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- Volume 51, 2020
Annual Review of Ecology, Evolution, and Systematics - Volume 51, 2020
Volume 51, 2020
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Arthropod Origins: Integrating Paleontological and Molecular Evidence
Vol. 51 (2020), pp. 1–25More LessPhylogenomics underpins a stable and mostly well-resolved hypothesis for the interrelationships of extant arthropods. Exceptionally preserved fossils are integrated into this framework by coding their morphological characters, as exemplified by total-evidence dating approaches that treat fossils as dated tips in analyses numerically dominated by molecular data. Cambrian fossils inform on the sequence of character acquisition in the arthropod stem group and in the stems of its main extant clades. The arthropod head problem incorporates unique appendage combinations and remains of the nervous system in fossils into a scheme mostly based on neuroanatomy and Hox expression domains for extant forms. Molecular estimates of arthropod origins in the Cryogenian or Ediacaran predate a coherent picture from the arthropod fossil record, which commences as trace fossils in the earliest Cambrian. Probabilistic morphological clock analysis of trilobites, which exemplify the earliest arthropod body fossils, supports a Cambrian origin, without the need to posit an unfossilized Ediacaran history.
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Diversification of Neotropical Freshwater Fishes
Vol. 51 (2020), pp. 27–53More LessNeotropical freshwater fishes (NFFs) constitute the most diverse continental vertebrate fauna on Earth, with more than 6,200 named species compressed into an aquatic footprint <0.5% of the total regional land-surface area and representing the greatest phenotypic disparity and functional diversity of any continental ichthyofauna. Data from the fossil record and time-calibrated molecular phylogenies indicate that most higher taxa (e.g., genera, families) diversified relatively continuously through the Cenozoic, across broad geographic ranges of the South American platform. Biodiversity data for most NFF clades support a model of continental radiation rather than adaptive radiation, in which speciation occurs mainly in allopatry, and speciation and adaptation are largely decoupled. These radiations occurred under the perennial influence of river capture and sea-level oscillations, which episodically fragmented and merged portions of adjacent river networks. The future of the NFF fauna into the Anthropocene is uncertain, facing numerous threats at local, regional, and continental scales.
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Resolving Food-Web Structure
Vol. 51 (2020), pp. 55–80More LessFood webs are a major focus and organizing theme of ecology, but the data used to assemble them are deficient. Early debates over food-web data focused on taxonomic resolution and completeness, lack of which had produced spurious inferences. Recent data are widely believed to be much better and are used extensively in theoretical and meta-analytic research on network ecology. Confidence in these data rests on the assumptions (a) that empiricists correctly identified consumers and their foods and (b) that sampling methods were adequate to detect a near-comprehensive fraction of the trophic interactions between species. Abundant evidence indicates that these assumptions are often invalid, suggesting that most topological food-web data may remain unreliable for inferences about network structure and underlying ecological and evolutionary processes. Morphologically cryptic species are ubiquitous across taxa and regions, and many trophic interactions routinely evade detection by conventional methods. Molecular methods have diagnosed the severity of these problems and are a necessary part of the cure.
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Hedgerows as Ecosystems: Service Delivery, Management, and Restoration
Vol. 51 (2020), pp. 81–102More LessHedge density, structure, and function vary with primary production and slope gradient and are subject to other diverse factors. Hedgerows are emerging ecosystems with both above- and belowground components. Functions of hedges can be categorized as provisioning, regulating, cultural, and supporting ecosystem services; these functions include food production, noncrop food and wood production, firewood production, pollination, pest control, soil conservation and quality improvement, mitigation of water flux and availability, carbon sequestration, landscape connectivity and character maintenance, and contributions to biodiversity. Urban hedges provide a relatively equitable microclimate and critical connections between green spaces and enhance human health and well-being through contact with biodiversity. Soil and water conservation are well researched in tropical hedges but less is known about their contribution to pollination, pest control, and biodiversity. Establishing a minimum hedge width and longer intervals between cutting of temperate hedges would enhance biosecurity and promote carbon sequestration and biodiversity. Hedges have a global role in mitigating biodiversity loss and climate change, which restoration should maximize, notwithstanding regional character.
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What We Don't Know About Diet-Breadth Evolution in Herbivorous Insects
Vol. 51 (2020), pp. 103–122More LessHalf a million species of herbivorous insects have been described. Most of them are diet specialists, using only a few plant species as hosts. Biologists suspect that their specificity is key to their diversity. But why do herbivorous insects tend to be diet specialists? In this review, we catalog a broad range of explanations. We review the evidence for each and suggest lines of research to obtain the evidence we lack. We then draw attention to a second major question, namely how changes in diet breadth affect the rest of a species’ biology. In particular, we know little about how changes in diet breadth feed back on genetic architecture, the population genetic environment, and other aspects of a species’ ecology. Knowing more about how generalists and specialists differ should go a long way toward sorting out potential explanations of specificity, and yield a deeper understanding of herbivorous insect diversity.
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Extending Plant Defense Theory to Seeds
Vol. 51 (2020), pp. 123–141More LessPlant defense theory explores how plants invest in defenses against natural enemies but has focused primarily on the traits expressed by juvenile and mature plants. Here we describe the diverse ways in which seeds are chemically and physically defended. We suggest that through associations with other traits, seeds are likely to exhibit defense syndromes that reflect constraints or trade-offs imposed by selection to attract dispersers, enable effective dispersal, ensure appropriate timing of seed germination, and enhance seedling performance. We draw attention to seed and reproductive traits that are analogous to defense traits in mature plants and describe how the effectiveness of defenses is likely to differ at pre- and postdispersal stages. We also highlight recent insights into the mutualistic and antagonistic interactions between seeds and microbial communities, including fungi and endohyphal bacteria, that can influence seed survival in the soil and subsequent seedling vigor.
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Origin and Evolution of the Turtle Body Plan
Vol. 51 (2020), pp. 143–166More LessThe origin of turtles and their uniquely shelled body plan is one of the longest standing problems in vertebrate biology. The unfulfilled need for a hypothesis that both explains the derived nature of turtle anatomy and resolves their unclear phylogenetic position among reptiles largely reflects the absence of a transitional fossil record. Recent discoveries have dramatically improved this situation, providing an integrated, time-calibrated model of the morphological, developmental, and ecological transformations responsible for the modern turtle body plan. This evolutionary trajectory was initiated in the Permian (>260 million years ago) when a turtle ancestor with a diapsid skull evolved a novel mechanism for lung ventilation. This key innovation permitted the torso to become apomorphically stiff, most likely as an adaption for digging and a fossorial ecology. The construction of the modern turtle body plan then proceeded over the next 100 million years following a largely stepwise model of osteological innovation.
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Our Current Understanding of Commensalism
Vol. 51 (2020), pp. 167–189More LessCommensalisms, interactions between two species in which one species benefits and the other experiences no net effect, are frequently mentioned in the ecological literature but are surprisingly little studied. Here we review and synthesize our limited understanding of commensalism. We then argue that commensalism is not a single type of interaction; rather, it is a suite of phenomena associated with distinct ecological processes and evolutionary consequences. For each form of commensalism we define, we present evidence for how, where, and why it occurs, including when it is evolutionarily persistent and when it is an occasional outcome of interactions that are usually mutualistic or antagonistic. We argue that commensalism should be of great interest in the study of species interactions due to its location at the center of the continuum between positive and negative outcomes. Finally, we offer a roadmap for future research.
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Evolutionary Dynamics and Consequences of Parthenogenesis in Vertebrates
Vol. 51 (2020), pp. 191–214More LessParthenogenesis is asexual reproduction without any required participation from males and, as such, is a null model for sexual reproduction. In a comparative context, we can expand our understanding of the evolution and ecology of sex by investigating the consequences of parthenogenesis. In this review, we examine the theoretical predictions of and empirical results on the evolution of asexual reproduction in vertebrates, focusing on recent studies addressing the origins and geographic spread of parthenogenetic lineages and the genomic consequences of an asexual life history. With advances in computational methods and genome technologies, researchers are poised to make rapid and significant progress in studying the origin and evolution of parthenogenesis in vertebrates, thus providing an important perspective on understanding biodiversity patterns of both asexual and sexual populations.
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Ecological Interactions and Macroevolution: A New Field with Old Roots
Vol. 51 (2020), pp. 215–243More LessLinking interspecific interactions (e.g., mutualism, competition, predation, parasitism) to macroevolution (evolutionary change on deep timescales) is a key goal in biology. The role of species interactions in shaping macroevolutionary trajectories has been studied for centuries and remains a cutting-edge topic of current research. However, despite its deep historical roots, classic and current approaches to this topic are highly diverse. Here, we combine historical and contemporary perspectives on the study of ecological interactions in macroevolution, synthesizing ideas across eras to build a zoomed-out picture of the big questions at the nexus of ecology and macroevolution. We discuss the trajectory of this important and challenging field, dividing research into work done before the 1970s, research between 1970 and 2005, and work done since 2005. We argue that in response to long-standing questions in paleobiology, evidence accumulated to date has demonstrated that biotic interactions (including mutualism) can influence lineage diversification and trait evolution over macroevolutionary timescales, and we outline major open questions for future research in the field.
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Genomic Prediction of (Mal)Adaptation Across Current and Future Climatic Landscapes
Vol. 51 (2020), pp. 245–269More LessSignals of local adaptation have been found in many plants and animals, highlighting the heterogeneity in the distribution of adaptive genetic variation throughout species ranges. In the coming decades, global climate change is expected to induce shifts in the selective pressures that shape this adaptive variation. These changes in selective pressures will likely result in varying degrees of local climate maladaptation and spatial reshuffling of the underlying distributions of adaptive alleles. There is a growing interest in using population genomic data to help predict future disruptions to locally adaptive gene-environment associations. One motivation behind such work is to better understand how the effects of changing climate on populations’ short-term fitness could vary spatially across species ranges. Here we review the current use of genomic data to predict the disruption of local adaptation across current and future climates. After assessing goals and motivationsunderlying the approach, we review the main steps and associated statistical methods currently in use and explore our current understanding of the limits and future potential of using genomics to predict climate change (mal)adaptation.
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Food Webs and Ecosystems: Linking Species Interactions to the Carbon Cycle
Vol. 51 (2020), pp. 271–295More LessAll species within ecosystems contribute to regulating carbon cycling because of their functional integration into food webs. Yet carbon modeling and accounting still assumes that only plants, microbes, and invertebrate decomposer species are relevant to the carbon cycle. Our multifaceted review develops a case for considering a wider range of species, especially herbivorous and carnivorous wild animals. Animal control over carbon cycling is shaped by the animals’ stoichiometric needs and functional traits in relation to the stoichiometry and functional traits of their resources. Quantitative synthesis reveals that failing to consider these mechanisms can lead to serious inaccuracies in the carbon budget. Newer carbon-cycle models that consider food-web structure based on organismal functional traits and stoichiometry can offer mechanistically informed predictions about the magnitudes of animal effects that will help guide new empirical research aimed at developing a coherent understanding of the interactions and importance of all species within food webs.
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Ecology and Neurobiology of Fear in Free-Living Wildlife
Vol. 51 (2020), pp. 297–318More LessThe ecology of fear concerns the population-, community-, and ecosystem-level consequences of the behavioral interactions between predators and prey, i.e., the aggregate impacts of individual responses to life-threatening events. We review new experiments demonstrating that fear itself is powerful enough to affect the population growth rate in free-living wild birds and mammals, and fear of large carnivores—or the human super predator—can cause trophic cascades affecting plant and invertebrate abundance. Life-threatening events like escaping a predator can have enduring, even lifelong, effects on the brain, and new interdisciplinary research on the neurobiology of fear in wild animals is both providing insights into post-traumatic stress (PTSD) and reinforcing the likely commonality of population- and community-level effects of fear in nature. Failing to consider fear thus risks dramatically underestimating the total impact predators can have on prey populations and the critical role predator-prey interactions can play in shaping ecosystems.
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Predator Effects on Plant-Pollinator Interactions, Plant Reproduction, Mating Systems, and Evolution
Vol. 51 (2020), pp. 319–340More LessPlants are the foundation of the food web and therefore interact directly and indirectly with myriad organisms at higher trophic levels. They directly provide nourishment to mutualistic and antagonistic primary consumers (e.g., pollinators and herbivores), which in turn are consumed by predators. These interactions produce cascading indirect effects on plants (either trait-mediated or density-mediated). We review how predators affect plant-pollinator interactions and thus how predators indirectly affect plant reproduction, fitness, mating systems, and trait evolution. Predators can influence pollinator abundance and foraging behavior. In many cases, predators cause pollinators to visit plants less frequently and for shorter durations. This decline in visitation can lead to pollen limitation and decreased seed set. However, alternative outcomes can result due to differences in predator, pollinator, and plant functional traits as well as due to altered interaction networks with plant enemies. Furthermore, predators may indirectly affect the evolution of plant traits and mating systems.
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What Do We Really Know About Adaptation at Range Edges?
Vol. 51 (2020), pp. 341–361More LessRecent theory and empirical evidence have provided new insights regarding how evolutionary forces interact to shape adaptation at stable and transient range margins. Predictions regarding trait divergence at leading edges are frequently supported. However, declines in fitness at and beyond edges show that trait divergence has sometimes been insufficient to maintain high fitness, so identifying constraints to adaptation at range edges remains a key challenge. Indirect evidence suggests that range expansion may be limited by adaptive genetic variation, but direct estimates of genetic constraints at and beyond range edges are still scarce. Sequence data suggest increased genetic load in edge populations in several systems, but its causes and fitness consequences are usually poorly understood. The balance between maladaptive and positive effects of gene flow on fitness at range edges deserves further study. It is becoming increasingly clear that characterizations about degree of adaptation based solely on geographical peripherality are unsupported.
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The Floral Microbiome: Plant, Pollinator, and Microbial Perspectives
Vol. 51 (2020), pp. 363–386More LessFlowers at times host abundant and specialized communities of bacteria and fungi that influence floral phenotypes and interactions with pollinators. Ecological processes drive variation in microbial abundance and composition at multiple scales, including among plant species, among flower tissues, and among flowers on the same plant. Variation in microbial effects on floral phenotype suggests that microbial metabolites could cue the presence or quality of rewards for pollinators, but most plants are unlikely to rely on microbes for pollinator attraction or reproduction. From a microbial perspective, flowers offer opportunities to disperse between habitats, but microbial species differ in requirements for and benefits received from such dispersal. The extent to which floral microbes shape the evolution of floral traits, influence fitness of floral visitors, and respond to anthropogenic change is unclear. A deeper understanding of these phenomena could illuminate the ecological and evolutionary importance of floral microbiomes and their role in the conservation of plant–pollinator interactions.
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Parallelism in Flower Evolution and Development
Vol. 51 (2020), pp. 387–408More LessFlower evolution is characterized by widespread repetition, with adaptations to pollinator environment evolving in parallel. Recent studies have expanded our understanding of the developmental basis of adaptive floral novelties—petal fusion, bilateral symmetry, heterostyly, and floral dimensions. In this article, we describe patterns of trait evolution and review developmental genetic mechanisms underlying floral novelties. We discuss the diversity of mechanisms for parallel adaptation, the evidence for constraints on these mechanisms, and how constraints help explain observed macroevolutionary patterns. We describe parallel evolution resulting from similarities at multiple hierarchical levels—genetic, developmental, morphological, functional—which indicate general principles in floral evolution, including the central role of hormone signaling. An emerging pattern is mutational bias that may contribute to rapid patterns of parallel evolution, especially if the derived trait can result from simple degenerative mutations. We argue that such mutational bias may be less likely to govern the evolution of novelties patterned by complex developmental pathways.
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The Evolution of Mutualistic Dependence
Vol. 51 (2020), pp. 409–432More LessWhile the importance of mutualisms across the tree of life is recognized, it is not understood why some organisms evolve high levels of dependence on mutualistic partnerships, while other species remain autonomous or retain or regain minimal dependence on partners. We identify four main pathways leading to the evolution of mutualistic dependence. Then, we evaluate current evidence for three predictions: (a) Mutualisms with different levels of dependence have distinct stabilizing mechanisms against exploitation and cheating, (b) less dependent mutualists will return to autonomy more often than those that are highly dependent, and (c) obligate mutualisms should be less context dependent than facultative ones. Although we find evidence supporting all three predictions, we stress that mutualistic partners follow diverse paths toward—and away from—dependence. We also highlight the need to better examine asymmetry in partner dependence. Recognizing how variation in dependence influences the stability, breakdown, and context dependence of mutualisms generates new hypotheses regarding how and why the benefits of mutualistic partnerships differ over time and space.
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The Structure of Ecological Networks Across Levels of Organization
Vol. 51 (2020), pp. 433–460More LessInteractions connect the units of ecological systems, forming networks. Individual-based networks characterize variation in niches among individuals within populations. These individual-based networks merge with each other, forming species-based networks and food webs that describe the architecture of ecological communities. Networks at broader spatiotemporal scales portray the structure of ecological interactions across landscapes and over macroevolutionary time. Here, I review the patterns observed in ecological networks across multiple levels of biological organization. A fundamental challenge is to understand the amount of interdependence as we move from individual-based networks to species-based networks and beyond. Despite the uneven distribution of studies, regularities in network structure emerge across scales due to the fundamental architectural patterns shared by complex networks and the interplay between traits and numerical effects. I illustrate the integration of these organizational scales by exploring the consequences of the emergence of highly connected species for network structures across scales.
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The Evolution of Annual and Perennial Plant Life Histories: Ecological Correlates and Genetic Mechanisms
Vol. 51 (2020), pp. 461–481More LessFlowering plants exhibit two principal life-history strategies: annuality (living and reproducing in one year) and perenniality (living more than one year). The advantages of either strategy depend on the relative benefits of immediate reproduction balanced against survivorship and future reproduction. This trade-off means that life-history strategies are associated with particular environments, with annuals being found more often in unpredictable habitats. Annuality and perenniality are the outcome of developmental genetic programs responding to their environment, with perennials being distinguished by their delayed competence to flower and reversion to growth after flowering. Evolutionary transitions between these strategies are frequent and have consequences for mating systems and genome evolution, with perennials being more likely to outcross with higher inbreeding depression and lower rates of molecular evolution. Integrating expectations from life-history theory with knowledge of the developmental genetics of flowering and seasonality is required to understand the mechanisms involved in the evolution of annual and perennial life histories.
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Previous Volumes
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Volume 54 (2023)
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Volume 53 (2022)
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Volume 52 (2021)
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Volume 51 (2020)
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Volume 50 (2019)
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Volume 49 (2018)
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Volume 48 (2017)
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Volume 47 (2016)
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Volume 46 (2015)
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Volume 45 (2014)
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Volume 44 (2013)
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Volume 43 (2012)
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Volume 42 (2011)
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Volume 41 (2010)
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Volume 40 (2009)
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Volume 39 (2008)
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Volume 38 (2007)
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Volume 37 (2006)
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Volume 36 (2005)
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Volume 35 (2004)
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Volume 34 (2003)
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Volume 33 (2002)
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Volume 32 (2001)
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Volume 31 (2000)
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Volume 30 (1999)
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Volume 29 (1998)
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Volume 28 (1997)
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Volume 27 (1996)
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Volume 26 (1995)
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Volume 25 (1994)
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Volume 24 (1993)
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Volume 23 (1992)
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Volume 22 (1991)
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Volume 21 (1990)
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Volume 20 (1989)
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Volume 19 (1988)
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Volume 18 (1987)
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Volume 17 (1986)
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Volume 16 (1985)
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Volume 15 (1984)
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Volume 14 (1983)
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Volume 13 (1982)
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Volume 12 (1981)
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Volume 11 (1980)
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Volume 10 (1979)
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Volume 9 (1978)
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Volume 8 (1977)
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Volume 7 (1976)
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Volume 6 (1975)
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Volume 5 (1974)
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Volume 4 (1973)
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Volume 3 (1972)
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Volume 2 (1971)
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Volume 1 (1970)
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Volume 0 (1932)