underlying mechanisms. We examined whether evolutionary shifts in gene expression plasticity (‘genetic accommodation’) mediate character displacement in spadefoot toads. Where Spea bombifrons and S. multiplicata occur separately in allopatry (the ancestral condition), each produces alternative, diet‐induced, larval ecomorphs: omnivores, which eat detritus, and carnivores, which specialize on shrimp. By contrast, where these two species occur together in sympatry (the derived condition), selection to minimize competition for detritus has caused S. bombifrons to become nearly fixed for producing only carnivores, suggesting that character displacement might have arisen through an extreme form of genetic accommodation (‘genetic assimilation’) in which plasticity is lost. Here, we asked whether we could infer a signature of this process in regulatory changes of specific genes. In particular, we investigated whether genes that are normally expressed more highly in one morph (‘biased’ genes) have evolved reduced plasticity in expression levels among S. bombifrons from sympatry compared to S. bombifrons from allopatry. We reared individuals from sympatry vs. allopatry on detritus or shrimp and measured the reaction norms of nine biased genes. Although different genes displayed different patterns of gene regulatory evolution, the combined gene expression profiles revealed that sympatric individuals had indeed lost the diet‐induced gene expression plasticity present in allopatric individuals. Our data therefore provide one of the few examples from natural populations in which genetic accommodation/assimilation can be traced to regulatory changes of specific genes. Such genetic accommodation might mediate character displacement in many systems.     

Autumn-winter energetics of Holarctic tree squirrels:a review

Similarities in general size, geometry, lifestyle, and environment mean that certain energetic constraints are common and peculiar to Holarctic tree squirrels as a group. Holarctic tree squirrels are relatively small, diurnal mammals which, in association with their food niche, maintain activity throughout the autumn-winter period. Despite this, they exhibit no major morphological or physiological adaptations to minimize energy expenditure at low temperatures; on the contrary, both basal metabolism and conductance are higher than expected on the grounds of physical size. When they are active energy expenditure is therefore strongly influenced by effective ambient temperature for these species when active in their natural autumn-winter environments. Nest use allows near-basal metabolism at most natural ambient temperatures. The balance of economical inactivity against feeding rewards offset by cold exposure must therefore be a crucial aspect of the lifestyle of these squirrels.     

Latitudinal clines in the properties of a circadian pacemaker

The circadian rhythm of eclosion activity and its pacemaker were analyzed in a series of latitudinal races of Drosophila auraria ranging from 34.2 degrees to 42.9 degrees N in Japan. The phase of the rhythm (psi EL) to the daily photoperiod (PP) changes as daylength is increased, and the slope of psi EL (PP) changes with latitude. Is is sufficiently greater in the north to cause a phase reversal of northern and southern races on long versus short photoperiods. This reversal is found in assays of the pacemaker's phase (psi PL) as well as that of the rhythm (psi EL). Assay of the pacemaker shows that its period (tau) is longer in northern than in southern races, and that the amplitude of its phase response curve (PRC) is lower in the north. The period of the rhythm in all latitudinal races is longer than 24 hr in short photoperiods (LD 1:23), but is probably less than 24 hr (as an aftereffect of photoperiod) in longer days such as LD 14:10. The observed north-south differences in the phase relation of both pacemaker and rhythm to the light cycle are explained by the latitudinal clines in pacemaker properties and a postulated aftereffect of photoperiod on tau. It is suggested that the latitudinal cline in PRC amplitude has functional significance in conserving the amplitude of the pacemaker's signal to the rest of the system it times. Computer simulation shows that without such a reduction in the perceived light intensity, pacemaker amplitude will be lowered by the increase in duration of the daily light at higher latitudes.     

Reproductive character displacement and signal ontogeny in a sympatric assemblage of electric fish

The reproductive signals of two or more taxa may diverge in areas of sympatry, due to selection against costly reproductive interference. This divergence, termed reproductive character displacement (RCD), is expected in species-rich assemblages, where interspecific signal partitioning among closely related species is common. However, RCD is usually documented from simple two-taxon cases, via geographical tests for greater divergence of reproductive traits in sympatry than in allopatry. We propose a novel approach to recognizing and understanding RCD in multi-species communities--one that traces the displacement of signals within multivariate signal space during the ontogeny of individual animals. We argue that a case for RCD can be made if the amount of signal displacement between a pair of species after maturation is negatively correlated to distance in signal space before maturation. Our application of this approach, using a dataset of communication signals from a sympatric Amazonian assemblage of the electric fish genus Gymnotus, provides strong evidence for RCD among multiple species. We argue that RCD arose from the costs of heterospecific mismating, but interacted with sexual selection--favoring the evolution of conspicuous male signals that not only serve for mate-choice, but which simultaneously facilitate species recognition.     

Existence of a pattern of reproductive character displacement in Homobasidiomycota but not in Ascomycota

Generally, stronger reproductive isolation is expected between sympatric than between allopatric sibling species. Such reproductive character displacement should predominantly affect premating reproductive isolation and can be due to several mechanisms, including population extinction, fusion of insufficiently isolated incipient species and reinforcement of reproductive isolation in response to low hybrid fitness. Experimental data on several taxa have confirmed these theoretical expectations on reproductive character displacement, but they are restricted to animals and a few plants. Using results reported in the literature on crossing experiments in fungi, we compared the degree and the nature of reproductive isolation between allopatric and sympatric species pairs. In accordance with theoretical expectations, we found a pattern of enhanced premating isolation among sympatric sibling species in Homobasidiomycota. By contrast, we did not find evidence for reproductive character displacement in Ascomycota at similar genetic distances. Both allopatric and sympatric species of Ascomycota had similarly low levels of reproductive isolation, being mostly post-zygotic. This suggests that some phylogeny-dependent life-history trait may strongly influence the evolution of reproductive isolation between closely related species. A significant correlation was found between degree of reproductive isolation and genetic divergence among allopatric species of Homobasidiomycota, but not among sympatric ones or among Ascomycota species.     

Parallel evolution and ecological selection: replicated character displacement in spadefoot toads

Ecological character displacement—trait evolution stemming from selection to lessen resource competition between species—is most often inferred from a pattern in which species differ in resource-use traits in sympatry but not in allopatry, and in which sympatric populations within each species differ from conspecific allopatric populations. Yet, without information on population history, the presence of a divergent phenotype in multiple sympatric populations does not necessarily imply that there has been repeated evolution of character displacement. Instead, such a pattern may arise if there has been character displacement in a single ancestral population, followed by gene flow carrying the divergent phenotype into multiple, derived, sympatric populations. Here, we evaluate the likelihood of such historical events versus ongoing ecological selection in generating divergence in trophic morphology between multiple populations of spadefoot toad (Spea multiplicata) tadpoles that are in sympatry with a heterospecific and those that are in allopatry. We present both phylogenetic and population genetic evidence indicating that the same divergent trait, which minimizes resource competition with the heterospecific, has arisen independently in multiple sympatric populations. These data, therefore, provide strong indirect support for competition''s role in divergent trait evolution.