Character displacement in Hydrobia

Summary Fenchel's study of size variation in Hydrobiid snails in the Limfjord, Denmark, provides one of the most convincing cases of ecological character displacement available. In order to assess the generality of the phenomena within the Hydrobia genus, allopatric and sympatric Hydrobia ventrosa, H. neglecta and H. ulvae were collected from 24 coastal sites around Eastern England in July and October, 1982. Shell heights of 5,850 snails from 55 samples were recorded. These data were analysed for intraspecific allopatric-sympatric, and interspecific height differences. Such differences were not significant for H. ventrosa/H. neglecta, but the species exhibited parallel variation at sympatric sites. H. ventrosa/H. ulvae showed only significant interspecific differences, but in both allopatry and sympatry. The pattern of mean size variation for these species resembles that in the Limfjord. However the statistical analyses fail to provide support for character displacement. Differences in character states attributed to the process of character displacement may result from a number of other causes. Environmental conditions at sympatric and allopatric sites may act differentially on the heights of H. ulvae and H. ventrosa. The conditions that prevail at sympatric H. ulvae sites appear to lead to increased size in this species irrespective of the presence of H. ventrosa.     

Character displacement and replicate adaptive radiation

Ecological character displacement is implicit in current views of adaptive radiation, but examples are few. The paucity of cases may be partly a problem of detection: statistical tests require that a consistent pattern of displacement be observed over independent lineages, yet opportunities for such replication may be few. Recent studies of fish in postglacial lakes and lizards on Caribbean islands reveal multiple speciation events accompanied by equivalent patterns of ecological and morphological divergence. These studies provide a wealth of new examples of apparent character displacement, and show that adaptive radiations may unfold in a predictable sequence.     

On the origins of triterpenoid skeletal diversity

The triterpenoids are a large group of natural products derived from C(30) precursors. Nearly 200 different triterpene skeletons are known from natural sources or enzymatic reactions that are structurally consistent with being cyclization products of squalene, oxidosqualene, or bis-oxidosqualene. This review categorizes each of these structures and provides mechanisms for their formation.     

Character displacement in flowering phenologies

Summary As an answer to a comment by Slansky (Oecologia (Berl) 46 (1980) 133–134) on a recent statement by the present authors (Oecologia (Berl) 43 (1979) 157–172), it is maintained that an evaluation of the relative consumption rate (RCR) of N. sertifer must be made in relation to other tree-feeding herbivores, not to herb-feeders. The average value of available RCR estimates for tree-feeders is 1.3 mg/mg/day compared to 1.9 mg/mg/day for N. sertifer. Thus, we still maintain that N. sertifer has a high consumption rate.Authors give alphabetically     

Character displacement in polyphenic tadpoles

Biologists have long known that closely related species are often phenotypically different where they occur together, but are indistinguishable where they occur alone. The causes of such character displacement are controversial, however. We used polyphenic spadefoot toad tadpoles (Spea bombifrons and S. multiplicata) to test the hypothesis that character displacement evolves to minimize competition for food. We also sought to evaluate the role of phenotypic plasticity in the mediation of competitive interactions between these species. Depending on their diet, individuals of both species develop into either a small-headed omnivore morph, which feeds mostly on detritus, or a large-headed carnivore morph, which specializes on shrimp. Laboratory experiments and surveys of natural ponds revealed that the two species were more dissimilar in their tendency to produce carnivores when they occurred together than when they occurred alone. This divergence in carnivore production was expressed as both character displacement (where S. multiplicata's propensity to produce carnivores was lower in sympatry than in allopatry) and as phenotypic plasticity (where S. multiplicata facultatively enhanced carnivore production in S. bombifrons, and S. bombifrons facultatively suppressed carnivore production in S. multiplicata). In separate experiments, we established that S. bombifrons (the species for which carnivore production was enhanced) was the superior competitor for shrimp. Conversely, S. multiplicata (the species for which carnivore production was suppressed and omnivore production enhanced) was the superior competitor for detritus. These results therefore demonstrate that selection to minimize competition for food can cause character displacement. They also suggest that both character displacement and phenotypic plasticity may mediate competitive interactions between species.     

African human diversity, origins and migrations

The continent of Africa is thought to be the site of origin of all modern humans and is the more recent origin of millions of African Americans. Although Africa has the highest levels of human genetic diversity both within and between populations, it is under-represented in studies of human genetics. Recent advances have been made in understanding the origins of modern humans within Africa, the rate of adaptations due to positive selection, the routes taken in the first migrations of modern humans out of Africa, and the degree of admixture with archaic populations. Africa is also in dire need of effective medical interventions, and studies of genetic variation in Africans will shed light on the genetic basis of diseases and resistance to infectious diseases. Thus, we have tremendous potential to learn about human variation and evolutionary history and to positively impact human health care from studies of genetic diversity in Africa.     

Character displacement in giant rhinoceros beetles

I quantitatively analyzed male morphology of two closely related rhinoceros beetles species (Chalcosoma caucasus F. and Chalcosoma atlas L.) in 12 allopatric and seven sympatric locations throughout Southeast Asia. The qualitative features and the magnitude of intraspecific variation of each species were unaltered between allopatric and sympatric locations. Across allopatric locations, body size, horn size, dimorphic dimension, and genitalia size nearly completely overlapped between C. caucasus and C. atlas. Yet, in all sympatric locations, the differences between the two species in these characters were highly significant. While the enlarged difference between the two species in body size in sympatry could be attributed to habitat differentiation, that in genitalia size far exceeded what was expected from the general body-size displacement. These results indicate that morphological character displacement in sympatry was most complete in sexual organs. This may account for the process of existing species conserving themselves as integrated units by avoiding interspecific competition and enhancing reproductive isolation.     

Historical origins and genetic diversity of wine grapes

The genomic resources that are available to the grapevine research community have increased enormously during the past five years, in parallel with a renewed interest in grapevine (Vitis vinifera L.) germplasm resources and analysis of genetic diversity in grapes. Genetic variation, either natural or induced, is invaluable for crop improvement and understanding gene function, and the same is true for the grapevine. The history and vineyard cultural practices have largely determined the genetic diversity that exists today in grapevines. In this article, we provide a synopsis of what is known about the origin and genetics of grapes and how molecular genetics is helping us understand more about this plant: its evolution, historical development, genetic diversity and potential for genetic improvement.     

Character displacement in the fighting colours of Hetaerina damselflies

Aggression between species is a seldom-considered but potentially widespread mechanism of character displacement in secondary sexual characters. Based on previous research showing that similarity in wing coloration directly influences interspecific territorial aggression in Hetaerina damselflies, we predicted that wing coloration would show a pattern of character displacement (divergence in sympatry). A geographical survey of four Hetaerina damselfly species in Mexico and Texas showed evidence for character displacement in both species pairs that regularly occurs sympatrically. Hetaerina titia, a species that typically has large black wing spots and small red wing spots, shifted to having even larger black spots and smaller red wing spots at sites where a congener with large red wing spots is numerically dominant (Hetaerina americana or Hetaerina occisa). Hetaerina americana showed the reverse pattern, shifting towards larger red wing spots where H. titia is numerically dominant. This pattern is consistent with the process of agonistic character displacement, but the ontogenetic basis of the shift remains to be demonstrated.     

Character displacement promotes cooperation in bacterial biofilms

Resource competition within a group of cooperators is expected to decrease selection for cooperative behavior but can also result in diversifying selection for the use of different resources, which in turn could retard the breakdown of cooperation. Diverse groups are likely to be less susceptible to invasion by noncooperating social cheats: First, competition repression resulting from character displacement may provide less of a selective advantage to cheating; second, cheats may trade off the ability to exploit cooperators that specialize in one type of resource against cooperators that specialize in another ; third, diverse communities of any kind may have higher invasion resistance because there are fewer resources available for an invader to use . Furthermore, diverse groups are likely to be more productive than clonal groups if a wider range of total resources are being used . We addressed these issues by using the cooperative trait of biofilm formation in Pseudomonas fluorescens. Character displacement through resource competition evolved within biofilms; productivity increased with increasing character displacement, and diverse biofilms were less susceptible to invasion by cheats. These results demonstrate that diversification into different ecological niches can minimize selection against cooperation in the face of local resource competition.     

Phylogenetic origins of local-scale diversity patterns and the causes of Amazonian megadiversity

What explains the striking variation in local species richness across the globe and the remarkable diversity of rainforest sites in Amazonia? Here, we apply a novel phylogenetic approach to these questions, using treefrogs (Hylidae) as a model system. Hylids show dramatic variation in local richness globally and incredible local diversity in Amazonia. We find that variation in local richness is not explained primarily by climatic factors, rates of diversification (speciation and extinction) nor morphological variation. Instead, local richness patterns are explained predominantly by the timing of colonization of each region, and Amazonian megadiversity is linked to the long-term sympatry of multiple clades in that region. Our results also suggest intriguing interactions between clade diversification, trait evolution and the accumulation of local richness. Specifically, sympatry between clades seems to slow diversification and trait evolution, but prevents neither the accumulation of local richness over time nor the co-occurrence of similar species.     

Extinction, ecological opportunity, and the origins of global snake diversity

Snake diversity varies by at least two orders of magnitude among extant lineages, with numerous groups containing only one or two species, and several young clades exhibiting exceptional richness (>700 taxa). With a phylogeny containing all known families and subfamilies, we find that these patterns cannot be explained by background rates of speciation and extinction. The majority of diversity appears to derive from a radiation within the superfamily Colubroidea, potentially stemming from the colonization of new areas and the evolution of advanced venom-delivery systems. In contrast, negative relationships between clade age, clade size, and diversification rate suggest the potential for possible bias in estimated diversification rates, interpreted by some recent authors as support for ecologically mediated limits on diversity. However, evidence from the fossil record indicates that numerous lineages were far more diverse in the past, and that extinction has had an important impact on extant diversity patterns. Thus, failure to adequately account for extinction appears to prevent both rate- and diversity-limited models from fully characterizing richness dynamics in snakes. We suggest that clade-level extinction may provide a key mechanism for explaining negative or hump-shaped relationships between clade age and diversity, and the prevalence of ancient, species-poor lineages in numerous groups.     

Genetic diversity and the origins of parthenogenesis in the teiid lizard Aspidoscelis laredoensis

Unisexual vertebrates typically form through hybridization events between sexual species in which reproductive mode transitions occur in the hybrid offspring. This evolutionary history is thought to have important consequences for the ecology of unisexual lineages and their interactions with congeners in natural communities. However, these consequences have proven challenging to study owing to uncertainty about patterns of population genetic diversity in unisexual lineages. Of particular interest is resolving the contribution of historical hybridization events versus post formational mutation to patterns of genetic diversity in nature. Here we use restriction site associated DNA genotyping to evaluate genetic diversity and demographic history in Aspidoscelis laredoensis, a diploid unisexual lizard species from the vicinity of the Rio Grande River in southern Texas and northern Mexico. The sexual progenitor species from which one or more lineages are derived also occur in the Rio Grande Valley region, although patterns of distribution across individual sites are quite variable. Results from population genetic and phylogenetic analyses resolved the major axes of genetic variation in this species and highlight how these match predictions based on historical patterns of hybridization. We also found discordance between results of demographic modelling using different statistical approaches with the genomic data. We discuss these insights within the context of the ecological and evolutionary mechanisms that generate and maintain lineage diversity in unisexual species. As one of the most dynamic, intriguing, and geographically well investigated groups of whiptail lizards, these species hold substantial promise for future studies on the constraints of diversification in unisexual vertebrates.     

Rhamnolipids: diversity of structures, microbial origins and roles

Rhamnolipids are glycolipidic biosurfactants produced by various bacterial species. They were initially found as exoproducts of the opportunistic pathogen Pseudomonas aeruginosa and described as a mixture of four congeners: α-L-rhamnopyranosyl-α-L-rhamnopyranosyl-β-hydroxydecanoyl-β-hydroxydecanoate (Rha-Rha-C₁₀-C₁₀), α-L-rhamnopyranosyl-α-L-rhamnopyranosyl-β-hydroxydecanoate (Rha-Rha-C₁₀), as well as their mono-rhamnolipid congeners Rha-C₁₀-C₁₀ and Rha-C₁₀. The development of more sensitive analytical techniques has lead to the further discovery of a wide diversity of rhamnolipid congeners and homologues (about 60) that are produced at different concentrations by various Pseudomonas species and by bacteria belonging to other families, classes, or even phyla. For example, various Burkholderia species have been shown to produce rhamnolipids that have longer alkyl chains than those produced by P. aeruginosa. In P. aeruginosa, three genes, carried on two distinct operons, code for the enzymes responsible for the final steps of rhamnolipid synthesis: one operon carries the rhlAB genes and the other rhlC. Genes highly similar to rhlA, rhlB, and rhlC have also been found in various Burkholderia species but grouped within one putative operon, and they have been shown to be required for rhamnolipid production as well. The exact physiological function of these secondary metabolites is still unclear. Most identified activities are derived from the surface activity, wetting ability, detergency, and other amphipathic-related properties of these molecules. Indeed, rhamnolipids promote the uptake and biodegradation of poorly soluble substrates, act as immune modulators and virulence factors, have antimicrobial activities, and are involved in surface motility and in bacterial biofilm development.     

Character displacement and the evolution of niche complementarity in a model biofilm community

Colonization of vacant environments may catalyze adaptive diversification and be followed by competition within the nascent community. How these interactions ultimately stabilize and affect productivity are central problems in evolutionary ecology. Diversity can emerge by character displacement, in which selection favors phenotypes that exploit an alternative resource and reduce competition, or by facilitation, in which organisms change the environment and enable different genotypes or species to become established. We previously developed a model of long‐term experimental evolution in which bacteria attach to a plastic bead, form a biofilm, and disperse to a new bead. Here, we focus on the evolution of coexisting mutants within a population of Burkholderia cenocepacia and how their interactions affected productivity. Adaptive mutants initially competed for space, but later competition declined, consistent with character displacement and the predicted effects of the evolved mutations. The community reached a stable equilibrium as each ecotype evolved to inhabit distinct, complementary regions of the biofilm. Interactions among ecotypes ultimately became facilitative and enhanced mixed productivity. Observing the succession of genotypes within niches illuminated changing selective forces within the community, including a fundamental role for genotypes producing small colony variants that underpin chronic infections caused by B. cenocepacia.     

Mitochondrial DNA diversity and origins of South and Central American goats

We have analysed the genetic diversity of South and Central American (SCA) goats by partially sequencing the mitochondrial control region of 93 individuals with a wide geographical distribution. Nucleotide and haplotype diversities reached values of 0.020 ± 0.00081 and 0.963 ± 0.0012 respectively. We have also observed a rather weak phylogeographic structure, with almost 69% of genetic variation included in the within-breed variance component. The topology of a median-joining network analysis including 286 European, Iberian, Atlantic and SCA mitochondrial sequences was very complex, with most of the haplotypes forming part of independent small clusters. SCA sequences showed a scattered distribution throughout the network, and clustering with Spanish and Portuguese sequences occurred only occasionally, not allowing the distinguishing of a clear Iberian signature. Conversely, we found a prominent cluster including Canarian, Chilean, Argentinian and Bolivian mitochondrial haplotypes. This result was independently confirmed by constructing a Bayesian phylogenetic tree (posterior probability of 0.97). Sharing of mitochondrial haplotypes by SCA and Canarian goats suggests that goat populations from the Atlantic archipelagos, where Spanish and Portuguese ships en route to the New World used to stow food and supplies, participated in the foundation of SCA caprine breeds.     

Character displacement and coexistence in mud snails (Hydrobiidae)

Summary Populations of coexisting and of allopatrically occurring species of hydrobiid snails (Hydrobia ulvae, H. neglecta, H. ventrosa and Potamopyrgus jenkinsi) have been studied in 90 localities within three different areas. When H. ventrosa coexists with H. ulvae they show character displacement, i.e., the average body size of the former is smaller and that of the latter is larger. When these species live alone they are of approximately the same size. It is shown here that the size ratio between the coexisting species usually found (1.3–1.5) allows stable coexistence based on food praticles size selectivity alone. Variation in the degree of character displacement from locality to locality is explained by different degrees of genetical isolation of the populations. Coexisting H. ventrosa and H. ulvae have shorter, more well-defined periods of reproduction than they do when they occur alone. H. neglecta is larger than coexisting H. ventrosa and smaller than coexisting H. ulvae. Due to the patchy distribution and the fluctuating populations of this species, and due to the fact that pure H. neglecta populations are rare, data on this species are difficult to interpret. Potamopyrgus jenkinsi shows a different food particle size selection than the Hydrobia spp. of identical sizes. Competitive interactions between P. jenkinsi and coexisting Hydrobia spp. are therefore probably weak. In accordance with this, P. jenkinsi does not show character displacement when coexisting with, e.g., H. ventrosa.The fact that the major study area, the Limfjord, is only 150 years old as a marine habitat as well as various more recent man-made changes of the coast line allow estimates of the time scale of the microevolutionary changes which lead to character displacement.