Quantitative genetics of response to competitors in Nemophila menziesii: a field experiment

Integrating eco-evolutionary dynamics and modern coexistence theory

Community ecology typically assumes that competitive exclusion and species coexistence are unaffected by evolution on the time scale of ecological dynamics. However, recent studies suggest that rapid evolution operating concurrently with competition may enable species coexistence. Such findings necessitate general theory that incorporates the coexistence contributions of eco-evolutionary processes in parallel with purely ecological mechanisms and provides metrics for quantifying the role of evolution in shaping competitive outcomes in both modelling and empirical contexts. To foster the development of such theory, here we extend the interpretation of the two principal metrics of modern coexistence theory-niche and competitive ability differences-to systems where competitors evolve. We define eco-evolutionary versions of these metrics by considering how invading and resident species adapt to conspecific and heterospecific competitors. We show that the eco-evolutionary niche and competitive ability differences are sums of ecological and evolutionary processes, and that they accurately predict the potential for stable coexistence in previous theoretical studies of eco-evolutionary dynamics. Finally, we show how this theory frames recent empirical assessments of rapid evolution effects on species coexistence, and how empirical work and theory on species coexistence and eco-evolutionary dynamics can be further integrated.

Lifetime Fitness through Female and Male Function: Influences of Genetically Effective Population Size and Density

AbstractAn individual's lifetime fitness and patterns of mating between individuals are interdependent features of sexual organisms. Mating systems (outcrossing vs. selfing or mating between close relatives) can affect the distribution of offspring fitness, which generally declines with inbreeding, which in turn is related to a population's genetically effective size (N_e). Fitness and mating patterns are also expected to vary with proximity of mates (i.e., population density). Consequently, density and N_e may influence demographic and genetic changes over generations and interact in their effects. Here, we report an experiment designed to assess the influence of these two population-level properties on mating system and lifetime fitness. In experimental arrays under quasi-natural conditions, we varied the density and N_e of the hermaphroditic annual legume Chamaecrista fasciculata. We recorded components of fitness for each individual and employed microsatellite markers to estimate outcrossing and assign paternity. We used aster analyses to estimate lifetime fitness for genetic families using female (seeds set) and male (seeds sired) reproduction as fitness measures. With estimates from these analyses, we assessed the evidence for a trade-off between fitness attained through female versus male function, but we found none. Lifetime fitness increased with density, especially under high N_e. Outcrossing rates increased with density under high N_e but declined modestly with density under low N_e. Our results show that density and N_e have strong direct effects on fitness and mating systems, with negative fitness effects of low N_e limiting the positive effects of increasing density. These findings highlight the importance of the interactive effects of density and N_e on lifetime fitness.

Evolution of asexual Daphnia pulex in Japan: variations and covariations of the digestive, morphological and life history traits

Background

Several genetic lineages of obligate parthenogenetic Daphnia pulex, a common zooplankton species, have invaded Japan from North America. Among these, a lineage named JPN1 is thought to have started colonization as a single genotype several hundred to thousand years ago and subsequently produced many genotypes in Japan. To examine the phenotypic variations due to ecological drivers diverging the genotypes in new habitats, we measured heritability and variation in 17 traits, including life history, morphology and digestive traits, and the genetic distance among the D. pulex JPN1 genotypes in Japan.

Results

We found that most of the traits measured varied significantly among the genotypes and that heritability was highest in the morphological traits, followed by the digestive and life history traits. In addition, 93% of the variation in these traits was explained by the first three components in the principal component analysis, implying that variation of these heritable traits is not random but rather converged into a few directions. These relations among traits revealed the potential importance of predation pressures and food conditions as factors for diverging and selecting different genotypes. However, the magnitude of the difference in any single trait group did not correlate with the genetic distance.

Conclusions

Our findings show that the divergent traits evolved within D. pulex JPN1 lineage without genetic recombination, since their ancestral clone invaded Japan. Large variations and covariations of the phenotypic traits, irrespective of the genetic distance among the genotypes, support the view that the invasive success of D. pulex JPN1 was promoted by a genetic architecture that allowed for large phenotypic variations with a limited number of functionally important mutations without recombination.

Electronic supplementary material

The online version of this article (10.1186/s12862-019-1453-9) contains supplementary material, which is available to authorized users.

Paradoxical effects of density on measurement of copper tolerance in Silene paradoxa L

This work investigated if the assessment of tolerance to trace metals can depend on plant density in the experimental design. A non-metallicolous and a metallicolous populations of Silene paradoxa were hydroponically cultivated at increasing density and in both the absence (-Cu conditions) and excess of copper (+Cu conditions). In -Cu conditions, the metallicolous population showed a lower susceptibility to plant density in comparison to the non-metallicolous one, explained by a higher capacity of the metallicolous population to exploit resources. In +Cu conditions, an alleviating effect of increasing density was found in roots. Such effect was present to a greater extent in the non-metallicolous population, thus making the populations equally copper-tolerant at the highest density used. In shoots, an additive effect of increasing plant density to copper toxicity was reported. Its higher intensity in the metallicolous population reverted the copper tolerance relationship at the highest plant densities used. In both populations, a density-induced decrease in root copper accumulation was observed, thus concurring to the reported mitigation in +Cu conditions. Our work revealed the importance of density studies on the optimization of eco-toxicological bioassays and of metal tolerance assessment and it can be considered the first example of an alleviating effect of increasing plant number on copper stress in a metallophyte.

Evidence for Selection-by-Environment but Not Genotype-by-Environment Interactions for Fitness-Related Traits in a Wild Mammal Population

How do environmental conditions influence selection and genetic variation in wild populations? There is widespread evidence for selection-by-environment interactions (S*E), but we reviewed studies of natural populations estimating the extent of genotype-by-environment interactions (G*E) in response to natural variation in environmental conditions and found that evidence for G*E appears to be rare within single populations in the wild. Studies estimating the simultaneous impact of environmental variation on both selection and genetic variation are especially scarce. Here, we used 24 years of data collected from a wild Soay sheep population to quantify how an important environmental variable, population density, impacts upon (1) selection through annual contribution to fitness and (2) expression of genetic variation, in six morphological and life history traits: body weight, hind leg length, parasite burden, horn length, horn growth, and testicular circumference. Our results supported the existence of S*E: selection was stronger in years of higher population density for all traits apart from horn growth, with directional selection being stronger under more adverse conditions. Quantitative genetic models revealed significant additive genetic variance for body weight, leg length, parasite burden, horn length, and testes size, but not for horn growth or our measure of annual fitness. However, random regression models found variation between individuals in their responses to the environment in only three traits, and did not support the presence of G*E for any trait. Our analyses of St Kilda Soay sheep data thus concurs with our cross-study review that, while natural environmental variation within a population can profoundly alter the strength of selection on phenotypic traits, there is less evidence for its effect on the expression of genetic variance in the wild.

VARIATION IN SEED CHARACTERS IN NEMOPHILA MENZIESII: EVIDENCE OF A GENETIC BASIS FOR MATERNAL EFFECT

A growing body of evidence indicates that phenotypic selection on juvenile traits of both plants and animals may be considerable. Because juvenile traits are typically subject to maternal effects and often have low heritabilities, adaptive responses to natural selection on these traits may seem unlikely. To determine the potential for evolutionary response to selection on juvenile traits of Nemophila menziesii (Hydrophyllaceae), we conducted two quantitative genetic studies. A reciprocal factorial cross, involving 16 parents and 1960 progeny, demonstrated a significant maternal component of variance in seed mass and additive genetic component of variance in germination time. This experiment also suggested that interaction between parents, though small, provides highly significant contributions to the variance of both traits. Such a parental interaction could arise by diverse mechanisms, including dependence of nuclear gene expression on cytoplasmic genotype, but the design of this experiment could not distinguish this from other possible causes, such as effects on progeny phenotype of interaction between the environmental conditions of both parents. The second experiment, spanning three generations with over 11,000 observations, was designed for investigation of the additive genetic variance in maternal effect, assessment of paternal effects, as well as further partitioning of the parental interaction identified in the reciprocal factorial experiment. It yielded no consistent evidence of paternal effects on seed mass, nor of parental interactions. Our inference of such interaction effects from the first experiment was evidently an artifact of failing to account for the substantial variance among fruits within crosses. The maternal effect was found to have a large additive genetic component, accounting for at least 20% of the variation in individual seed mass. This result suggests that there is appreciable potential for response to selection on seed mass through evolution of the maternal effect. We discuss aspects that may nevertheless limit response to individual selection on seed mass, including trade-offs between the size of individual seeds and germination time and between the number of seeds a maternal plant can mature and their mean size.

Natural quantitative genetic variance in plant growth differs in response to ecologically relevant temperature heterogeneity

Adaptation to large‐scale spatial heterogeneity in the environment accounts for a major proportion of genetic diversity within species. Theory predicts the erosion of adaptive genetic variation on a within‐population level, but considerable genetic diversity is often found locally. Genetic diversity could be expected to be maintained within populations in temporally or spatially variable conditions if genotypic rank orders vary across contrasting microenvironmental settings. Taking advantage of fine‐resolution environmental data, we tested the hypothesis that temperature heterogeneity among years could be one factor maintaining quantitative genetic diversity within a natural and genetically diverse plant population. We sampled maternal families of Boechera stricta, an Arabidopsis thaliana relative, at one location in the central Rocky Mountains and grew them in three treatments that, based on records from an adjacent weather station, simulated hourly temperature changes at the native site during three summers with differing mean temperatures. Treatment had a significant effect on all traits, with 2–3‐fold increase in above‐ and belowground biomass and the highest allocation to roots observed in the treatment simulating the warmest summer on record at the site. Treatment affected bivariate associations between traits, with the weakest correlation between above‐ and belowground biomass in the warmest treatment. The magnitude of quantitative genetic variation for all traits differed across treatments: Genetic variance of biomass was 0 in the warmest treatment, while highly significant diversity was found in average conditions, resulting in broad‐sense heritability of 0.31. Significant genotype × environment interactions across all treatments were found only in root‐to‐shoot ratio. Therefore, temperature variation among summers appears unlikely to account for the observed levels of local genetic variation in size in this perennial species, but may influence family rank order in growth allocation. Our results indicate that natural environmental fluctuations can have a large impact on the magnitude of within‐population quantitative genetic variance.

Geographic variation in resistance to nectar robbing and consequences for pollination

PREMISE OF THE STUDY

Floral evolution is frequently ascribed to selection by pollinators, but may also be shaped by antagonists. However, remarkably few studies have examined geographic mosaics in resistance to floral antagonists or the consequences for other floral interactions.

METHODS

Gelsemium sempervirens experiences frequent nectar robbing in northern Georgia, but rarely in southern Georgia. We conducted common-garden experiments in both locations using genotypes from each region and measured robbing, pollinator attraction, floral attractive and defensive traits, and plant reproduction.

KEY RESULTS

Nectar robbing was more than four times higher in the north vs. south, and pollinator visits did not differ between gardens. Across both gardens, northern genotypes were half as likely to be nectar-robbed but received half as many pollinator visits as southern genotypes, suggesting evolution of resistance to robbing at a cost of reduced pollinator attraction. Plant-level traits, such as height and number of flowers, were more closely associated with resistance to robbing than floral size, shape, or chemistry. Northern genotypes had lower female and estimated male reproduction compared to southern genotypes at both locations, which could be due to costs of resistance to nectar robbing, or costs of adaptations to other biotic or abiotic differences between regions.

CONCLUSIONS

Our study indicates that geographic variation can play a strong role structuring interactions with floral antagonists and mutualists and provides evidence consistent with the hypothesis that local resistance to nectar robbing imposes costs in terms of decreased pollinator attraction and reproduction.

Spatial pattern of invasion and the evolutionary responses of native plant species

Invasive plant species can have a strong negative impact on the resident native species, likely imposing new selective pressures on them. Altered selective pressures may result in evolutionary changes in some native species, reducing competitive exclusion and allowing for coexistence with the invader. Native genotypes that are able to coexist with strong invaders may represent a valuable resource for management efforts. A better understanding of the conditions under which native species are more, or less, likely to adapt to an invader is necessary to incorporate these eco-evolutionary dynamics into management strategies. We propose that the spatial structure of invasion, in particular the size and isolation of invaded patches, is one factor which can influence the evolutionary responses of native species through modifying gene flow and the strength of selection. We present a conceptual model in which large, dense, and well-connected patches result in a greater likelihood of native species adaptation. We also identify characteristics of the interacting species that may influence the evolutionary response of native species to invasion and outline potential management implications. Identifying areas of rapid evolutionary change may offer one additional tool to managers in their effort to conserve biodiversity in the face of invasion.

Spatiotemporal environmental heterogeneity and the maintenance of the tailspot polymorphism in the variable platyfish (Xiphophorus variatus)

Genetic variation is critical for adaptive evolution. Despite its importance, there is still limited evidence in support of some prominent theoretical models explaining the maintenance of genetic polymorphism within populations. We examined 84 populations of Xiphophorus variatus, a livebearing fish with a genetic polymorphism associated with physiological performance, to test: (1) whether niche differentiation explains broad-scale maintenance of polymorphism, (2) whether polymorphism is maintained among populations by local adaptation and migration, or (3) whether heterogeneity in explicit environmental variables could be linked to levels of polymorphism within populations. We found no evidence of climatic niche differentiation that could generate or maintain broad geographic variation in polymorphism. Subsequently, hierarchical partitioning of genetic richness and partial mantel tests revealed that 76% of the observed genetic richness was partitioned within populations with no effect of geographic distance on polymorphism. These results strongly suggest a lack of migration-selection balance in the maintenance of polymorphism, and model selection confirmed a significant relationship between environmental heterogeneity and genetic richness within populations. Few studies have demonstrated such effects at this scale, and additional studies in other taxa should examine the generality of gene-by-environment interactions across populations to better understand the dynamics and scale of balancing selection.

Environmental heterogeneity generates opposite gene-by-environment interactions for two fitness-related traits within a population

Theory predicts that environmental heterogeneity offers a potential solution to the maintenance of genetic variation within populations, but empirical evidence remains sparse. The live-bearing fish Xiphophorus variatus exhibits polymorphism at a single locus, with different alleles resulting in up to five distinct melanistic "tailspot" patterns within populations. We investigated the effects of heterogeneity in two ubiquitous environmental variables (temperature and food availability) on two fitness-related traits (upper thermal limits and body condition) in two different tailspot types (wild-type and upper cut crescent). We found gene-by-environment (G × E) interactions between tailspot type and food level affecting upper thermal limits (UTL), as well as between tailspot type and thermal environment affecting body condition. Exploring mechanistic bases underlying these G × E patterns, we found no differences between tailspot types in hsp70 gene expression despite significant overall increases in expression under both thermal and food stress. Similarly, there was no difference in routine metabolic rates between the tailspot types. The reversal of relative performance of the two tailspot types under different environmental conditions revealed a mechanism by which environmental heterogeneity can balance polymorphism within populations through selection on different fitness-related traits.

Newly rare or newly common: evolutionary feedbacks through changes in population density and relative species abundance, and their management implications

Environmental management typically seeks to increase or maintain the population sizes of desirable species and to decrease population sizes of undesirable pests, pathogens, or invaders. With changes in population size come long-recognized changes in ecological processes that act in a density-dependent fashion. While the ecological effects of density dependence have been well studied, the evolutionary effects of changes in population size, via changes in ecological interactions with community members, are underappreciated. Here, we provide examples of changing selective pressures on, or evolution in, species as a result of changes in either density of conspecifics or changes in the frequency of heterospecific versus conspecific interactions. We also discuss the management implications of such evolutionary responses in species that have experienced rapid increases or decreases in density caused by human actions.

The contribution of spontaneous mutations to thermal sensitivity curve variation in Drosophila serrata

Many traits studied in ecology and evolutionary biology change their expression in response to a continuously varying environmental factor. One well-studied example are thermal performance curves (TPCs); continuous reaction norms that describe the relationship between organismal performance and temperature and are useful for understanding the trade-offs involved in thermal adaptation. We characterized curves describing the thermal sensitivity of voluntary locomotor activity in a set of 66 spontaneous mutation accumulation lines in the fly Drosophila serrata. Factor-analytic modeling of the mutational variance-covariance matrix, M, revealed support for three axes of mutational variation in males and two in females. These independent axes of mutational variance corresponded well to the major axes of TPC variation required for different types of thermal adaptation; "faster-slower" representing changes in performance largely independent of temperature, and the "hotter-colder" and "generalist-specialist" axes, representing trade-offs. In contrast to its near-absence from standing variance in this species, a "faster-slower" axis, accounted for most mutational variance (75% in males and 66% in females) suggesting selection may easily fix or remove these types of mutations in outbred populations. Axes resembling the "hotter-colder" and "generalist-specialist" modes of variation contributed less mutational variance but nonetheless point to an appreciable input of new mutations that may contribute to thermal adaptation.

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.

Indirect effects drive evolutionary responses to global change

Anthropogenic environmental changes pose significant threats to plant and animal populations. These changes also may affect the evolution of natural populations either directly or indirectly by altering the outcome of species interactions that are important drivers of evolution. This latter indirect pathway may be especially important for evolutionary responses to elevated atmospheric CO2 concentrations (eCO2), which appear to have minimal direct effects on plant evolution but have large effects on interspecific interactions, such as competition. We manipulated competitive and CO2 environments of experimental Arabidopsis thaliana populations to test whether eCO2 alters evolutionary trajectories indirectly by altering selection imposed by competitors. We found that interspecific competition increased selection on growth traits, reduced heritabilities, and altered genetic covariances between traits and that the magnitude of these effects depended upon the CO2 environment. Although eCO2 had minimal direct effects on evolutionary processes, eCO2 typically reduced the strength of selection imposed by competitors and, therefore, relaxed selection on plant traits when competitors were present. Our results indicate that global changes may affect plant evolution indirectly by altering competitive interactions and underscore the importance of conducting research in natural communities when attempting to predict population responses to global change.

Field-based insights to the evolution of specialization: plasticity and fitness across habitats in a specialist/generalist species pair

Factors promoting the evolution of specialists versus generalists have been little studied in ecological context. In a large-scale comparative field experiment, we studied genotypes from naturally evolved populations of a closely related generalist/specialist species pair (Polygonum persicaria and P. hydropiper), reciprocally transplanting replicates of multiple lines into open and partially shaded sites where the species naturally co-occur. We measured relative fitness, individual plasticity, herbivory, and genetic variance expressed in the contrasting light habitats at both low and high densities. Fitness data confirmed that the putative specialist out-performed the generalist in only one environment, the favorable full sun/low-density environment to which it is largely restricted in nature, while the generalist had higher lifetime reproduction in both canopy and dense neighbor shade. The generalist, P. persicaria, also expressed greater adaptive plasticity for biomass allocation and leaf size in shaded conditions than the specialist. We found no evidence that the ecological specialization of P. hydropiper reflects either genetically based fitness trade-offs or maintenance costs of plasticity, two types of genetic constraint often invoked to prevent the evolution of broadly adaptive genotypes. However, the patterns of fitness variance and herbivore damage revealed how release from herbivory in a new range can cause an introduced species to evolve as a specialist in that range, a surprising finding with important implications for invasion biology. Patterns of fitness variance between and within sites are also consistent with a possible role for the process of mutation accumulation (in this case, mutations affecting shade-expressed phenotypes) in the evolution and/or maintenance of specialization in P. hydropiper.

Fine-scale community and genetic structure are tightly linked in species-rich grasslands

Recent evidence indicates that grassland community structure and species diversity are influenced by genetic variation within species. We review what is known regarding the impact of intraspecific diversity on grassland community structure, using an ancient limestone pasture as a focal example. Two genotype-dependent effects appear to modify community structure in this system. First, the abundance of individual constituent species can depend upon the combined influence of direct genetic effects stemming from individuals within the population. Second, the outcome of localized interspecific interactions occurring within the community can depend on the genotypes of participating individuals (indicating indirect genetic effects). Only genotypic interactions are thought to be capable of allowing the long-term coexistence of both genotypes and species. We discuss the implications of these effects for the maintenance of diversity in grasslands. Next, we present new observations indicating that losses of genotypic diversity from each of two species can be predicted by the abundance of other coexisting species within experimental grassland communities. These results suggest genotype-specific responses to abundance in other coexisting species. We conclude that both direct and indirect genetic effects are likely to shape community structure and species coexistence in grasslands, implying tight linkage between fine-scale genetic and community structure.

Fitness variation and local distribution limits in an annual plant population

Understanding how genetic variation shapes species' distributions involves examining how variation is distributed across a species' range as well as how it responds to underlying environmental heterogeneity. We examined patterns of fitness variation across the local distribution of an annual composite (Lasthenia fremontii) spanning a small-scale inundation gradient in a California vernal pool wetland. Using seeds collected from the center and edge of a population, paternal half-sib families were generated and transplanted back to the center and edge of the original population. All transplants were adapted to the conditions at the center of the population. The effect of the environment on the opportunity for selection depended on the model of selection assumed. Under a model of hard selection, variance in absolute fitness was lower among transplants at the edge of the population than at the center. Under a model of soft selection, the variance in relative fitness was similar between center and edge microhabitats. Given that this population is likely well-mixed, differences in habitat quality between center and edge microhabitats will likely cause selection at the center of the population to dominate the evolutionary trajectory of this population.

Heritability, covariation and natural selection on 24 traits of common evening primrose (Oenothera biennis) from a field experiment

This study explored genetic variation and co-variation in multiple functional plant traits. Our goal was to characterize selection, heritabilities and genetic correlations among different types of traits to gain insight into the evolutionary ecology of plant populations and their interactions with insect herbivores. In a field experiment, we detected significant heritable variation for each of 24 traits of Oenothera biennis and extensive genetic covariance among traits. Traits with diverse functions formed several distinct groups that exhibited positive genetic covariation with each other. Genetic variation in life-history traits and secondary chemistry together explained a large proportion of variation in herbivory (r(2) = 0.73). At the same time, selection acted on lifetime biomass, life-history traits and two secondary compounds of O. biennis, explaining over 95% of the variation in relative fitness among genotypes. The combination of genetic covariances and directional selection acting on multiple traits suggests that adaptive evolution of particular traits is constrained, and that correlated evolution of groups of traits will occur, which is expected to drive the evolution of increased herbivore susceptibility. As a whole, our study indicates that an examination of genetic variation and covariation among many different types of traits can provide greater insight into the evolutionary ecology of plant populations and plant-herbivore interactions.

Resource specialization in a phytophagous insect: no evidence for genetically based performance trade-offs across hosts in the field or laboratory

We present a field test of the genetically based performance trade-off hypothesis for resource specialization in a population of the moth Rothschildia lebeau whose larvae primarily feed on three host plant species. Pairwise correlations between growth vs. growth, survival vs. survival and growth vs. survival across the different hosts were calculated, using families (sibships) as the units of analysis. Of 15 pairwise correlations, 14 were positive, 5 significantly so and none were negative. The same pattern was found using complementary growth and survival data from the laboratory. Overall, we found no evidence of negative genetic correlations in cross-host performance that would be indicative of performance trade-offs in this population. Rather, variation among families in performance appears to reflect 'general vigour' whereby families that perform well on one host perform well across multiple hosts. We discuss the implications of positive genetic correlations in cross-host performance in terms of the ecology and evolution of host range. We argue that this genetic architecture facilitates colonization of novel hosts and recolonization of historical hosts, therefore contributing to host shifts, host range expansions, biological invasions and introductions, and host ranges that are regionally broad but locally narrow.

Facilitation across stress gradients: the importance of local adaptation

While there is some information on genetic variation in response to competition in plants, we know nothing about intraspecific variation in facilitation. Previous studies suggest that facilitation should increase fitness in stressful environments. However, whether a plant experiences an environment as stressful may depend on prior adaptive responses to stressors at a site. Local adaptation to stress at a site may reduce the likelihood of facilitation. Seeds of Plantago erecta from stressful (serpentine soil) and non-stressful (non-serpentine soil) edaphic environments were reciprocally planted into these two soil types. Although competition did not differ significantly among seed sources, there was evidence for a local adaptation effect on facilitation. Non-serpentine seeds planted into serpentine soil exhibited greater individual plant biomass at higher densities. The interaction between population source and growth environment indicates a role for evolutionary processes such as local adaptation in the expression of facilitation in plants.

A potential resolution to the lek paradox through indirect genetic effects

Females often prefer males with elaborate traits, even when they receive no direct benefits from their choice. In such situations, mate discrimination presumably has genetic advantages; selective females will produce offspring of higher genetic quality. Over time, persistent female preferences for elaborate secondary-sexual traits in males should erode genetic variance in these traits, eventually eliminating any benefit to the preferences. Yet, strong female preferences persist in many taxa. This puzzle is called the lek paradox and raises two primary questions: do females obtain genetic benefits for offspring by selecting males with elaborate secondary-sexual characteristics and, if so, how is the genetic variation in these male traits maintained? We suggest that indirect genetic effects may help to resolve the lek paradox. Maternal phenotypes, such as habitat selection behaviours and offspring provisioning, often influence the condition and the expression of secondary-sexual traits in sons. These maternal influences are commonly genetic based (i.e. they are indirect genetic effects). Females choosing mates with elaborate traits may receive 'good genes' for daughters in the form of effective maternal characteristics. Recognizing the significance of indirect genetic effects may be important to our understanding of the process and consequences of sexual selection.

Quantitative trait loci with age-specific effects on fecundity in Drosophila melanogaster

Life-history theory and evolutionary theories of aging assume the existence of alleles with age-specific effects on fitness. While various studies have documented age-related changes in the genetic contribution to variation in fitness components, we know very little about the underlying genetic architecture of such changes. We used a set of recombinant inbred lines to map and characterize the effects of quantitative trait loci (QTL) affecting fecundity of Drosophila melanogaster females at 1 and 4 weeks of age. We identified one QTL on the second chromosome and one or two QTL affecting fecundity on the third chromosome, but these QTL affected fecundity only at 1 week of age. There was more genetic variation for fecundity at 4 weeks of age than at 1 week of age and there was no genetic correlation between early and late-age fecundity. These results suggest that different loci contribute to the variation in fecundity as the organism ages. Our data provide support for the mutation accumulation theory of aging as applied to reproductive senescence. Comparing the results from this study with our previous work on life-span QTL, we also find evidence that antagonistic pleiotropy may contribute to the genetic basis of senescence in these lines as well.

Evolution in heterogeneous environments and the potential of maintenance of genetic variation in traits of adaptive significance

The maintenance of genetic variation in traits of adaptive significance has been a major dilemma of evolutionary biology. Considering the pattern of increased genetic variation associated with environmental clines and heterogeneous environments, selection in heterogeneous environments has been proposed to facilitate the maintenance of genetic variation. Some models examining whether genetic variation can be maintained, in heterogeneous environments are reviewed. Genetic mechanisms that constrain evolution in quantitative genetic traits indicate that genetic variation can be maintained but when is not clear. Furthermore, no comprehensive models have been developed, likely due to the genetic and environmental complexity of this issue. Therefore, I have suggested two empirical approaches to provide insight for future theoretical and empirical research. Traditional path analysis has been a very powerful approach for understanding phenotypic selection. However, it requires substantial information on the biology of the study system to construct a causal model and alternatives. Exploratory path analysis is a data driven approach that uses the statistical relationships in the data to construct a set of models. For example, it can be used for understanding phenotypic selection in different environments, where there is no prior information to develop path models in the different environments. Data from Brassica rapa grown in different nutrients indicated that selection changed in the different environments. Experimental evolutionary studies will provide direct tests as to when genetic variation is maintained.

Statistical convenience vs biological insight: consequences of data transformation for the analysis of fitness variation in heterogeneous environments

In plants, more favourable environmental conditions can lead to dramatic increases in both mean fitness and variance in fitness. This results in data that violate the equality-of-variance assumption of anova, a problem that most empiricists would address by log-transforming fitness values. Using heuristic data sets and simple simulations, we show that anova on log-transformed fitness consistently fails to match the outcome of selection in a heterogeneous environment or its sensitivity to environmental frequency. Only anova based on relative fitness within environments accurately predicts the sensitivity of genotype selection to the frequency of alternative environments. Parallel analyses of variance based on absolute fitness and relative fitness can bracket the expected success of alternative genotypes under hard and soft selection, respectively. For example, for Sinapis arvensis growing in full sun and partial shade treatments, families achieving high fitness in the best environment are favoured under hard selection, whereas soft selection favours different families that achieve consistently good performance across environments. Based on these findings, we recommend that log-transformation of fitness should no longer be standard practice in ecological genetics studies. Weighted anova is a preferable method for dealing with unequal variances, and investigators should also make greater use of techniques such as quantile regression or resampling to describe and evaluate fitness variation across heterogeneous environments.

Polygenic variation maintained by balancing selection: pleiotropy, sex-dependent allelic effects and G x E interactions

We investigate three alternative selection-based scenarios proposed to maintain polygenic variation: pleiotropic balancing selection, G x E interactions (with spatial or temporal variation in allelic effects), and sex-dependent allelic effects. Each analysis assumes an additive polygenic trait with n diallelic loci under stabilizing selection. We allow loci to have different effects and consider equilibria at which the population mean departs from the stabilizing-selection optimum. Under weak selection, each model produces essentially identical, approximate allele-frequency dynamics. Variation is maintained under pleiotropic balancing selection only at loci for which the strength of balancing selection exceeds the effective strength of stabilizing selection. In addition, for all models, polymorphism requires that the population mean be close enough to the optimum that directional selection does not overwhelm balancing selection. This balance allows many simultaneously stable equilibria, and we explore their properties numerically. Both spatial and temporal G x E can maintain variation at loci for which the coefficient of variation (across environments) of the effect of a substitution exceeds a critical value greater than one. The critical value depends on the correlation between substitution effects at different loci. For large positive correlations (e.g., rho(ij)2>3/4), even extreme fluctuations in allelic effects cannot maintain variation. Surprisingly, this constraint on correlations implies that sex-dependent allelic effects cannot maintain polygenic variation. We present numerical results that support our analytical approximations and discuss our results in connection to relevant data and alternative variance-maintaining mechanisms.

Polygenic Variation Maintained by Balancing Selection: Pleiotropy, Sex-Dependent Allelic Effects and G × E Interactions

We investigate three alternative selection-based scenarios proposed to maintain polygenic variation: pleiotropic balancing selection, G × E interactions (with spatial or temporal variation in allelic effects), and sex-dependent allelic effects. Each analysis assumes an additive polygenic trait with n diallelic loci under stabilizing selection. We allow loci to have different effects and consider equilibria at which the population mean departs from the stabilizing-selection optimum. Under weak selection, each model produces essentially identical, approximate allele-frequency dynamics. Variation is maintained under pleiotropic balancing selection only at loci for which the strength of balancing selection exceeds the effective strength of stabilizing selection. In addition, for all models, polymorphism requires that the population mean be close enough to the optimum that directional selection does not overwhelm balancing selection. This balance allows many simultaneously stable equilibria, and we explore their properties numerically. Both spatial and temporal G × E can maintain variation at loci for which the coefficient of variation (across environments) of the effect of a substitution exceeds a critical value greater than one. The critical value depends on the correlation between substitution effects at different loci. For large positive correlations (e.g., ρij2>3∕4), even extreme fluctuations in allelic effects cannot maintain variation. Surprisingly, this constraint on correlations implies that sex-dependent allelic effects cannot maintain polygenic variation. We present numerical results that support our analytical approximations and discuss our results in connection to relevant data and alternative variance-maintaining mechanisms.

The contribution of spontaneous mutation to variation in environmental response in Arabidopsis thaliana: responses to nutrients

Although the evolutionary importance of spontaneous mutation is evident, its contribution to the evolution of ecological specificity remains unclear, because the environmental sensitivity of effects of new mutations has received little empirical attention. To address this issue, we report a greenhouse in which we grew plants from 20 mutation-accumulation (MA) lines, advanced by selfing and single-seed descent from a single common founder to generation 17, as well as plants from five lines representing the founder, in high and low nutrient conditions. We examined 11 traits throughout life history, including germination, survivorship, bolting date, flowering date, leaf number, leaf size, early and late height, mean fruit size, total seed weight, and reproductive biomass. Comparison of trait means between the two generations did not support the commonly held view that new mutations affecting fitness in these MA lines are strongly biased toward deleterious effects. We detected significant variance among MA lines for one fitness component, mean fruit size, but we did not detect a significant contribution of mutations accumulated in these MA lines to genotype by environment interaction (GEI). These results suggest that other evolutionary mechanisms play a more important role than spontaneous mutation alone in establishing the GEI found for wild collections and lab accessions of Arabidopsis thaliana in previous studies.

Expression of additive genetic variances and covariances for wild radish floral traits: comparison between field and greenhouse environments

Measurements of the genetic variation and covariation underlying quantitative traits are crucial to our understanding of current evolutionary change and the mechanisms causing this evolution. This fact has spurred a large number of studies estimating heritabilities and genetic correlations in a variety of organisms. Most of these studies have been done in laboratory or greenhouse settings, but it is not well known how accurately these measurements estimate genetic variance and covariance expressed in the field. We conducted a quantitative genetic half-sibling analysis on six floral traits in wild radish. Plants were grown from seed in the field and were exposed to natural environmental variation throughout their lives, including herbivory and intra- and interspecific competition. The estimates of heritabilities and the additive genetic variance-covariance matrix (G) obtained from this analysis were then compared to previous greenhouse estimates of the same floral traits from the same natural population. Heritabilities were much lower in the field for all traits, and this was due to both large increases in environmental variance and decreases in additive genetic variance. Additive genetic covariance expressed was also much lower in the field. These differences resulted in highly significant differences in the G matrix between the greenhouse and field environments using two complementary testing methods. Although the G matrices shared some principal components in common, they were not simply proportional to each other. Therefore, the greenhouse results did not accurately depict how the floral traits would respond to natural selection in the field.