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.     

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.     

Mixed responses to resource densities and their implications for character displacement

Summary 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.     

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.     

Ecological and morphological diversification within single species and character displacement in Mandarina,endemic land snails of the Bonin Islands

The endemic land snail species Mandarina hahajimana has undergone extensive habitat and morphological diversification within the Hahajima islands in the Bonin archipelago. This species has diversified into populations with ground, arboreal and semi-arboreal life histories. In addition, arboreal populations and semi-arboreal populations show diversification in preferences of species and positions of the tree on which they are found. Shell morphologies of M. hahajimana exhibit remarkable geographical variation, and they have a clear relationship with their life histories. The morphological variation of M. hahajimana results from adaptation to different lifestyles. The habitats of these populations influence the relationships with other species of Mandarina coexisting with M. hahajimana. This suggests that the morphological and ecological divergence within M. hahajimana has been induced by competitive interaction with other species of Mandarina. Character displacement may have played an important role in promoting adaptive radiation of Mandarina in the Bonin Islands.     

Genetics of insular populations of mammals, with particular reference to differentiation and founder effects in British small mammals

Island populations are of interest for their differentiation as well as their species diversity; some of the earliest biological interest in islands was concerned with the number of 'endemics' thereon. There is dispute about the long-term evolutionary importance of island forms, but they are rich sources of data for studying the under-exploited interface of genetics, ecology and physiology. Differentiation of island populations may arise from genetic change after isolation, or from the chance collection of alleles carried by the colonizing group itself. The general reduction of genetic variance in island populations compared to continental forms of the same species suggests that founder events have played a major role in the formation of most island forms. However, there is ample evidence of adaptation in island populations despite this lower variation; this is relevant when using island biology as a base for the deriving of rules for genetic conservation.     

Extracellular release of photosynthetic products by freshwater phytoplankton populations, with special reference to the algal species involved

SUMMARY. Over three successive years, depth profiles of C-fixation and excretion, chlorophyll- a concentrations, phytoplankton species composition and bacterial numbers were determined in Lake Vechten, a slightly eutrophic lake in The Netherlands. Special attention was given to the method used to measure extracellular release.
Excretion of dissolved organic ¹⁴C depended largely upon the photo-synthetic activity of the phytoplankton, ranging from 0–2.5 mg m^-1 h^-1, representing a percentage extracellular release (PER) of 0–25%.
During a period in August, however, a subsurface chlorophyll- a maximum at 5–7 m depth coincided with high excretion rates of up to 10 mg Cm^-3 h^-1 (PER = 55%). Phytoplankton analysis revealed a stratification in numbers of Mallomonas caudata with a maximum at 5–7 m depth.
The results suggest that in these water layers bacterial populations grew at the expense of the dissolved organic carbon compounds excreted by Mallomonas caudata. This means that extracellular release can temporarily function as an important nutrient source for the heterotrophie community in addition to the more or less constant dissolved organic carbon pool.     

Character displacement among bat-pollinated flowers of the genus Burmeistera: analysis of mechanism, process and pattern

Coexisting plants that share pollinators can compete through interspecific pollen transfer. A long-standing idea holds that divergence in floral morphology may reduce this competition by placing pollen on different regions of the pollinator's bodies. However, surprisingly little empirical support for this idea exists. Burmeistera is a diverse neotropical genus that exhibits wide interspecific variation in the degree to which the reproductive parts are exserted outside the corolla. Coexisting Burmeistera share bats as their primary pollinators, and the degree of exsertion determines the site of pollen deposition on the bats' heads. Here we study the mechanism, process and pattern of floral character displacement for assemblages of coexisting Burmeistera. Flight cage experiments with bats and pairs of Burmeistera species demonstrate that the greater the divergence in exsertion length, the less pollen transferred interspecifically. Null model analyses of exsertion lengths for 19 species of Burmeistera across 18 sites (each containing two to four species) demonstrate that observed assemblage structure is significantly overdispersed relative to what would be expected by chance. Local evolution, rather than ecological sorting, appears to be the primary process driving this pattern of overdispersion because local adaptation of the nine widespread species accounts for a large portion of the observed pattern. Taken together, results of this study provide strong support for the idea that competition through interspecific pollen transfer can drive character displacement in plants.     

Diagnostic and prognostic criteria for heat stroke with special reference to plasma enzyme and isoenzyme release patterns.

1. The aim of this study was to investigate the potential value of isoenzyme release patterns as parameters of hyperthermic injury in improving the diagnostic protocol for heat stroke. 2. Rats were exposed to combinations of exercise and hyperthermic stress. Following exposure, respective release patterns of isoenzymes were determined. 3. Isoenzyme release patterns following hyperthermic injury portray both the type and extent of tissue damage more clearly than do plasma total enzyme release patterns. 4. The sensitivity of isoenzymes as a diagnostic criterion is extended to include both a temporal component (i.e. an accurate diagnosis will be possible at an earlier stage subsequent to hyperthermic exposure) and at the same time to be indicative of the exposure.     

Divergent echolocation call frequencies in insular rhinolophids (Chiroptera): a case of character displacement?

Aims Because rhinolophids have been hypothesized to use echolocation call frequency to recognize conspecifics, sympatric species calling at similar frequencies should be subject to acoustic character displacement, i.e. a drift in frequency values to minimize the risk of misidentification of conspecifics. However, it has been proposed that insufficient geographical separation between populations in sympatry and allopatry may counter the establishment of frequency differences by character displacement. Here we tested the hypothesis that insular populations should exhibit acoustic divergence, and this should be revealed by comparing call frequencies with those observed in mainland, allopatric populations of conspecifics. We also tested whether the evolutionary pressure towards acoustic divergence should be especially strong at sites where rhinolophid species emitting similar call frequencies roost together in order to minimize interspecific frequency overlap. Location Sardinia and southern Italy (Campania, Lazio, Abruzzo). Methods Time‐expanded echolocation calls and body size were recorded from Sardinian populations of Rhinolophus mehelyi Matschie, Rhinolophus hipposideros (Bechstein) and Rhinolophus euryale Blasius. Both call frequencies and forearm length of insular R. hipposideros and R. euryale were compared with those of populations from mainland areas of Italy where R. mehelyi is absent, to explore the hypothesis that the presence of the latter species (which calls at frequency values intermediate between the other two) may determine acoustic divergence in the other species. For Sardinian R. mehelyi and R. euryale, we also carried out intraspecific comparisons of call frequencies between bats from monospecific colonies and those from mixed colonies. Results As hypothesized, Sardinian R. hipposideros and R. euryale called at frequencies higher and lower, respectively, than in the peninsula. In this way, overlap with R. mehelyi is avoided. Body size showed no difference between insular and peninsular populations, i.e. frequency differences are not a by‐product of difference in body size determined by insularity. Frequency values in Sardinian R. euryale from monospecific colonies did not differ from those of bats roosting together with R. mehelyi. However, R. mehelyi showed frequency values significantly higher when associated with R. euryale, possibly to minimize the risk of species misrecognition. Main conclusions At least under geographical isolation, character displacement may be a causal mechanism for shifts in call frequency of sympatric rhinolophids. Species recognition and facilitation of intraspecific communication (with possible implications for mate recognition) constitute the best candidate factors for the phenomenon we observed.     

Comparative observations on corneas,with special reference to Bowman's layer and Descemet's membrane in mammals and amphibians

Corneas of tadpole, mouse, rat, guinea pig, rabbit, cat, cattle, and human were examined by TEM and SEM in a comparative study. The differences between species were noted mainly by using TEM. Bowman's layer showed a tendency to be well developed in higher mammals. Tadpoles lack a Bowman's layer, lower mammals have a thin Bowman's layer, and higher mammals have a thick Bowman's layer. The boundary between the substantia propria and Descemet's membrane was distinct in higher mammals. On the other hand, there are no differences in thickness of the collagen fibrils that constitute Bowman's layer and those of the substantia propria. NaOH digestion was utilized for SEM preparation. SEM imaging revealed a textured appearance of the epithelial side of Bowman's layer. In Descemet's membrane, fibrous long spacing (FLS) fiber-like structures, which are arranged in parallel to the endothelium, were observed by both TEM and SEM. To our knowledge, this is the first report of SEM observations of FLS fiber-like structures on the endothelial surface of Descemet's membrane. SEM at a plane normal to the plane of the cornea showed that Descemet's membrane has a piled laminar structure. Descemet's membrane is closely associated with the collagen layer of the substantia propria. Collagen fibrils invading from the substantia propria into Descemet's membrane were observed with both TEM and SEM.     

Macroevolution in progress: competition between semislugs and slugs resulting in ecological displacement and ecological release

The vitrinids, which represent a probably competitively inferior transitional stage between shelled snails and slugs, and their sister group, the limacoid slugs, are used to investigate a macroevolutionary effect of a microevolutionary process, competition between individuals resulting in ecological displacement of one clade by another, based on patterns in the present-day world. The activity period and the altitudinal distribution of the vitrinids has shifted - to the cold season or to higher altitudes, respectively - in those regions where their range overlaps with that of the limacoid slugs. The diversity of the vitrinids is lower in regions with limacoid slugs. These patterns can be ascribed to the ecological displacement or the exclusion of the vitrinids from some habitats by limacoid slugs. Two vitrinid clades which independently colonized regions without limacoid slugs show patterns attributable to ecological release. The ecological displacement or exclusion of the vitrinids by the slugs is probably due to competition for shelter.     

Evidence for large-scale effects of competition: niche displacement in Canada lynx and bobcat

Determining the patterns, causes and consequences of character displacement is central to our understanding of competition in ecological communities. However, the majority of competition research has occurred over small spatial extents or focused on fine-scale differences in morphology or behaviour. The effects of competition on broad-scale distribution and niche characteristics of species remain poorly understood but critically important. Using range-wide species distribution models, we evaluated whether Canada lynx (Lynx canadensis) or bobcat (Lynx rufus) were displaced in regions of sympatry. Consistent with our prediction, we found that lynx niches were less similar to those of bobcat in areas of sympatry versus allopatry, with a stronger reliance on snow cover driving lynx niche divergence in the sympatric zone. By contrast, bobcat increased niche breadth in zones of sympatry, and bobcat niches were equally similar to those of lynx in zones of sympatry and allopatry. These findings suggest that competitively disadvantaged species avoid competition at large scales by restricting their niche to highly suitable conditions, while superior competitors expand the diversity of environments used. Our results indicate that competition can manifest within climatic niche space across species’ ranges, highlighting the importance of biotic interactions occurring at large spatial scales on niche dynamics.     

Palynological characters of Glycyrrhiza,Glycyrrhizopsis,and Meristotropis(Leguminosae),with special reference to their taxonomic significance

Li X-Y.1992.Studies on germplasm of Glycyrrhiza by using different taxonomic methods.Advances in Plant Taxonomy in Northwest China 1:7-24.

Li X-Y.1993.A study of the system and new taxa of genus Glycyrrhiza L.Bulletin of Botanical Research 13(1):14-43.

Turrill WB.1937.Glycyrrhizopsis syriaca Turrill.Bulletin of Miscellaneous Information 2:79.

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Molecular phylogeny, diversification and character evolution in Lupinus (Fabaceae) with special attention to Mediterranean and African lupines

Phylogenetic relationships within the complex genus Lupinus are estimated from internal transcribed spacer (ITS) sequences of the nuclear ribosomal DNA repeat. The molecular data supports Lupinus as a distinct monophyletic group within the tribe Genisteae. Different geographical lineages are revealed within Lupinus, which are each restricted to either the Old or the New World. In the New World, the ITS data support an eastern-western geographic disjunction of the lupines and the recognition of some well-supported clades. In the Old World, almost all the previously recognized taxa are taxonomically well differentiated. The homogeneous African rough-seeded lupines, Scabrispermae, are strongly supported as a monophyletic group, which is distinct from the diverse and heterogeneous circum-Mediterranean smooth-seeded ones. The latter appear to have evolved as two lineages, in which are revealed some intersectional relationships. Also ITS data allow the assessment of the phylogenetic position of the newly discovered species, L. anatolicus (in the Old World) and L. jaimehintoniana (the Mexican tree lupin). The ITS phylogeny suggests a rapid initial radiation of the lupines subsequent to their divergence from a common ancestor. Moreover, the results indicate that the annual and perennial habits have evolved many times in Lupinus and suggest a role for generation time in affecting the evolutionary history of lupines. Data on adaptive processes and character evolution are re-examined and discussed in the light of the ITS phylogeny.These studies were supported by the research unit UMR-CNRS 6553 Ecobio – University of Rennes (France), and by the NSERC of Canada (grant to R.J. Bayer, at the University of Alberta). They are greatfully acknowledged. All the persons that have contributed in different ways to this work on Lupinus, summarized in this presentation, are greatly thanked. Particularly, we would like to mention Malika Aïnouche, Roland Greinwald, André Huon, W.K. Swiecicki, Billie L. Turner and Ludger Witte for their contributions.