Non‐linear relationship between intensity of plant–animal interactions and selection strength

The shape of the relationship between intensity of biotic interactions and strength of selection is important for spatial variation in selection, but is little explored. We quantified interactions and selection in 69 populations of the short‐lived herb Primula farinosa. As predicted because of saturation and depletion effects, the strength of selection on a discrete and on a continuously varying floral display trait were in several cases significantly non‐linearly related to the mean intensity of mutualistic and antagonistic interactions. Strength of selection was strongest at low levels of fruit initiation and at high intensities of seed predation. Seed predation varied more among populations than did fruit initiation and could explain a larger proportion of the among‐population variation in strength of selection. Our results support the contention that interaction intensity affects selection strength, and suggests that for mutualistic and antagonistic interactions that can be saturated or depleted, this relationship is sometimes non‐linear.     

Sexual selection and sexual size dimorphism in animals

Sexual selection is often considered as a critical evolutionary force promoting sexual size dimorphism (SSD) in animals. However, empirical evidence for a positive relationship between sexual selection on males and male-biased SSD received mixed support depending on the studied taxonomic group and on the method used to quantify sexual selection. Here, we present a meta-analytic approach accounting for phylogenetic non-independence to test how standardized metrics of the opportunity and strength of pre-copulatory sexual selection relate to SSD across a broad range of animal taxa comprising up to 95 effect sizes from 59 species. We found that SSD based on length measurements was correlated with the sex difference in the opportunity for sexual selection but showed a weak and statistically non-significant relationship with the sex difference in the Bateman gradient. These findings suggest that pre-copulatory sexual selection plays a limited role for the evolution of SSD in a broad phylogenetic context.     

Impact of selection on genes involved in regulatory network: a modelling study

Complex phenotypes are often controlled by many interacting genes. One question emerging from such organization is how selection, acting at the phenotypic level, shapes the evolution of genes involved in regulatory networks controlling the phenotypes. We studied this issue through a matrix model of such networks. In a population submitted to selection, we simulated the evolution of a quantitative trait controlled by a set of loci that regulate each other through positive or negative interactions. Investigating several levels of selection intensity on the trait, we studied the evolution of regulation intensity between the genes and the evolution of the genetic diversity of those genes as an indirect measure of the strength of selection acting on them. We show that an increasing intensity of selection on the phenotype leads to an increased level of regulation between the loci. Moreover, we found that the genes responding more strongly to selection within the network were those evolving towards stronger regulatory action on the other genes and/or those that are the less regulated by the other genes. This observation is strongest for an intermediate level of selection. This may explain why several experimental studies have shown evidence of selection on regulatory genes inside gene networks.     

Why do Tree Species Affect Soils? The Warp and Woof of Tree-soil Interactions

Many ideas have been advanced regarding how trees affect soils. Enough evidence is now available to evaluate the strength of these ideas and to consider interactions between tree species and soils in an evolutionary context. Forest floor mass commonly differs by about 20% for different species growing on the same site; differences of up to 5-fold have been reported. Litterfall mass and N content commonly differ by 20 to 30%, but larger differences are also common (especially with N-fixing species). The net mineralization of soil N typically differs by 50% or more among species, indicating very strong feedback possibilities. We evaluate the evolutionary context of tree effects on soils by considering 3 degrees of coupling of trees to soils: tightly woven connections where the fitness of the tree is enhanced by its effect on soils; loosely woven interactions where selection for tree fitness unrelated to soil properties leads to indirect effects on soils (either enhancing or impairing fitness); and frayed interactions where the effects of trees on soil derive from features of the ecosystem that do not involve direct selection for tree fitness. Evidence supports each of these degrees of interaction for at least some cases, and no single context explains all the interactions between trees and soils. Important areas for further work include: next-generation assessments of the effects of trees on soil suitability for the same (and different) species, and the role of soil organisms in developing and modifying the effects of trees on soils.     

Species interactions and plant polyploidy

Polyploidy is a common mode of speciation that can have far‐reaching consequences for plant ecology and evolution. Because polyploidy can induce an array of phenotypic changes, there can be cascading effects on interactions with other species. These interactions, in turn, can have reciprocal effects on polyploid plants, potentially impacting their establishment and persistence. Although there is a wealth of information on the genetic and phenotypic effects of polyploidy, the study of species interactions in polyploid plants remains a comparatively young field. Here we reviewed the available evidence for how polyploidy may impact many types of species interactions that range from mutualism to antagonism. Specifically, we focused on three main questions: (1) Does polyploidy directly cause the formation of novel interactions not experienced by diploids, or does it create an opportunity for natural selection to then form novel interactions? (2) Does polyploidy cause consistent, predictable changes in species interactions vs. the evolution of idiosyncratic differences? (3) Does polyploidy lead to greater evolvability in species interactions? From the scarce evidence available, we found that novel interactions are rare but that polyploidy can induce changes in pollinator, herbivore, and pathogen interactions. Although further tests are needed, it is likely that selection following whole‐genome duplication is important in all types of species interaction and that there are circumstances in which polyploidy can enhance the evolvability of interactions with other species.     

CONSUMER–RESOURCE INTERACTIONS AND THE EVOLUTION OF MIGRATION

Theoretical studies have demonstrated that selection will favor increased migration when fitnesses vary both temporally and spatially, but it is far from clear how pervasive those theoretical conditions are in nature. Although consumer–resource interactions are omnipresent in nature and can generate spatial and temporal variation, it is unknown even in theory whether these dynamics favor the evolution of migration. We develop a mathematical model to address whether and how migration evolves when variability in fitness is determined at least in part by consumer–resource coevolutionary interactions. Our analyses show that such interactions can drive the evolution of migration in the resource, consumer, or both species and thus supplies a general explanation for the pervasiveness of migration. Over short time scales, we show the direction of change in migration rate is determined primarily by the state of local adaptation of the species involved: rates increase when a species is locally maladapted and decrease when locally adapted. Our results reveal that long‐term evolutionary trends in migration rates can differ dramatically depending on the strength or weakness of interspecific interactions and suggest an explanation for the evolutionary divergence of migration rates among interacting species.     

A conceptual framework for studying the strength of plant–animal mutualistic interactions

The strength of species interactions influences strongly the structure and dynamics of ecological systems. Thus, quantifying such strength is crucial to understand how species interactions shape communities and ecosystems. Although the concepts and measurement of interaction strength in food webs have received much attention, there has been comparatively little progress in the context of mutualism. We propose a conceptual scheme for studying the strength of plant–animal mutualistic interactions. We first review the interaction strength concepts developed for food webs, and explore how these concepts have been applied to mutualistic interactions. We then outline and explain a conceptual framework for defining ecological effects in plant–animal mutualisms. We give recommendations for measuring interaction strength from data collected in field studies based on a proposed approach for the assessment of interaction strength in plant–animal mutualisms. This approach is conceptually integrative and methodologically feasible, as it focuses on two key variables usually measured in field studies: the frequency of interactions and the fitness components influenced by the interactions.     

The ecology of predispersal insect herbivory on tree reproductive structures in natural forest ecosystems

Plant-insect interactions are key model systems to assess how some species affect the distribution, the abundance, and the evolution of others. Tree reproductive structures represent a critical resource for many insect species, which can be likely drivers of demography, spatial distribution, and trait diversification of plants. In this review, we present the ecological implications of predispersal herbivory on tree reproductive structures by insects (PIHR) in forest ecosystems. Both insect's and tree's perspectives are addressed with an emphasis on how spatiotemporal variation and unpredictability in seed availability can shape such particular plant-animal interactions. Reproductive structure insects show strong trophic specialization and guild diversification. Insects evolved host selection and spatiotemporal dispersal strategies in response to variable and unpredictable abundance of reproductive structures in both space and time. If PIHR patterns have been well documented in numerous systems, evidences of the subsequent demographic and evolutionary impacts on tree populations are still constrained by time-scale challenges of experimenting on such long-lived organisms, and modeling approaches of tree dynamics rarely consider PIHR when including biotic interactions in their processes. We suggest that spatially explicit and mechanistic approaches of the interactions between individual tree fecundity and in sect dynamics will clarify predictions of the demogenetic implications of PIHR in tree populations. In a global change context, further experimental and theoretical contributions to the likelihood of life-cycle disruptions between plants and their specialized herbivores, and to how these changes may gen erate novel dynamic patterns in each partner of the interaction are increasingly critical.     

Experimental evidence that sexual conflict influences the opportunity, form and intensity of sexual selection

Sexual interactions are often rife with conflict. Conflict between members of the same sex over opportunities to mate has long been understood to effect evolution via sexual selection. Although conflict between males and females is now understood to be widespread, such conflict is seldom considered in the same light as a general agent of sexual selection. Any interaction between males or females that generates variation in fitness, whether due to conflict, competition or mate choice, can potentially influence sexual selection acting on a range of male traits. Here we seek to address a lack of direct experimental evidence for how sexual conflict influences sexual selection more broadly. We manipulate a major source of sexual conflict in the black field cricket, Teleogryllus commodus, and quantify the resulting changes in the nature of sexual selection using formal selection analysis to statistically compare multivariate fitness surfaces. In T. commodus, sexual conflict occurs over the attachment time of an external spermatophore. By experimentally manipulating the ability of males and females to influence spermatophore attachment, we found that sexual conflict significantly influences the opportunity, form, and intensity of sexual selection on male courtship call and body size. When males were able to harass females, the opportunity for selection was smaller, the form of selection changed, and sexual selection was weaker. We discuss the broader evolutionary implications of these findings, including the contributions of sexual conflict to fluctuating sexual selection and the maintenance of additive genetic variation.     

Pollen limitation and its influence on natural selection through seed set

Stronger pollen limitation should increase competition among plants, leading to stronger selection on traits important for pollen receipt. The few explicit tests of this hypothesis, however, have provided conflicting support. Using the arithmetic relationship between these two quantities, we show that increased pollen limitation will automatically result in stronger selection (all else equal) although other factors can alter selection independently of pollen limitation. We then tested the hypothesis using two approaches. First, we analysed the published studies containing information on both pollen limitation and selection. Second, we explored how natural selection measured in one Ontario population of Lobelia cardinalis over 3 years and two Michigan populations in 1 year relates to pollen limitation. For the Ontario population, we also explored whether pollinator‐mediated selection is related to pollen limitation. Consistent with the hypothesis, we found an overall positive relationship between selection strength and pollen limitation both among species and within L. cardinalis. Unexpectedly, this relationship was found even for vegetative traits among species, and was not found in L. cardinalis for pollinator‐mediated selection on nearly all trait types.     

Multiple infections and the evolution of virulence

Infections that consist of multiple parasite strains or species are common in the wild and are a major public health concern. Theory suggests that these infections have a key influence on the evolution of infectious diseases and, more specifically, on virulence evolution. However, we still lack an overall vision of the empirical support for these predictions. We argue that within‐host interactions between parasites largely determine how virulence evolves and that experimental data support model predictions. Then, we explore the main limitation of the experimental study of such ‘mixed infections’, which is that it draws conclusions on evolutionary outcomes from studies conducted at the individual level. We also discuss differences between coinfections caused by different strains of the same species or by different species. Overall, we argue that it is possible to make sense out of the complexity inherent to multiple infections and that experimental evolution settings may provide the best opportunity to further our understanding of virulence evolution.     

植物的亲缘选择

亲缘选择是指在一个随机交配群体中的个体基于亲缘关系而以一种非随机性的方式相互作用,其作用结果是亲缘个体得到更大的广义适合度。综述了亲缘选择和亲缘竞争两种观点以及各自的试验支持证据;分析了导致亲缘选择试验结果出现分歧的原因,认为这主要是由于对亲缘选择理解上的模糊以及试验设计的不严谨所致。植物间的亲缘选择研究不仅相对较少,对亲缘选择的机制研究更为欠缺,这就造成了目前对此问题在科学认识上出现不少盲点。综合前期研究,提出今后对亲缘选择的研究应该首先界定"亲缘"程度,同时改良试验设计方案,选择多种不同生境下的物种对亲缘选择进行深入研究,并且考虑环境因子对植物亲缘选择的影响。同时,对植物亲缘识别机制的研究应该从生理生化方面出发,通过定性定量地分析探索植物根系分泌物在植物亲缘识别中的作用和作用途径。     

EVOLUTION OF RESISTANCE TO A MULTIPLE‐HERBIVORE COMMUNITY: GENETIC CORRELATIONS,DIFFUSE COEVOLUTION,AND CONSTRAINTS ON THE PLANT'S RESPONSE TO SELECTION

Although plants are generally attacked by a community of several species of herbivores, relatively little is known about the strength of natural selection for resistance in multiple‐herbivore communities—particularly how the strength of selection differs among herbivores that feed on different plant organs or how strongly genetic correlations in resistance affect the evolutionary responses of the plant. Here, we report on a field study measuring natural selection for resistance in a diverse community of herbivores of Solanum carolinense. Using linear phenotypic‐selection analyses, we found that directional selection acted to increase resistance to seven species. Selection was strongest to increase resistance to fruit feeders, followed by flower feeders, then leaf feeders. Selection favored a decrease in resistance to a stem borer. Bootstrapping analyses showed that the plant population contained significant genetic variation for each of 14 measured resistance traits and significant covariances in one‐third of the pairwise combinations of resistance traits. These genetic covariances reduced the plant's overall predicted evolutionary response for resistance against the herbivore community by about 60%. Diffuse (co)evolution was widespread in this community, and the diffuse interactions had an overwhelmingly constraining (rather than facilitative) effect on the plant's evolution of resistance.     

Extreme environmental variation sharpens selection that drives the evolution of a mutualism

The importance of infrequent events for both adaptive evolution and the evolution of species interactions is largely unknown. We investigated how the infrequent production of large seed crops (masting) of a bird-dispersed tree (whitebark pine, Pinus albicaulis) influenced phenotypic selection exerted by its primary avian seed predator-disperser, the Clark's nutcracker (Nucifraga columbiana). Selection was not evident during common years of low seed abundance, whereas it was replicated among areas and favoured traits facilitating seed dispersal during infrequent years of high seed abundance. Since nutcrackers act mostly as seed predators during small seed crops but as seed dispersers during the largest seed crops, trees experienced strong selection from nutcrackers only during infrequent years when the interaction was most strongly mutualistic. Infrequent events can thus be essential to both adaptive evolution and the evolutionary dynamics of species interactions.     

Plant–plant facilitation increases with reduced phylogenetic relatedness along an elevation gradient

Environmental conditions can modify the intensity and sign of ecological interactions. The stress gradient hypothesis (SGH) predicts that facilitation becomes more important than competition under stressful conditions. To properly test this hypothesis, it is necessary to account for all (not a subset of) interactions occurring in the communities and consider that species do not interact at random but following a specific pattern. We aim to assess elevational changes in facilitation, in terms of species richness, frequency and intensity of the interaction as a function of the evolutionary relatedness between nurses and their associated species. We sampled nurse and their facilitated plant species in two 1000–2000 m. elevation gradients in Mediterranean Chile where low temperature imposes a mortality filter on seedlings. We first estimated the relative importance of facilitation as a mechanism adding new species to communities distributed along these gradients. We then tested whether the frequency and intensity of facilitation increases with elevation, taking into account the evolutionary relatedness of the nurse species and the facilitated species. We found that nurses increase the species richness of the community by up to 35%. Facilitative interactions are more frequent than competitive interactions (56% versus 44%) and facilitation intensity increased with elevation for interactions involving distantly related lineages. Our results highlight the importance of including an evolutionary dimension in the study of facilitation to have a clearer picture of the mechanisms enabling species to coexist and survive under stressful conditions. This knowledge is especially relevant to conserve vulnerable and threatened communities facing new climate scenarios, such as those located in Mediterranean-type ecosystems.     

PHYTOPHAGOUS INSECT–MICROBE MUTUALISMS AND ADAPTIVE EVOLUTIONARY DIVERSIFICATION

Adaptive diversification is a process intrinsically tied to species interactions. Yet, the influence of most types of interspecific interactions on adaptive evolutionary diversification remains poorly understood. In particular, the role of mutualistic interactions in shaping adaptive radiations has been largely unexplored, despite the ubiquity of mutualisms and increasing evidence of their ecological and evolutionary importance. Our aim here is to encourage empirical inquiry into the relationship between mutualism and evolutionary diversification, using herbivorous insects and their microbial mutualists as exemplars. Phytophagous insects have long been used to test theories of evolutionary diversification; moreover, the diversification of a number of phytophagous insect lineages has been linked to mutualisms with microbes. In this perspective, we examine microbial mutualist mediation of ecological opportunity and ecologically based divergent natural selection for their insect hosts. We also explore the conditions and mechanisms by which microbial mutualists may either facilitate or impede adaptive evolutionary diversification. These include effects on the availability of novel host plants or adaptive zones, modifying host-associated fitness trade-offs during host shifts, creating or reducing enemy-free space, and, overall, shaping the evolution of ecological (host plant) specialization. Although the conceptual framework presented here is built on phytophagous insect–microbe mutualisms, many of the processes and predictions are broadly applicable to other mutualisms in which host ecology is altered by mutualistic interactions.     

Socially flexible female choice differs among populations of the Pacific field cricket: geographical variation in the interaction coefficient psi (Ψ)

Indirect genetic effects (IGEs) occur when genes expressed in one individual affect the phenotype of a conspecific. Theoretical models indicate that the evolutionary consequences of IGEs critically depend on the genetic architecture of interacting traits, and on the strength and direction of phenotypic effects arising from social interactions, which can be quantified by the interaction coefficient Ψ. In the context of sexually selected traits, strong positive Ψ tends to exaggerate evolutionary change, whereas negative Ψ impedes sexual trait elaboration. Despite its theoretical importance, whether and how Ψ varies among geographically distinct populations is unknown. Such information is necessary to evaluate the potential for IGEs to contribute to divergence among isolated or semi-isolated populations. Here, we report substantial variation in Ψ for a behavioural trait involved in sexual selection in the field cricket Teleogryllus oceanicus: female choosiness. Both the strength and direction of Ψ varied among geographically isolated populations. Ψ also changed over time. In a contemporary population of crickets from Kauai, experience of male song increased female choosiness. In contrast, experience of male song decreased choosiness in an ancestral population from the same location. This rapid change corroborates studies examining the evolvability of Ψ and demonstrates how interpopulation variation in the interaction coefficient might influence sexual selection and accelerate divergence of traits influenced by IGEs that contribute to reproductive isolation in nascent species or subspecies.     

CAUSES OF VARIATION IN BIOTIC INTERACTION STRENGTH AND PHENOTYPIC SELECTION ALONG AN ALTITUDINAL GRADIENT

Understanding the causes of variation in biotic interaction strength and phenotypic selection remains one of the outstanding goals of evolutionary ecology. Here we examine the variation in strength of interactions between two seed predators, common crossbills (Loxia curvirostra) and European red squirrels (Sciurus vulgaris), and mountain pine (Pinus uncinata) at and below tree limit in the Pyrenees, and how this translates into phenotypic selection. Seed predation by crossbills increased whereas seed predation by squirrels decreased with increasing elevation and as the canopy became more open. Overall, seed predation by crossbills averaged about twice that by squirrels, and the intensity of selection exerted by crossbills averaged between 2.6 and 7.5 times greater than by squirrels. The higher levels of seed predation by crossbills than squirrels were related to the relatively open nature of most of the forests, and the higher intensity of selection exerted by crossbills resulted from their higher levels of seed predation. However, most of the differences in selection intensity between crossbills and squirrels were the result of habitat features having a greater effect on the foraging behavior of squirrels than of crossbills, causing selection to be much lower for squirrels than for crossbills.     

Natural selection in mimicry

Biological mimicry has served as a salient example of natural selection for over a century, providing us with a dazzling array of very different examples across many unrelated taxa. We provide a conceptual framework that brings together apparently disparate examples of mimicry in a single model for the purpose of comparing how natural selection affects models, mimics and signal receivers across different interactions. We first analyse how model–mimic resemblance likely affects the fitness of models, mimics and receivers across diverse examples. These include classic Batesian and Müllerian butterfly systems, nectarless orchids that mimic Hymenoptera or nectar‐producing plants, caterpillars that mimic inert objects unlikely to be perceived as food, plants that mimic abiotic objects like carrion or dung and aggressive mimicry where predators mimic food items of their own prey. From this, we construct a conceptual framework of the selective forces that form the basis of all mimetic interactions. These interactions between models, mimics and receivers may follow four possible evolutionary pathways in terms of the direction of selection resulting from model–mimic resemblance. Two of these pathways correspond to the selective pressures associated with what is widely regarded as Batesian and Müllerian mimicry. The other two pathways suggest mimetic interactions underpinned by distinct selective pressures that have largely remained unrecognized. Each pathway is characterized by theoretical differences in how model–mimic resemblance influences the direction of selection acting on mimics, models and signal receivers, and the potential for consequent (co)evolutionary relationships between these three protagonists. The final part of this review describes how selective forces generated through model–mimic resemblance can be opposed by the basic ecology of interacting organisms and how those forces may affect the symmetry, strength and likelihood of (co)evolution between the three protagonists within the confines of the four broad evolutionary possibilities. We provide a clear and pragmatic visualization of selection pressures that portrays how different mimicry types may evolve. This conceptual framework provides clarity on how different selective forces acting on mimics, models and receivers are likely to interact and ultimately shape the evolutionary pathways taken by mimetic interactions, as well as the constraints inherent within these interactions.