Evaluating the targets of selection during character displacement

Genetic Variants Underlying Plasticity in Natural Populations of Spadefoot Toads: Environmental Assessment versus Phenotypic Response

Many organisms facultatively produce different phenotypes depending on their environment, yet relatively little is known about the genetic bases of such plasticity in natural populations. In this study, we describe the genetic variation underlying an extreme form of plasticity--resource polyphenism--in Mexican spadefoot toad tadpoles, Spea multiplicata. Depending on their environment, these tadpoles develop into one of two drastically different forms: a carnivore morph or an omnivore morph. We collected both morphs from two ponds that differed in which morph had an adaptive advantage and performed genome-wide association studies of phenotype (carnivore vs. omnivore) and adaptive plasticity (adaptive vs. maladaptive environmental assessment). We identified four quantitative trait loci associated with phenotype and nine with adaptive plasticity, two of which exhibited signatures of minor allele dominance and two of which (one phenotype locus and one adaptive plasticity locus) did not occur as minor allele homozygotes. Investigations into the genetics of plastic traits in natural populations promise to provide novel insights into how such complex, adaptive traits arise and evolve.

Below-ground competition favors character convergence but not character displacement in root traits

Character displacement can play a major role in species ecology and evolution; however, research testing whether character displacement can influence the evolution of root traits in plant systems remains scarce in the literature. Here we investigated the potential that character displacement may influence the evolution of root traits using two closely related morning glory species, Ipomoea purpurea and Ipomoea hederacea. We performed a field experiment where we grew the common morning glory, I. purpurea, in the presence and absence of competition from I. hederacea and examined the potential that the process of character displacement could influence the evolution of root traits. We found maternal line variation in root phenotypes and evidence that below-ground competition acts as an agent of selection on these traits. Our test of character displacement, however, showed evidence of character convergence on our measure of root architecture rather than displacement. These results suggest that plants may be constrained by their local environments to express a phenotype that enhances fitness. Therefore, the conditions of the competitive environment experienced by a plant may influence the potential for character convergence or displacement to influence the evolution of root traits.

Phylogeographical analysis of character displacement in feeding phenotypes of snail-feeding Acoptolabrus ground beetles

Ecological character displacement predicts that interspecific resource competition results in greater trait divergence between species in sympatry than in allopatry. However, other processes, such as ecological sorting, result in the same pattern of trait variation. In this study, we characterize character displacement in eight species of snail-feeding Acoptolabrus ground beetles in the Far East. Acoptolabrus exhibit divergent feeding phenotypes, including species with a slender forebody that is able to intrude into large shells and species with stout heads and mandibles for crushing small shells. The pattern of character displacement in feeding phenotypes was confirmed by multivariate analysis of body dimensions. Molecular phylogenetic analysis, divergence time estimation and biogeographical analysis revealed that sympatry and phenotypic divergence occurred repeatedly during the Pleistocene and almost simultaneously within each geographical area. Comparative analysis revealed that the evolution of feeding phenotypes best fitted a selective model with three adaptive optima, corresponding to the three cases of sympatry with a congener. Repeated coincidences of sympatry and adaptive differentiation in feeding phenotypes suggested causal relationships, although the precise order of events was difficult to discriminate. This study provides insight into the spatiotemporal dynamics of interspecific interactions and adaptive phenotypic diversification.

Identification of candidate loci for adaptive phenotypic plasticity in natural populations of spadefoot toads

Phenotypic plasticity allows organisms to alter their phenotype in direct response to changes in the environment. Despite growing recognition of plasticity's role in ecology and evolution, few studies have probed plasticity's molecular bases-especially using natural populations. We investigated the genetic basis of phenotypic plasticity in natural populations of spadefoot toads (Spea multiplicata). Spea tadpoles normally develop into an "omnivore" morph that is favored in long-lasting, low-density ponds. However, if tadpoles consume freshwater shrimp or other tadpoles, they can alternatively develop (via plasticity) into a "carnivore" morph that is favored in ephemeral, high-density ponds. By combining natural variation in pond ecology and morph production with population genetic approaches, we identified candidate loci associated with each morph (carnivores vs. omnivores) and loci associated with adaptive phenotypic plasticity (adaptive vs. maladaptive morph choice). Our candidate morph loci mapped to two genes, whereas our candidate plasticity loci mapped to 14 genes. In both cases, the identified genes tended to have functions related to their putative role in spadefoot tadpole biology. Our results thereby form the basis for future studies into the molecular mechanisms that mediate plasticity in spadefoots. More generally, these results illustrate how diverse loci might mediate adaptive plasticity.

Patterns of morphological diversification in giant Berberomeloe blister beetles (Coleoptera: Meloidae) reveal an unexpected taxonomic diversity concordant with mtDNA phylogenetic structure

Delimiting species boundaries is a complex challenge usually hindered by overlooked morphological diversification or misinterpretation of geographically structured phenotypic variability. Independent molecular data are extremely useful to characterize and understand such morphological diversity. Morphological and molecular variability of the non-phoretic and apterous, widely distributed, giant blister beetles of the genus Berberomeloe, were investigated within and between lineages across most of the distributional range of the genus. We used two mtDNA gene fragments to characterize genetic variability and to produce a time-calibrated phylogeny of the genus. Our results reveal several mitochondrial lineages, allopatrically, parapatrically and sympatrically distributed. Most clades are not distinguishable between each other based on morphometrics. However, no morphometric overlap is observed between two closely related clades, one of them occurring in sympatry with a distantly congeneric species (B. insignis), suggesting that sympatry could trigger morphological diversification. Although most species share a morphometric space, they can be morphologically identified by a combination of easily observed characteristic qualitative features. Based on the concordance between mtDNA clades and morphological units, we describe six new species of Berberomeloe (B. castuo sp. nov., B. comunero sp. nov., B. indalo sp. nov, B. yebli sp. nov., B. payoyo sp. nov. and B. tenebrosus sp. nov.), revalidate two taxa (B. maculifrons comb. nov. and B. laevigatus comb. nov.) and redefine B. majalis.

Ecological Character Displacement in Non-Congeneric Frogs

Character displacement is phenotypic divergence driven by competition (ecological character displacement) or reproductive interference (reproductive character displacement). Although previous studies have examined these phenomena separately, recent evidence suggests that reproductive interference can drive both reproductive and ecological character displacement, in that certain traits are related to both competition and reproduction. Thus, to evaluate the effect of competition, the effect of reproductive interference must be excluded. Here, we analysed ecological character displacement between non-congeneric frogs, which show little reproductive interference. Odorrana amamiensis inhabits the Amami and Tokunoshima Islands, Japan, whereas its non-congeneric competitor Babina subaspera inhabits the Amami Island. We tested three of the Schluter (2000) criteria for ecological character displacement: phenotypic changes in O. amamiensis between the two islands, phenotypic change related to prey preference, prey availability between the two islands. We demonstrated that the three criteria in Schluter (2000) were likely to be satisfied, indicating the occurrence of ecological character displacement in non-congeners without reproductive interference. Thus, we conclude that competition is potentially the main driver of this phenotypic divergence, and that non-congeners may be a suitable model for evaluating ecological character displacement in a variety of organisms, as the influence of reproductive interference can be excluded.

Tadpoles of three western African frog genera: Astylosternus Werner, 1898, Nyctibates Boulenger, 1904, and Scotobleps Boulenger, 1900 (Amphibia, Anura, Arthroleptidae)

Herein, we describe the tadpoles of six Astylosternus species, A.fallax, A.cf.fallax, A.laurenti, A.montanus, A.perreti, A.ranoides, and Scotoblepsgabonicus, and redescribe the tadpoles of A.batesi, A.diadematus, A.laticephalus, A.occidentalis, A.rheophilus, and Nyctibatescorrugatus. All Astylosternus tadpoles are adapted to torrent currents and share a long, oval body, slightly flattened in lateral view, with very long muscular tails with narrow fins. The jaws are massive, serrated, and often show a tooth-like medial projection (fang) in the upper jaw. Body proportions of Astylosternus tadpoles are extremely similar. The best characters to distinguish species might be life coloration and potentially the shape of labial papillae. The tadpole of Scotoblepsgabonicus is similar to Astylosternus and differs only slightly by a narrower body with a shorter and rounder head. The upper jaw of Scotobleps carries two or three lateral fangs instead of one medial one. The tadpole of Nyctibatescorrugatus is easily distinguishable from the other two genera on the basis of their very long, eel-shaped body and tail and the bluish-black color.

Male sexual signal predicts phenotypic plasticity in offspring: implications for the evolution of plasticity and local adaptation

In a rapidly changing world, understanding the processes that influence a population's ability to respond to natural selection is critical for identifying how to preserve biodiversity. Two such processes are phenotypic plasticity and sexual selection. Whereas plasticity can facilitate local adaptation, sexual selection potentially impedes local adaptation, especially in rapidly changing or variable environments. Here we hypothesize that, when females preferentially choose males that sire plastic offspring, sexual selection can actually facilitate local adaptation to variable or novel environments by promoting the evolution of adaptive plasticity. We tested this hypothesis by evaluating whether male sexual signals could indicate plasticity in their offspring and, concomitantly, their offspring's ability to produce locally adapted phenotypes. Using spadefoot toads ( Spea multiplicata) as our experimental system, we show that a male sexual signal predicts plasticity in his offspring's resource-use morphology. Specifically, faster-calling males (which are preferred by females) produce more plastic offspring; such plasticity, in turn, enables these males' offspring to respond adaptively to the spadefoots' highly variable environment. The association between a preferred male signal and adaptive plasticity in his offspring suggests that female mate choice can favour the evolution and maintenance of phenotypic plasticity and thereby foster adaptation to a variable environment. This article is part of the theme issue 'The role of plasticity in phenotypic adaptation to rapid environmental change'.

Plasticity-led evolution: evaluating the key prediction of frequency-dependent adaptation

Plasticity-led evolution occurs when a change in the environment triggers a change in phenotype via phenotypic plasticity, and this pre-existing plasticity is subsequently refined by selection into an adaptive phenotype. A critical, but largely untested prediction of plasticity-led evolution (and evolution by natural selection generally) is that the rate and magnitude of evolutionary change should be positively associated with a phenotype's frequency of expression in a population. Essentially, the more often a phenotype is expressed and exposed to selection, the greater its opportunity for adaptive refinement. We tested this prediction by competing against each other spadefoot toad tadpoles from different natural populations that vary in how frequently they express a novel, environmentally induced carnivore ecomorph. As expected, laboratory-reared tadpoles whose parents were derived from populations that express the carnivore ecomorph more frequently were superior competitors for the resource for which this ecomorph is specialized-fairy shrimp. These tadpoles were better at using this resource both because they were more efficient at capturing and consuming shrimp and because they produced more exaggerated carnivore traits. Moreover, they exhibited these more carnivore-like features even without experiencing the inducing cue, suggesting that this ecomorph has undergone an extreme form of plasticity-led evolution-genetic assimilation. Thus, our findings provide evidence that the frequency of trait expression drives the magnitude of adaptive refinement, thereby validating a key prediction of plasticity-led evolution specifically and adaptive evolution generally.

Phenotypic plasticity, canalization, and the origins of novelty: Evidence and mechanisms from amphibians

A growing number of biologists have begun asking whether environmentally induced phenotypic change--'phenotypic plasticity'--precedes and facilitates the origin and canalization of novel, complex phenotypes. However, such 'plasticity-first evolution' (PFE) remains controversial. Here, we summarize the PFE hypothesis and describe how it can be evaluated in natural systems. We then review the evidence for PFE from amphibians (a group in which phenotypic plasticity is especially widespread) and describe how phenotypic plasticity might have facilitated macroevolutionary change. Finally, we discuss what is known about the proximate mechanisms of PFE in amphibians. We close with suggestions for future research. As we describe, amphibians offer some of the best support for plasticity's role in the origin of evolutionary novelties.

Intraspecific adaptive radiation: Competition, ecological opportunity, and phenotypic diversification within species

Intraspecific variation in resource-use traits can have profound ecological and evolutionary implications. Among the most striking examples are resource polymorphisms, where alternative morphs that utilize different resources evolve within a population. An underappreciated aspect of their evolution is that the same conditions that favor resource polymorphism-competition and ecological opportunity-might foster additional rounds of diversification within already existing morphs. We examined these issues in spadefoot toad tadpoles that develop into either a generalist "omnivore" or a specialist "carnivore" morph. Specifically, we assessed the morphological diversity of tadpoles from natural ponds and experimentally induced carnivores reared on alternative diets. We also surveyed natural ponds to determine if the strength of intramorph competition and the diversity and abundance of dietary resources (measures of ecological opportunity) influenced the diversity of within-morph variation. We found that five omnivore and four carnivore types were present in natural ponds; alternative diets led to shape differences, some of which mirrored variation in the wild; and both competition and ecological opportunity were associated with enhanced morphological diversity in natural ponds. Such fine-scale intraspecific variation might represent an underappreciated form of biodiversity and might constitute a crucible of evolutionary innovation and diversification.

Choosy Cannibals Preferentially Consume Siblings with Relatively Low Fitness Prospects

When an individual can selfishly cannibalize a relative or altruistically set it free, the benefits of altruism will be positively associated with the relative's fitness prospects (the benefits it receives from altruism). We tested the prediction that altruism should be preferentially directed toward high-quality relatives using larvae of the New Mexican spadefoot toad (Spea multiplicata), a species in which tadpoles plastically express omnivore and carnivore ecomorphs. In a no-choice design, we presented carnivores with sibling or nonsibling omnivores varying in developmental stage, which is positively associated with survival in this toad's ephemeral larval environment. There was a significant interaction between relatedness and developmental stage on the probability of cannibalism: carnivores were overall more likely to cannibalize less developed omnivores, but this effect was exaggerated when the potential victim was a sibling. This evidence that altruists favor relatives with high fitness prospects highlights the numerous factors shaping altruism's payoffs.

Sexual selection's impacts on ecological specialization: an experimental test

In many species, individuals specialize on different resources, thereby reducing competition. Such ecological specialization can promote the evolution of alternative ecomorphs-distinct phenotypes adapted for particular resources. Elucidating whether and how this process is influenced by sexual selection is crucial for understanding how ecological specialization promotes the evolution of novel traits and, potentially, speciation between ecomorphs. We evaluated the population-level effects of sexual selection (as mediated by mate choice) on ecological specialization in spadefoot toad tadpoles that express alternative ecomorphs. We manipulated whether sexual selection was present or reversed by mating females to their preferred versus non-preferred males, respectively. We then exposed their tadpoles to resource competition in experimental mesocosms. The resulting distribution of ecomorphs was similar between treatments, but sexual selection generated poorer trait integration in, and lower fitness of, the more specialized carnivore morph. Moreover, disruptive and directional natural selection were weaker in the sexual selection present treatment. Nevertheless, this effect on disruptive selection was smaller than previously documented effects of ecological opportunity and competitor density. Thus, sexual selection can inhibit adaptation to resource competition and thereby hinder ecological specialization, particularly when females obtain fitness benefits from mate choice that offset the cost of producing competitively inferior offspring.

Habitat segregation and ecological character displacement in cryptic African malaria mosquitoes

Understanding how divergent selection generates adaptive phenotypic and population diversification provides a mechanistic explanation of speciation in recently separated species pairs. Towards this goal, we sought ecological gradients of divergence between the cryptic malaria vectors Anopheles coluzzii and An. gambiae and then looked for a physiological trait that may underlie such divergence. Using a large set of occurrence records and eco-geographic information, we built a distribution model to predict the predominance of the two species across their range of sympatry. Our model predicts two novel gradients along which the species segregate: distance from the coastline and altitude. Anopheles coluzzii showed a ‘bimodal’ distribution, predominating in xeric West African savannas and along the western coastal fringe of Africa. To test whether differences in salinity tolerance underlie this habitat segregation, we assessed the acute dose–mortality response to salinity of thirty-two larval populations from Central Africa. In agreement with its coastal predominance, Anopheles coluzzii was overall more tolerant than An. gambiae. Salinity tolerance of both species, however, converged in urban localities, presumably reflecting an adaptive response to osmotic stress from anthropogenic pollutants. When comparing degree of tolerance in conjunction with levels of syntopy, we found evidence of character displacement in this trait.

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.

Ecological character displacement: glass half full or half empty?

Ecological character displacement (ECD), the evolutionary divergence of competing species, has oscillated wildly in scientific opinion. Initially thought to play a central role in community assembly and adaptive radiation, ECD recovered from a 1980s nadir to present-day prominence on the strength of many case studies compiled in several influential reviews. However, we reviewed recent studies and found that only nine of 144 cases are strong examples that have ruled out alternative explanations for an ECD-like pattern. We suggest that the rise in esteem of ECD has outpaced available data and that more complete, rather than simply more, case studies are needed. Recent years have revealed that evolutionary change can be observed as it occurs, opening the door to experimental field studies as a new approach to studying ECD.

Widespread disruptive selection in the wild is associated with intense resource competition

Background

Disruptive selection has been documented in a growing number of natural populations. Yet, its prevalence within individual systems remains unclear. Furthermore, few studies have sought to identify the ecological factors that promote disruptive selection in the wild. To address these issues, we surveyed 15 populations of Mexican spadefoot toad tadpoles, Spea multiplicata, and measured the prevalence of disruptive selection acting on resource-use phenotypes. We also evaluated the relationship between the strength of disruptive selection and the intensity of intraspecific competition—an ecological agent hypothesized to be an important driver of disruptive selection.

Results

Disruptive selection was the predominant mode of quadratic selection across all populations. However, a directional component of selection favoring an extreme ecomorph—a distinctive carnivore morph—was also common. Disruptive selection was strongest in populations experiencing the most intense intraspecific competition, whereas stabilizing selection was only found in populations experiencing relatively weak intraspecific competition.

Conclusions

Disruptive selection can be common in natural populations. Intraspecific competition for resources may be a key driver of such selection.

Development and evolution of character displacement

Character displacement occurs when competition for either resources or successful reproduction imposes divergent selection on interacting species, causing divergence in traits associated with resource use or reproduction. Here, we describe how character displacement can be mediated either by genetically canalized changes (i.e., changes that reflect allelic or genotype frequency changes) or by phenotypic plasticity. We also discuss how these two mechanisms influence the tempo of character displacement. Specifically, we suggest that, under some conditions, character displacement mediated by phenotypic plasticity might occur more rapidly than that mediated by genetically canalized changes. Finally, we describe how these two mechanisms may act together and determine character displacement's mode, such that it proceeds through an initial phase in which trait divergence is environmentally induced to a later phase in which divergence becomes genetically canalized. This plasticity-first hypothesis predicts that character displacement should be generally mediated by ancestral plasticity and that it will arise similarly in multiple, independently evolving populations. We conclude by highlighting future directions for research that would test these predictions.