Annual Review of Ecology, Evolution, and Systematics - Volume 40, 2009

Specific plant associations may decrease (associational resistance, AR) or increase (associational susceptibility, AS) the likelihood of detection by, and/or vulnerability to, herbivores. We discuss presumed mechanisms leading to AR and AS, suggest others, and conduct meta-analyses on plant and herbivore traits affecting AR and AS, and the effects of habitat.

Specific plant associations determine the likelihood of detection and/or vulnerability of focal plants to herbivores. AS is more likely with insects and AR more likely with mammals. Unpalatable neighbors increase the likelihood of AR. An herbivore's feeding guild, diet breadth, and habitat type do not influence the likelihood of AR or AS. The effectiveness of AR in reducing herbivore abundance is independent of whether neighboring plants are within a plot of focal crops or along the edge of a plot. AR and AS may be applicable to associations among herbivores, and may be appropriately studied from a landscape perspective.

Sexual cannibalism, the consumption of the male by the female before, during, or after mating, can be a striking example of sexual conflict with potentially large fitness consequences for males and females. In this review, we examine how ecological and phylogenetic factors may affect the occurrence and frequency of sexual cannibalism within and among species. Ecological factors such as food and mate availability may primarily influence cannibalism by affecting the benefit of cannibalism for females. Phylogenetic factors such as feeding mode, sexual size dimorphism, certain mating behaviors, and genetic constraints may influence the vulnerability of the male to the female or the propensity of females to attack males. Although in some cases it may be difficult to separate the effects of co-occurring factors, in other cases comparative and phylogenetic approaches may aid in determining the influence of ecological and phylogenetic factors for the evolution of sexual cannibalism.

Evolutionary biology has struggled to explain the coexistence of two basic observations: Genetic variation is found in almost all traits in the presence of strong natural and sexual selection in natural populations. These two observations are in direct conflict as such selection should deplete genetic variation. Furthermore, the presence of genetic variation in a trait, and selection acting on that trait, is often not sufficient for the trait to respond to selection. Here, we bring together geometric perspectives on mutation, selection, and genetic variation and show how the perceived incompatibility between these two observations is a consequence of taking a trait-by-trait approach to the multivariate problem of genetic variation and selection. We conclude that the simultaneous presence of widespead genetic variation in, and strong selection on, individual traits indicates that substantial multivariate genetic constraints are likely to be present in natural populations.

Although no mature tropical tree had ever been exposed to an experimentally CO2-enriched atmosphere, evidence from works with seedlings and saplings, theory, models, and proxy data suggest effects on biodiversity and enhanced forest dynamics. Tropical forest carbon stocking is unlikely to increase, given that carbon pool size is driven by tree and patch demography, with tree longevity unlikely to increase. Unlike epiphytes, tropical lianas are likely to play a more important role in a CO2-rich future.

Although most studies of factors contributing to successful establishment and spread of non-native species have focused on species traits and characteristics (both biotic and abiotic), increasing empirical and statistical evidence implicates propagule pressure—propagule sizes, propagule numbers, and temporal and spatial patterns of propagule arrival—as important in both facets of invasion. Increasing propagule size enhances establishment probability primarily by lessening effects of demographic stochasticity, whereas propagule number acts primarily by diminishing impacts of environmental stochasticity. A continuing rain of propagules, particularly from a variety of sources, may erase or vitiate the expected genetic bottleneck for invasions initiated by few individuals (as most are), thereby enhancing likelihood of survival. For a few species, recent molecular evidence suggests ongoing propagule pressure aids an invasion to spread by introducing genetic variation adaptive for new areas and habitats. This phenomenon may also explain some time lags between establishment of a non-native species and its spread to become an invasive pest.

Modern evolutionary biology is founded on the Mendelian-genetic model of inheritance, but it is now clear that this model is incomplete. Empirical evidence shows that environment (encompassing all external influences on the genome) can impose transgenerational effects and generate heritable variation for a broad array of traits in animals, plants, and other organisms. Such effects can be mediated by the transmission of epigenetic, cytoplasmic, somatic, nutritional, environmental, and behavioral variation. Building on the work of many authors, we outline a general framework for conceptualizing nongenetic inheritance and its evolutionary implications. This framework shows that, by decoupling phenotypic change from the genotype, nongenetic inheritance can circumvent the limitations of genetic inheritance and thereby influence population dynamics and alter the fitness landscape. The weight of theory and empirical evidence indicates that nongenetic inheritance is a potent factor in evolution that can engender outcomes unanticipated under the Mendelian-genetic model.

Inherited microorganisms that manipulate the reproduction of their host are a common feature in arthropod biology. Although research initially concentrated on why these manipulations were observed, more recent study has emphasized the profound effects they may have on the ecology and evolution of their host. We review the natural history and evolutionary ecology of inherited reproductive parasites, before examining their impact on host ecology and evolution. We posit that sex-ratio distorting microorganisms sometimes dominate their host's microevolution and reproductive ecology, driving extremely rapid natural selection, altering the molecular evolution landscape, and potentially causing evolution in conserved systems such as sex determination. The evolutionary importance of symbionts inducing cytoplasmic incompatibility lies more in the barriers to gene flow they can produce, which may then contribute to reproductive isolation and speciation. Throughout, we link theory with empirical data, point to areas of ignorance, and identify promising avenues of future research.

Mutation accumulation (MA) experiments, in which mutations are allowed to drift to fixation in inbred lines, have been a principal way of studying the rates and properties of new spontaneous mutations. Phenotypic assays of MA lines inform us about the nature of new mutational variation for quantitative traits and provide estimates of the genomic rate and the distribution of effects of new mutations. Parameter estimates compared for a range of species suggest that the genomic mutation rate varies by several orders of magnitude and that the distribution of effects tends to be dominated by large-effect mutations. Some experiments suggest synergistic interactions between the effects of spontaneous deleterious mutations, whereas others do not. There is little reliable information on the distribution of dominance effects of new mutations. Most evidence does not suggest strong dependency of the effects of new mutations on the environment. Information from phenotypic assays has recently been augmented by direct molecular estimates of the mutation rate.

The shallow, tropical reef environment differs from other marine environments in its more intense competition for space, more limited nutrient concentrations, proliferation of clonal animals, and greater habitat complexity. The evolutionary consequences of these ecologic peculiarities are still poorly understood, but they seem to cause greater turnover rates of reef taxa than nonreef taxa and an especially volatile record of reefs on geologic timescales. The boom and bust pattern of Phanerozoic reef construction is impossible to explain by linear responses to physicochemical changes. Threshold effects appear to be involved not only in reef crises but also in reef expansions. Long-term climate change seems to influence the biotic composition of reefs, but neither climate nor sea-level nor chemical changes in the oceans can elucidate the waxing and waning of reefs. Biological factors affecting spatial competition are thus probably more important than geologic controls on reef evolution.

The dispersal process, by which individuals or other dispersing agents such as gametes or seeds move from birthplace to a new settlement locality, has important consequences for the dynamics of genes, individuals, and species. Many of the questions addressed by ecology and evolutionary biology require a good understanding of species’ dispersal patterns. Much effort has thus been devoted to overcoming the difficulties associated with dispersal measurement. In this context, genetic tools have long been the focus of intensive research, providing a great variety of potential solutions to measuring dispersal. This methodological diversity is reviewed here to help (molecular) ecologists find their way toward dispersal inference and interpretation and to stimulate further developments.

Developmental genetic pathways involved in flower formation in model plants such as Arabidopsis and maize enable us to identify genes, gene families, and gene networks that are involved in the regulation of flower initiation, growth and differentiation. These genes can then function as “candidate genes” and their expression, function, and biochemical interactions can be explored in other lineages to determine if they provide a necessary and sufficient toolkit for the development of the flower. Likewise, a view to the fossil record can provide documentation of reproductive innovations occurring within gymnosperms or along the stem lineage leading to angiosperms, elucidating the transitions required for the evolution of the angiosperm flower from an ancestral reproductive strobilus. Here we discuss the origin and subsequent evolution in form of the flower, highlighting recent studies in paleobotany, morphology, evolution, and developmental genetics with the goal of outlining advances in our understanding of flower evolution.

Biotic interactions are believed to play a role in the origin and maintenance of species diversity, and multiple hypotheses link the latitudinal diversity gradient to a presumed gradient in the importance of biotic interactions. Here we address whether biotic interactions are more important at low latitudes, finding support for this hypothesis from a wide range of interactions. Some of the best-supported examples are higher herbivory and insect predation in the tropics, and predominantly tropical mutualisms such as cleaning symbioses and ant-plant interactions. For studies that included tropical regions, biotic interactions were never more important at high latitudes. Although our results support the hypothesis that biotic interactions are more important in the tropics, additional research is needed, including latitudinal comparisons of rates of molecular evolution for genes involved in biotic interactions, estimates of gradients in interaction strength, and phylogenetic comparisons of the traits that mediate biotic interactions.

The placenta is a complex organ that mediates all physiological and endocrine interactions between mother and developing embryos. Placentas have evolved throughout the animal kingdom, but little is known about how or why the placenta evolved. We review hypotheses about the evolution of placentation and examine empirical evidence in support for these hypotheses by drawing on insights from the fish family Poeciliidae. The placenta evolved multiple times within this family, and there is a remarkable diversity in its form and function among closely related species, thus providing us with ideal material for studying its evolution. Current hypotheses fall into two categories: adaptive hypotheses, which propose that the placenta evolved as an adaptation to environmental pressures, and conflict hypotheses, which posit that the placenta evolved as a result of antagonistic coevolution. These hypotheses are not mutually exclusive. Each may have played a role at different stages of the evolutionary process.

Successful dispersal between populations leaves a genetic wake that can reveal historical and contemporary patterns of connectivity. Genetic studies of differentiation in the sea suggest the role of larval dispersal is often tempered by adult ecology, that changes in differentiation with geographic distance are limited by disequilibrium between drift and migration, and that phylogeographic breaks reflect shared barriers to movement in the present more than common historical divisions. Recurring complications include the presence of cryptic species, selection on markers, and a failure to account for differences in heterozygosity among markers and species. A better understanding of effective population sizes is needed. Studies that infer parentage or kinship and coalescent analyses employing more markers are both likely to spur progress, with analyses based on linkage disequilibrium potentially bridging results from these studies and reconciling patterns that vary at ecological and evolutionary timescales.

Latex is a sticky emulsion that exudes upon damage from specialized canals in about 10% of flowering plant species. Latex has no known primary metabolic function and has been strongly implicated in defense against herbivorous insects. Here we review historical hypotheses about the function of latex, evidence that it serves as a potent defense, and the chemistry and mode of action of the major constituent defense chemicals and proteins across a diversity of plant species. We further attempt to synthesize the characteristics of latex as a coordinated plant defense system. Herbivores that feed on latex-bearing plants typically evade contact with latex by severing the laticifers or feeding intercellularly, or may possess physiological adaptations. Convergent evolution appears to be rampant both in plants with latex and insects that exploit latex-bearing plants. Because latex shows phenotypic plasticity, heritability, and macoevolutionary lability, it is an ideal system to study plant-herbivore interactions using evolutionary approaches.

Taxon-based research in evolution permits the development of a multidimensional approach, illustrated here with lessons learned from research on salamanders. The clade is widespread and diverse, yet sufficiently small that one can keep all of the species in mind. This facilitates research from diverse perspectives: systematics and phylogenetics, morphology, development, ecology, neurobiology, behavior, and physiology. Different avenues of research offer unique perspectives on how a relatively old vertebrate clade has diversified. An integrated, hierarchically organized, multidimensional program of research on a taxon illuminates many general principles and processes. Among these are the nature of species and homology, adaptation and adaptive radiations, size and shape in relation to issues in organismal integration, ontogeny and development in relation to phylogeny, the ubiquity of homoplasy, ecological niche conservation, species formation, biodiversity, and conservation. Opportunities for future research and threats to the continued existence of salamanders are briefly outlined.

RNA viruses are the main agents of emerging disease. To understand how RNA viruses are able to jump species boundaries and spread in new hosts it is essential to determine the basic processes of evolutionary change in these infectious agents. RNA virus evolution is largely shaped by very high rates of mutation. This, coupled with potentially enormous intra- and interhost population sizes and continual replication, allows the rapid production of genetic diversity, including those mutations that facilitate host adaptation. However, high mutation rates also act to constrain aspects of RNA virus evolution, as the majority of mutations, including many at synonymous sites, are deleterious, which in turn places an upper limit on genome size. Ironically, although RNA viruses are characterized by their mutation rates, these rates may not be high enough to allow the onset of quasispecies dynamics, in which natural selection acts on the viral population as a whole.

Belowground-feeding herbivores may be very destructive to plants. Roots are known to produce various defense compounds to protect themselves against these herbivores, both with direct and indirect—inducible—defense compounds. Recent literature reviews reveal no overall pattern for root-shoot defense allocation. Optimal defense allocation patterns within roots may be predicted with an ecophysiological model taking into account the value and vulnerability of root classes. Induced responses elicited by root herbivores are likely to result in systemic responses in the shoots. These systemic responses may affect aboveground herbivores and higher trophic levels. This calls into question whether root-to-shoot systemic induction is an adaptive response. Physiological responses conferring tolerance may co-occur with resistance responses, depending on the biotic and abiotic environment of the roots. More detailed analyses of root defenses and the feeding sites of herbivores are needed to gain a better understanding of optimal defense allocation in roots.

Ecological communities are constantly responding to environmental change. Theory and evidence suggest that the loss or decline of stress-intolerant species can be compensated for by the growth of other species. Compensatory dynamics are a long-term feature of community dynamics across a broad range of models, and they can have strong stabilizing effects at the community level. Coexistence theory indicates that distinct environmental responses are required for compensatory dynamics and deemphasizes competition. Compensatory dynamics have been detected under experimental conditions, but are not dominant in a metanalysis of field surveys. Recent progress has been made in quantitative methods that detect compensatory dynamics at different temporal scales. Appropriate null models are required to sharpen our understanding of compensatory dynamics in nature. An integrated theory of compensation and compensatory dynamics will improve our ability to understand when communities maintain sufficient response diversity to buffer the effects of environmental change and anthropogenic stress.

Species range limits involve many aspects of evolution and ecology, from species distribution and abundance to the evolution of niches. Theory suggests myriad processes by which range limits arise, including competitive exclusion, Allee effects, and gene swamping; however, most models remain empirically untested. Range limits are correlated with a number of abiotic and biotic factors, but further experimentation is needed to understand underlying mechanisms. Range edges are characterized by increased genetic isolation, genetic differentiation, and variability in individual and population performance, but evidence for decreased abundance and fitness is lacking. Evolution of range limits is understudied in natural systems; in particular, the role of gene flow in shaping range limits is unknown. Biological invasions and rapid distribution shifts caused by climate change represent large-scale experiments on the underlying dynamics of range limits. A better fusion of experimentation and theory will advance our understanding of the causes of range limits.