Example Of Character Displacement Research Articles

Biotic interactions vary across species' ranges and are likely conserved through geological time

AbstractAimThe evolutionary interactions between western spadefoot toads (genus Spea) represent a textbook example of character displacement, facilitated by dietary specialization of one Spea species on fairy shrimp (Anostraca) when all three co‐occur. The aim of this study is to understand the covariation between predator (Spea) and prey (Anostraca) range shifts in response to climate change oscillations, and whether biotic interactions can be used to project species distribution models on different time scales when studying species with dietary specialization. Taxon: Amphibia: Spea spp. and Crustacea: Anostraca.LocationNorth America.MethodsUsing multiple modelling techniques, we first estimated the potential distribution of central and western North American fairy shrimp species (Crustacea: Anostraca) and two western spadefoot toad species (Spea bombifrons and Spea multiplicata). We then created a shrimp species richness map by aggregating individual species estimates. Third, we studied the relationship between the probability of spadefoot toad presence and fairy shrimp species richness during the present and Last Glacial Maximum conditions. Finally, we estimated the strength and direction of the co‐occurrence between spadefoot toads and fairy shrimp sampled at the level of entire predicted range and at the regional level (allopatric and sympatric).ResultsFirst, the same abiotic environmental variables shape spadefoot toad and fairy shrimp species' distributions in central and western North America across time. Second, areas of sympatry of Spea bombifrons and Spea multiplicata correspond with dry conditions and higher shrimp richness. Finally, the spatial patterns of predator–prey co‐occurrence are highly variable across geography, forming a spatial mosaic over the species' ranges.Main ConclusionPredator–prey relationships form a spatial mosaic across geography and species ranges. Including biotic interactions into species distribution estimates for organisms with dietary specialization is highly recommended. Biotic interactions can be projected across different time frames for organisms with dietary specialization as they are likely conserved.

Comparative Morphological Analysis of Two Species of Turtle Leeches Coexisting in North America (Hirudinea: Glossiphoniidae): Embryological Evidence for Character Displacement

The genus Placobdella (Glossiphoniidae) has a centre of species concentration in North America. The type species P. costata is the only representative in the Palaearctic region. American Placobdella which feed on turtles are represented predominantly by two common species, P. parasitica and P. rugosa, which geographically overlap in eastern USA and southern Canada. The latter species is morphologically indistinguishable from P. multilineata of southeast USA. These two tuberculated forms are recognised herein as a clade and treated as a single ‘species’ for comparative purposes. Both P. parasitica and P. rugosa clade commonly coexist and feed on the same turtle species without host preference. This paper addresses morphological differences between them. An unexpected finding is that hatchlings of both species are very difficult to distinguish. A morphologically significant observation is that developmental divergence results in tubercle prominence in P. rugosa clade, but tubercle suppression in P. parasitica, the first example of character displacement in the Hirudinea. Morphological differences are interpreted as reflecting interspecific competition, a phenomenon not found in their Palaearctic counterpart. Why do the two American turtle leech species coexist rather than reduce competition by partitioning their food supply?

Experimental evolution of competing bean beetle species reveals long-term reversals of short-term evolution, but no consistent character displacement.

Interspecific competition for shared resources should select for evolutionary divergence in resource use between competing species, termed character displacement. Many purported examples of character displacement exist, but few completely rule out alternative explanations. We reared genetically diverse populations of two species of bean beetles, Callosobruchus maculatus and Callosobruchus chinensis, in allopatry and sympatry on a mixture of adzuki beans and lentils, and assayed oviposition preference and other phenotypic traits after four, eight, and twelve generations of (co)evolution. C. maculatus specializes on adzuki beans; the generalist C. chinensis uses both beans. C. chinensis growing in allopatry emerged equally from both bean species. In sympatry, the two species competing strongly and coexisted via strong realized resource partitioning, with C. chinensis emerging almost exclusively from lentils and C. maculatus emerging almost exclusively from adzuki beans. However, oviposition preferences, larval survival traits, and larval development rates in both beetle species did not vary consistently between allopatric versus sympatric treatments. Rather, traits evolved in treatment‐independent fashion, with several traits exhibiting reversals in their evolutionary trajectories. For example, C. chinensis initially evolved a slower egg‐to‐adult development rate on adzuki beans in both allopatry and sympatry, then subsequently evolved back toward the faster ancestral development rate. Lack of character displacement is consistent with a previous similar experiment in bean beetles and may reflect lack of evolutionary trade‐offs in resource use. However, evolutionary reversals were unexpected and remain unexplained. Together with other empirical and theoretical work, our results illustrate the stringency of the conditions for character displacement.

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.

Disassociation between weak sexual isolation and genetic divergence in a hermaphroditic land snail and implications about chirality

Examination of the association between reproductive isolation and genetic divergence in a variety of organisms is essential for elucidating the mechanisms causing speciation. However, such studies are lacking for hermaphrodites. We measured premating (sexual) isolation in species pairs of the hermaphroditic land snail Albinaria and we compared it with their genetic divergence. We did not find substantial sexual isolation barriers between the species studied. The absence of strong sexual isolation between species implies its minor effect in the evolution of this genus, because distributional, population and life-history characteristics of Albinaria make mate-choice possibly redundant. Furthermore, we found disassociation between genetic divergence and sexual isolation, suggesting that they do not form necessarily a cause-effect duet. However, Albinaria voithii, the only dextral Albinaria species, shows strong sexual isolation against the other sinistral species. We discuss whether change in coiling either has triggered instantaneous speciation, or is an example of character displacement.

The Evolution of Traits Affecting Resource Acquisition and Predator Vulnerability: Character Displacement under Real and Apparent Competition

This article investigates some simple models of the evolutionary interaction between two prey species that share a common resource and a common predator. Each prey species is characterized by a trait that determines both the rate of resource capture and vulnerability to a predator. In a simple model of a three-species food chain, such traits usually increase in response to an imposed reduction in resource density. When the per capita growth rates of each of two prey species depend linearly on resource density, such traits will change in opposite directions when the two prey come into sympatry. In addition, the ratio of the effect of the predator on prey fitness to the effect of the resource on prey fitness will diverge from the corresponding ratio in a second prey species when those species coexist in sympatry. These simple predictions need not hold under several alternative assumptions, which may be more common in biological systems. Parallel changes in sympatry may occur if the relationship between resource consumption and prey growth is nonlinear, if the prey species have partial overlap in the set of resources used or in the set of predators that consume them, or if prey experience direct intraspecific competition. The responses to a second prey can also differ significantly from those predicted by the simplest model if separate traits affect vulnerability to predators and resource acquisition rate. It is important to determine whether examples of character displacement previously interpreted as responses to competition for resources might also reflect responses to altered predation risks in sympatry.

The Evolution of Traits Affecting Resource Acquisition and Predator Vulnerability: Character Displacement under Real and Apparent Competition

This article investigates some simple models of the evolutionary interaction between two prey species that share a common resource and a common predator. Each prey species is characterized by a trait that determines both the rate of resource capture and vulnerability to a predator. In a simple model of a three‐species food chain, such traits usually increase in response to an imposed reduction in resource density. When the per capita growth rates of each of two prey species depend linearly on resource density, such traits will change in opposite directions when the two prey come into sympatry. In addition, the ratio of the effect of the predator on prey fitness to the effect of the resource on prey fitness will diverge from the corresponding ratio in a second prey species when those species coexist in sympatry. These simple predictions need not hold under several alternative assumptions, which may be more common in biological systems. Parallel changes in sympatry may occur if the relationship between resource consumption and prey growth is nonlinear, if the prey species have partial overlap in the set of resources used or in the set of predators that consume them, or if prey experience direct intraspecific competition. The responses to a second prey can also differ significantly from those predicted by the simplest model if separate traits affect vulnerability to predators and resource acquisition rate. It is important to determine whether examples of character displacement previously interpreted as responses to competition for resources might also reflect responses to altered predation risks in sympatry.

Ecological character displacement in Plethodon: biomechanical differences found from a geometric morphometric study.

Ecological character displacement describes a pattern where morphological differences between sympatric species are enhanced through interspecific competition. Although widely considered a pervasive force in evolutionary ecology, few clear-cut examples have been documented. Here we report a case of ecological character displacement between two salamander species, Plethodon cinereus and Plethodon hoffmani. Morphology was quantified by using linear measurements and landmark-based geometric morphometric methods for specimens from allopatric and sympatric populations from two geographic transects in south-central Pennsylvania, and stomach contents were assayed to quantify food resource use. Morphological variation was also assessed in 13 additional allopatric populations. In both transects, we found significant morphological differentiation between sympatric populations that was associated with a reduction in prey consumption in sympatry and a segregation of prey according to prey size. No trophic morphological or resource use differences were found between allopatric populations, and comparisons of sympatric populations with randomly paired allopatric populations revealed that the observed sympatric morphological differentiation was greater than expected by chance. The major trophic anatomical differences between sympatric populations relates to functional and biomechanical differences in jaw closure: sympatric P. hoffmani have a faster closing jaw, whereas sympatric P. cinereus have a slower, stronger jaw. Because salamanders immobilize prey of different sizes in different ways, and because the observed sympatric biomechanical differences in jaw closure are associated with the differences in prey consumption, the observed character displacement has a functional ecological correlate, and we can link changes in form with changes in function in this apparent example of character displacement.

Size of mudsnails, Hydrobia ulvae (Pennant) and H. ventrosa (Montagu), in allopatry and sympatry: conclusions from field distributions and laboratory growth experiments

In most areas, sizes of the two mudsnails Hydrobia ulvae and H. ventrosa diverge when their distributions overlap, while in allopatry they are about equally large. This has been suggested as an example of character displacement, although later studies suggest alternative interpretations. In this study, distribution and snail size of both species was assessed in 30 localities on the Swedish West coast, both allopatric sites of each species and sympatric sites. Overall, mean sizes varied considerably among populations; in H. ulvae mean size range was 2.0–4.5 mm and in H. ventrosa 2.2–3.6 mm. Mean sizes in allopatry were roughly similar, while in sympatry H. ulvae was always larger than H. ventrosa. In the field, snail size of both species increased with increasing silt content of the sediment. However, silt content alone seemed unable to explain the character displacement-like pattern as growth in the laboratory was not differently affected by sandy or silty sediments. In the laboratory, however, snails of both species grew at various rates over sediments from different bays, suggesting environmental effects other than grain size. Furthermore, H. ulvae constantly grew at about 150% the rate of H. ventrosa when grown under the same conditions. This suggests that the divergence in size in sympatric sites is due to intrinsic growth rate differences, which persist over a range of environments. The similar sizes in allopatric sites may be explained by phenotypic adjustments to environmental factors. Our results reject inherited differences in growth rate among populations as an explanation of the character displacement-like pattern of size variation. We also argue that differences in recruitment time seems an unlikely explanation.

Evolution and the Consequences of Species Introductions and Deletions

The addition or deletion of a species from a community is likely to have effects on the trait values of other species and on their population densities. This article argues that current theory is insufficiently developed to provide guidance in predicting what might happen to either traits or population densities. In addition, there has been relatively little empirical work to examine many of the phenomena that have been predicted by the limited available theory. The example of character displacement of competitors is reviewed to reveal some of the gaps in our knowledge about the evolutionary consequences of additions or deletions. The example of evolution of traits related to predation in food webs is used to reveal gaps in our knowledge of the population—level consequences of evolutionary changes initiated by a species addition or deletion. Several approaches to studying combined evolutionary and ecological processes in multispecies communities are discussed. Some previous multispecies models have been too abstract to be easily related to more mechanistic two—species models, but recent methods derived from quantitative genetics may result in significant advances in understanding multispecies systems and their relationship to communities with fewer species.

A Critical Comparison of the Taxon-Cycle and Character-Displacement Models for Size Evolution of Anolis Lizards in the Lesser Antilles

revealed an interesting pattern: on most islands with only one species, that species is of intermediate size, but two-species islands usually contain both a large and a small species (Schoener, 1970; Table 1). The evolution of large and small size has often been interpreted as an example of character displacement (e.g., Schoener, 1988b; Williams 1972; Losos, 1990). Roughgarden and associates (e.g., Roughgarden et al., 1987; Roughgarden and Pacala, 1989; Rummel and Roughgarden 1985a) have proposed an alternative hypothesis; they argue that a taxon cycle is operating in the northern Lesser Antilles in which a larger species invades an island occupied by an intermediate-sized species, and both evolve smaller size until the former species is intermediate in size and the latter species is extinct (Fig. 1). Roughgarden and Pacala (1989) reviewed the pertinent data and concluded that the available evidence overwhelmingly favors the taxon-cycle model. Here I argue that much of this evidence is either inconclusive, incorrect, or more consistent with the character-displacement model.

Geographic Variation in the Kentropyx calcarata Species Group (Sauria: Teiidae): A Possible Example of Morphological Character Displacement

Morphological variation of the South American lizard genus Kentropyxis discussed, with particular emphasis on the Amazonian distributed calcarata species group. Ecological and zoogeographical data are analyzed. Analysis of geographic variation of meristic characters, in con- junction with color and striping pattern variation, supports the recognition of three Amazonian species of Kentropyx with small dorsal scales. A dramatic morphological change is noted for Ken- tropyx calcarata, and it is suggested that the morphological divergence might be an example of character displacement.

Visual and Acoustic Display in Uca Mordax and U. Burgersi, Sibling Species of Neotropical Fiddler Crabs. Ii. Vibration Signals

AbstractThe sibling species of fiddler crabs, Uca mordax and U. burgersi, which are very similar in respect to morphology and many traits of their waving display (see part I of this study), were studied bioacoustically in Trinidad (W.I.). When recording their vibration signals (mainly in the laboratory) it was, at the same time, possible to directly observe the animals' underground activities. The dominant vibration signals of the two species were found to be extremely different: males of U. mordax emit rapping sounds ("drumwhirls") by ambulatory percussion, whereas males of U. burgersi produce "howling" sounds (with a varying number of harmonics) by cheliped convulsion, i.e. inconspicuous quivering movements of this appendage. Similar interspecific differences exist in the fainter vibration signals of females. Each of the two species is capable of producing other signals in addition to the prevalent ones mentioned: U. mordax can emit cheliped vibrations as well (though percussive ones) and U. burgersi can also produce ambulatory "drumwhirls". From these and other basic similarities and from comparisons with recordings of burgersi sounds from Colombia and allopatric populations in Florida, the common starting-point and the different evolutionary pathways leading to the two species-specific termini of acoustic display are reconstructed. The extreme differences that were found for the sympatric situation in Trinidad are regarded as an example of character displacement. Apparently, the acoustic communication system of these crabs is much more affected by character displacement than the visual one.

The genus Partula on Moorea: speciation in progress

Land snails of the genus Partula Fer. inhabiting the island of Moorea in French Polynesia show complex patterns of partial speciation. The late H. E. Crampton described eleven species from the island. Two have since been relegated to the genus Samoana . Of the remaining nine taxa, as many as four may coexist at a single locality without evidence of hybridization. Nevertheless an allele appearing in any of the taxa is potentially capable of spreading to any of the others, although in the process it may have to pass through several intervening taxa. Large morphological, ecological and behavioural differences have evolved despite this potential for gene flow. The biological differences are not reflected in 20 enzyme loci surveyed by conventional starch-gel electrophoresis, nor in the numbers of chromosomes. It is clear that a significant part of the morphological divergence between taxa has taken place of Moorea. Our observations are consistent with the view that all the Moorean taxa (excepting Samoana ) are descendants of a single ancestral invasion, although the possibility of more than one invasion cannot be excluded. The pattern of differentiation shows at least two cases of circular overlap (‘ring species’), with diameters of 2 and 3 km, and several apparent examples of character displacement. Coexisting taxa tend to partition the available habitat, although the separation is never complete.

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

Effect of Sympatry on Meadowlark Vocalizations

When the ranges of two similar species partially overlap, differences between the two species are usually greater in the areas of overlap than where they are separate. This is character displacement as described by Brown and Wilson (1956). Lack (1971) cited many examples of character displacement in such features as morphology, habitat, and feeding behavior. Contrary to the theory of character displacement, Rohwer (1973) suggested that, under certain conditions, interspecific territoriality and convergence in display characters can be expected in an interacting species pair. In his study of the two species of meadowlark (Sturnella) he found convergence in characters used in posturing displays. He also noted a loss of information content in vocalizations when territorial males of one species were exposed to the primary song of the other species. This resulted in increased fighting as a compensation for the ineffectiveness of song for maintaining territorial possession. Cody (1969) stated that similarities are often less pronounced or even absent where two species are allopatric and that sympatry is necessary for retention of any similarities. He proposed that songs, specifically in the case of the two meadowlarks, show convergence in the zone of sympatry in response to selection for interspecific territoriality. Moynihan (1968) predicted that related species which are sympatric will usually be more similar in overall appearance than equally closely related species of similar habits which are allopatric. Frequent contacts between individuals of sympatric species result in closer resemblances between them to facilitate social reactions.

Reproductive Isolation and the Potential for Character Displacement in the Damselflies, Calopteryx Maculata and C. Aequabilis (Odonata: Calopterygidae)

Waage, J. K. (Division of Biological and Medical Sciences, Brown University, Providence, Rhode Island 02912) 1975. Reproductive isolation and the potential for character displacement in the damselflies, Calopteryx maculata and C. aequabilis (Odonata: Calopterygidae). Syst. Zool. 24:24-36.-Potential reproductive isolating mechanisms between sympatric Calopteryx maculata (Beauvois) and C. aequabilis Say are examined and evidence is presented for the reinforcement of species discrimination during pair formation and for a selective basis for changes in female wing pigmentation in the context of reproductive isolation between these species. The species are sympatric across the northeastern United States and southeastern Canada. In the area of sympatry C. aequabilis shows nearly complete ecological and temporal overlap with C. maculata. Mechanical barriers to interspecific copulations are apparently absent and no hybrids are known despite several observed interspecific copulations. Visual discrimination based on female wing pigmentation is identified as a major component in the reproductive isolation between C. maculata and C. aequabilis. C. maculata males discriminate between their females and C. aequabilis females by responding to the females with the darker of two alternative wing colors. Differences in wing pigmentation between sexes and among Calopteryx species are accentuated and displayed during pair formation and courtship. There is experimental evidence for the reinforcement of C. maculata male species discrimination ability in the area of sympatry with C. aequabilis. Experiments also indicate a selective disadvantage against dark winged sympatric C. aequabilis females. Coupled with the fact that sympatric C. aequabilis females have more lightly pigmented wings than allopatric ones, this finding provides a potential example of character displacement in the context of reproductive isolation between these species. [Reproductive isolation; damselflies; character displacement.] Of particular interest in the study of speciation is the origin of effective barriers to gene exchange between sympatric or parapatric populations of closely related (cognate) species. Whether these barriers result from genetic divergence during the allopatric stage of speciation, or from selection in the context of reproductive isolation during sympatry has been the subject of debate among systematists since the exchanges of Darwin and Wallace on the evolution of sterility (Dobzhansky, 1937, 1958; Fisher, 1930; Mayr, 1959). The opposing views are partially resolved (Littlejohn, 1969; Mecham, 1961) by noting that postmating incompatibility must normally arise as an incidental byproduct of divergence, either during the allopatric stage of speciation, or, less likely, during sympatry when there are sufficient restrictions to gene flow. The question then becomes: do premating isolating mechanisms also originate as incidental byproducts of divergence, or are they selected for in the context of reproductive isolation? Since these are not mutually exclusive alternatives, the question is more one of degree rather than kind. For example, a premating isolating mechanism might evolve between two species A and B in sympatry and incidentally function between one of the species and a third species C at a later time or in another part of its range (Littlejohn, 1969). The isolating mechanism would then be of incidental origin in the context of A and C or B and C, even though its origin was a consequence of selection in the context of isolation between A and B. A striking feature of the five North American Calopteryx species is the difference in wing pigmentation between sexes and among species (Howe, 1917; Walker, 1953).

Character Displacement and Acoustic Insects

Crickets, katydids, grasshoppers, and cicadas should be ideal for illustrating character displacement. Their species-specific calling songs are of direct importance to reproductive success, and species with similar songs and overlapping ranges should develop greater differences in their songs in areas of sympatry. Although many pairs of species have suitably similar songs and overlapping ranges and the data concerning songs are extensive for crickets and katydids, no convincing example of character displacement has been found in acoustic insects. Lack of such examples can be attributed to false premises, to divergence completed in allopatry. to elimination of geographical variation in song by gene flow between the zone of sympatry and the zones of allopatry, and to the scanty sample that has been intensively studied.

On the Ecological Significance of Bergmann's Rule

A positive correlation of weight with latitude in homoiotherms (Bergmann's rule) cannot normally depend upon the physics of heat exchange. Most latitudinally widespread mammals in North America do not follow this rule. Those that do are usually carnivores or granivores; a change in their body size reflects a change in the size of their prey. A latitudinal change in the size of available prey is due either to the distribution of the prey species or to the distribution of other predators utilizing the same prey species. Only the smallest species of a set of similar predators normally will conform to Bergmanns rule, and then only beyond the limits of distribution of the largest species. These changes in size seem to be another example of character displacement.