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Am Nat 2020 May 8;195(5):833-850. Epub 2020 Apr 8.

Global change may induce changes in savanna and forest distributions, but the dynamics of these changes remain unclear. Classical biome theory suggests that climate is predictive of biome distributions, such that shifts will be continuous and reversible. This view, however, cannot explain the overlap in the climatic ranges of tropical biomes, which some argue may result from fire-vegetation feedbacks, maintaining savanna and forest as bistable states. Read More

Am Nat 2020 May 12;195(5):927-933. Epub 2020 Mar 12.

Mating signals have evolved to attract target receivers, even to the point of exploiting receivers through perceptual manipulation. Signals, however, can also expose signalers to nontarget receivers, including predators and parasites, and thus have also evolved to decrease enemy attraction. Here we show that male tree frogs () reduce their attractiveness to eavesdropping enemies (bats and midges) by overlapping their calls at near-perfect synchrony with the calls of neighboring conspecifics. Read More

Am Nat 2020 May 24;195(5):772-787. Epub 2020 Mar 24.

Evolution is never truly predictable, in part because the process of selection is recursive: it operates on its own output to generate historical contingencies, so emergent traits can reshape how others evolve in the future. Studies rarely attempt to directly trace how recursion underlies present-day phenotypic pattern on a macroevolutionary basis. To address this gap, we examined how different selection regimes-each operating on a different timescale-guide the evolution of the woodpecker drum display. Read More

Am Nat 2020 May 18;195(5):868-875. Epub 2020 Mar 18.

Inclusive fitness theory predicts that individuals can increase their indirect fitness by grouping with kin. However, kin grouping also increases competition between kin, which potentially outweighs its benefits. The level of kin competition is contingent on environmental conditions and thus highly variable. Read More

Am Nat 2020 May 20;195(5):788-801. Epub 2020 Mar 20.

Blood oxygen-carrying capacity is one of the important determinants of the amount of oxygen supplied to the tissue per unit time and plays a key role in oxidative metabolism. In wild vertebrates, blood oxygen-carrying capacity is most commonly measured with the total blood hemoglobin concentration (Hb) and hematocrit (Hct), which is the volume percentage of red blood cells in blood. Here, I used published estimates of avian Hb and Hct (nearly 1,000 estimates from 300 species) to examine macroevolutionary patterns in the oxygen-carrying capacity of blood in birds. Read More

Am Nat 2020 May 19;195(5):802-817. Epub 2020 Mar 19.

Variation in species richness across environmental gradients results from a combination of historical nonequilibrium processes (time, speciation, extinction) and present-day differences in environmental carrying capacities (i.e., ecological limits affected by species interactions and the abundance and diversity of resources). Read More

Am Nat 2020 May 17;195(5):899-917. Epub 2020 Mar 17.

Is speciation generally a "special time" in morphological evolution, or are lineage-splitting events just "more of the same" where the end product happens to be two separate lineages? Data on evolutionary dynamics during anagenetic and cladogenetic events among closely related lineages within a clade are rare, but the fossil record of the bryozoan genus is considered a textbook example of a clade where speciation causes rapid evolutionary change against a backdrop of morphological stasis within lineages. Here, we point to some methodological and measurement theoretical issues in the original work on . We then reanalyze a subset of the original data that can be meaningfully investigated using quantitative statistical approaches similar to those used in the original studies. Read More

Am Nat 2020 May 18;195(5):851-867. Epub 2020 Mar 18.

Emigration is a fundamental process affecting species' local, regional, and large-scale dynamics. The paradigmatic view in ecology is that emigration is density independent (DIE) or positive density dependent (+DDE). However, alternative forms are biologically plausible, including negative (-DDE), U-shaped (uDDE), and hump-shaped (hDDE) forms. Read More

Am Nat 2020 May 7;195(5):E132-E149. Epub 2020 Apr 7.

In animals, bright colors often evolve to mimic other species when a resemblance is selectively favored. Understanding the proximate mechanisms underlying such color mimicry can give insights into how mimicry evolves-for example, whether color convergence evolves from a shared set of mechanisms or through the evolution of novel color production mechanisms. We studied color production mechanisms in poison frogs (Dendrobatidae), focusing on the mimicry complex of . Read More

Am Nat 2020 May 30;195(5):753-771. Epub 2020 Mar 30.

The time taken for a selectively favorable allele to spread through a single population was investigated early in the history of population genetics. The resulting formulas are based on deterministic dynamics, leading to inaccuracies at allele frequencies close to 0 or 1. To remedy this problem, the properties of the stochastic phases at either end point of allele frequency need to be analyzed. Read More

Am Nat 2020 May 31;195(5):918-926. Epub 2020 Mar 31.

Parasites have evolved a diversity of lifestyles that exploit the biology of their hosts. Some nematodes that parasitize mammals pass via the placenta or milk from one host to another. Similar cases of vertical transmission have never been reported in avian and nonavian reptiles, suggesting that egg laying may constrain the means of parasite transmission. Read More

Am Nat 2020 May 17;195(5):876-885. Epub 2020 Mar 17.

When the strength or nature of a host-parasite interaction changes over the host life cycle, the consequences of parasitism can depend on host population age structure. Avian brood parasites reduce hosts' breeding success, and host age may play a role in this interaction if younger hosts are more likely parasitized and/or less able to defend themselves. We analyzed whether the age of female magpie () hosts is associated with parasite attack or their ability to reject foreign eggs. Read More

Am Nat 2020 May 20;195(5):818-832. Epub 2020 Mar 20.

Both benign and harsh environments promote the evolution of sociality. This paradox-societies occur in environments of such contrasting quality-may be explained by the different types of benefits that individuals receive from grouping: resource defense benefits that derive from group-defended critical resources versus collective action benefits that result from social cooperation among group members. Here, we investigate cooperative behavior in the burying beetle along an elevational gradient where environmental quality (climate and competition) varies with altitude. Read More

Am Nat 2020 May 24;195(5):886-898. Epub 2020 Mar 24.

Most theory on the evolution of senescence implicitly assumes that all offspring are of equal quality. However, in addition to age-related declines in survival and fecundity (classically defined senescence), many organisms exhibit age-related declines in offspring quality, a phenomenon known as a parental age effect. Theoretical work suggests that parental age effects may alter age trajectories of selection and therefore shape the evolution of senescence; however, to date, these analyses have been limited to idealized life cycles and models of maternal care in human populations. Read More

Am Nat 2020 May 2;195(5):E118-E131. Epub 2020 Apr 2.

Many parasites infect multiple species and persist through a combination of within- and between-species transmission. Multispecies transmission networks are typically constructed at the species level, linking two species if any individuals of those species interact. However, generalist species often consist of specialized individuals that prefer different subsets of available resources, so individual- and species-level contact networks can differ systematically. Read More

Am Nat 2020 Apr 2;195(4):664-677. Epub 2020 Mar 2.

Learning is an adaptation that allows individuals to respond to environmental stimuli in ways that improve their reproductive outcomes. The degree of sophistication in learning mechanisms potentially explains variation in behavioral responses. Here, we present a model of learning that is inspired by documented intra- and interspecific variation in the performance of a simultaneous two-choice task, the biological market task. Read More

Am Nat 2020 Apr 3;195(4):678-690. Epub 2020 Mar 3.

Life-history theory predicts that investment per offspring should correlate negatively with the quality of the environment that offspring are anticipated to encounter; parents may use their own experience as juveniles to predict this environment and may modulate offspring traits, such as growth capacity and initial size. We manipulated nutrient levels in the juvenile habitat of wild Atlantic salmon () to investigate the hypothesis that the egg size that maximizes juvenile growth and survival depends on environmental quality. We also tested whether offspring traits were related to parental growth trajectory. Read More

Am Nat 2020 Apr 28;195(4):E112-E117. Epub 2020 Feb 28.

In a recent modeling study ("Limiting Similarity? The Ecological Dynamics of Natural Selection among Resources and Consumers Caused by Both Apparent and Resource Competition") that appeared in the April 2019 issue of , Mark A. McPeek argued that ecologically equivalent species may emerge via competition-induced trait convergence, in conflict with naive expectations based on the limiting similarity principle. Although the emphasis on the possibility of the convergence of competitors is very timely, here we show that the proposed mechanism will only lead to actual coexistence in the converged state for specially chosen fine-tuned parameter settings. Read More

Am Nat 2020 Apr 13;195(4):636-648. Epub 2020 Feb 13.

The ability of prey to assess predation risk is fundamental to their success. It is routinely assumed that predator cues do not vary in reliability across levels of predation risk. We propose that cues can differ in how precisely they indicate different levels of predation risk. Read More

Am Nat 2020 Apr 3;195(4):616-635. Epub 2020 Mar 3.

A key assumption of epidemiological models is that population-scale disease spread is driven by close contact between hosts and pathogens. At larger scales, however, mechanisms such as spatial structure in host and pathogen populations and environmental heterogeneity could alter disease spread. The assumption that small-scale transmission mechanisms are sufficient to explain large-scale infection rates, however, is rarely tested. Read More

Am Nat 2020 Apr 3;195(4):603-615. Epub 2020 Mar 3.

Variable, changing climates may affect each participant in a biotic interaction differently. We explored the effects of temperature and plasticity on the outcome of a host-pathogen interaction to try to predict the outcomes of infection under fluctuating temperatures. We infected crickets with the entomopathogenic fungus under constant (6°, 12°, 18°, or 25°C) or fluctuating (from 6° to 18°C or from 6° to 25°C) temperatures. Read More

Am Nat 2020 Apr 4;195(4):577-590. Epub 2020 Mar 4.

Although root traits play a critical role in mediating plant-plant interactions and resource acquisition from the soil environment, research examining whether and how belowground competition can influence the evolution of root traits remains largely unexplored. Here we examine the possibility that root traits may evolve as a target of selection from interspecific competition using and , two closely related morning glory species that commonly co-occur in the United States, as a model system. We show that belowground competitive interactions between the two species can alter the pattern of selection on root traits in each species. Read More

Am Nat 2020 Apr 3;195(4):591-602. Epub 2020 Feb 3.

The potential for either pathogens or mutualists to alter the outcome of interactions between host species has been clearly demonstrated experimentally, but our understanding of their joint influence remains limited. Individually, pathogens and mutualists can each stabilize (via negative feedback) or destabilize (via positive feedback) host-host interactions. When pathogens and mutualists are both present, the potential for simultaneous positive and negative feedbacks can generate a wide range of possible effects on host species coexistence and turnover. Read More

Am Nat 2020 Apr 28;195(4):733-742. Epub 2020 Feb 28.

Communication requires both the encoding of information and its effective transmission, but little is known about display traits that primarily serve to enhance efficacy. Here we examined the visual courtships of , a cursorial fly that lives and mates in heterogeneous foreshores, and tested the prediction that males should seek to enhance signal salience and consequent fitness through the flexible choice of display locations. We show that courting males access the field of view of females by straddling them and holding their wings closed before moving ahead to present their structurally colored faces in ritualized dances. Read More

Am Nat 2020 Apr 28;195(4):705-716. Epub 2020 Jan 28.

The distribution of biodiversity depends on the combined and interactive effects of ecological and evolutionary processes. The joint contribution of these processes has focused almost exclusively on deterministic effects, even though mechanisms that increase the importance of random ecological processes are expected to also increase the importance of random evolutionary processes. Here we manipulate the sizes of old field fragments to generate correlated sampling effects for a focal population (a gall maker) and its enemy community. Read More

Am Nat 2020 Apr 4;195(4):717-732. Epub 2020 Mar 4.

Adaptation is often described in behavioral ecology as individuals maximizing their inclusive fitness. Under what conditions does this hold, and how does this relate to the gene-centered perspective of adaptation? We unify and extend the literature on these questions to class-structured populations. We demonstrate that the maximization (in the best-response sense) of class-specific inclusive fitness obtains in uninvadable population states (meaning that all deviating mutants become extinct). Read More

Am Nat 2020 Apr 9;195(4):691-704. Epub 2020 Mar 9.

Water limitation is a primary driver of plant geographic distributions and individual plant fitness. Drought resistance is the ability to survive and reproduce despite limited water, and numerous studies have explored its physiological basis in plants. However, it is unclear how drought resistance and trade-offs associated with drought resistance evolve within plant clades. Read More

Am Nat 2020 Apr 10;195(4):E100-E111. Epub 2020 Feb 10.

Trophic strategy determines stoichiometry of plankton. In general, heterotrophic zooplankton have lower and more stable C∶N and C∶P ratios than photoautotrophic phytoplankton, whereas mixotrophic protists, which consume prey and photosynthesize, have stoichiometry between zooplankton and phytoplankton. As trophic strategies change with cell size, body size may be a key trait influencing eukaryotic plankton stoichiometry. Read More

Am Nat 2020 Apr 9;195(4):743-751. Epub 2020 Mar 9.

Selfish genetic elements that gain a transmission advantage through the destruction of sperm have grave implications for drive male fertility. In the X-linked meiotic drive system (SR) of a stalk-eyed fly, we found that SR males have greatly enlarged testes and maintain high fertility despite the destruction of half of their sperm, even when challenged with fertilizing large numbers of females. Conversely, we observed reduced allocation of resources to the accessory glands that probably explains the lower mating frequency of SR males. Read More

Am Nat 2020 Apr 28;195(4):649-663. Epub 2020 Feb 28.

Shadow competition, the interception of prey by sit-and-wait predators closest to the source of prey arrival, is prevalent in clusters of sit-and-wait predators. Peripheral positions in the cluster receive more prey and should thus be more frequently occupied. Models predicting spatial positioning in groups, however, usually ignore variability among group members. Read More

Am Nat 2020 Mar 15;195(3):E67-E86. Epub 2020 Jan 15.

When environmental variation is spatially continuous, dispersing individuals move among nearby sites with similar habitat conditions. But as an environmental gradient becomes steeper, gene flow may connect more divergent habitats, and this is predicted to reduce the slope of the adaptive cline that evolves. We compared quantitative genetic divergence of frog populations along a 2,000-m elevational gradient in eastern Switzerland (new experimental results) with divergence along a 1,550-km latitudinal gradient in Fennoscandia (previously published results). Read More

Am Nat 2020 Mar 17;195(3):569-576. Epub 2020 Jan 17.

Extinction threatens many species yet is predicted by few factors across the plant tree of life (ToL). Taxon age is one factor that may associate with extinction if occupancy of geographic and adaptive zones varies with time, but evidence for such an association has been equivocal. Age-dependent occupancy can also influence diversification rates and thus extinction risk where new taxa have small range and population sizes. Read More

Am Nat 2020 Mar 17;195(3):432-444. Epub 2020 Jan 17.

A major focus of ecology is to understand and predict ecosystem function across scales. Many ecosystem functions are measured only at local scales, while their effects occur at a landscape level. Here we investigate how landscape-scale predictions of ecosystem function depend on intraspecific competition, a fine-scale process, by manipulating intraspecific density of shredding macroinvertebrates and examining effects on leaf litter decomposition, a key function in freshwater ecosystems. Read More

Am Nat 2020 Mar 15;195(3):561-568. Epub 2020 Jan 15.

Most population genetic theory assumes that populations adapt to an environmental change without a change in population size. However, environmental changes might be so severe that populations decline in size and, without adaptation, become extinct. This "evolutionary rescue" scenario differs from traditional models of adaptation in that rescue involves a race between adaptation and extinction. Read More

Am Nat 2020 Mar 30;195(3):445-462. Epub 2020 Jan 30.

Ecosystems are under threat from anthropogenic and natural disturbances, yet little is known about how these disturbances alter mutualistic interactions. Many mutualistic interactions are highly context dependent and dynamic due to "ongoing" partner choice, impeding our understanding of how disturbances might influence mutualistic systems. Previously we showed that in the absence of additional known mechanisms of competitive coexistence, mutualistic fungi can coexist in a system where the plant community associates dynamically with two empirically defined arbuscular mycorrhizal fungal types: a cheap kind that provides low nutrient benefits, and an expensive type that provides high nutrient benefits. Read More

Am Nat 2020 Mar 9;195(3):524-533. Epub 2020 Jan 9.

Egg trading-whereby simultaneous hermaphrodites exchange each other's eggs for fertilization-constitutes one of the few rigorously documented and most widely cited examples of direct reciprocity among unrelated individuals. Yet how egg trading may initially invade a population of nontrading simultaneous hermaphrodites is still unresolved. Here, we address this question with an analytical model that considers mate encounter rates and costs of egg production in a population that may include traders (who provide eggs for fertilization only if their partners also have eggs to reciprocate), providers (who provide eggs regardless of whether their partners have eggs to reciprocate), and withholders (cheaters who mate only in the male role and just use their eggs to elicit egg release from traders). Read More

Am Nat 2020 Mar 30;195(3):E87-E99. Epub 2020 Jan 30.

Understanding the dynamics of biological invasions is crucial for managing numerous phenomena, from invasive species to tumors. While the Allee effect (where individuals in low-density populations suffer lowered fitness) is known to influence both the ecological and the evolutionary dynamics of an invasion, the possibility that an invader's susceptibility to the Allee effect might itself evolve has received little attention. Since invasion fronts are regions of perpetually low population density, selection should be expected to favor vanguard invaders that are resistant to Allee effects. Read More

Am Nat 2020 Mar 16;195(3):463-484. Epub 2020 Jan 16.

In large clonal populations, several clones generally compete, resulting in complex evolutionary and ecological dynamics: experiments show successive selective sweeps of favorable mutations as well as long-term coexistence of multiple clonal strains. The mechanisms underlying either coexistence or fixation of several competing strains have rarely been studied altogether. Conditions for coexistence have mostly been studied by population and community ecology, while rates of invasion and fixation have mostly been studied by population genetics. Read More

Am Nat 2020 Mar 15;195(3):504-523. Epub 2020 Jan 15.

In deterministic models of epidemics, there is a host abundance threshold above which the introduction of a few infected individuals leads to a severe epidemic. Studies of weather-driven animal pathogens often assume that abundance thresholds will be overwhelmed by weather-driven stochasticity, but tests of this assumption are lacking. We collected observational and experimental data for a fungal pathogen, , that infects the gypsy moth, . Read More

Am Nat 2020 Mar 29;195(3):412-431. Epub 2020 Jan 29.

Understanding how spatially variable selection shapes adaptation is an area of long-standing interest in evolutionary ecology. Recent meta-analyses have quantified the extent of local adaptation, but the relative importance of abiotic and biotic factors in driving population divergence remains poorly understood. To address this gap, we combined a quantitative meta-analysis and a qualitative metasynthesis to (1) quantify the magnitude of local adaptation to abiotic and biotic factors and (2) characterize major themes that influence the motivation and design of experiments that seek to test for local adaptation. Read More

Am Nat 2020 Mar 30;195(3):395-411. Epub 2020 Jan 30.

Adaptation to local conditions can increase species' geographic distributions and rates of diversification, but which components of the environment commonly drive local adaptation-particularly the importance of biotic interactions-is unclear. Biotic interactions should drive local adaptation when they impose consistent divergent selection; if this is common, we expect transplant experiments to detect more frequent and stronger local adaptation when biotic interactions are left intact. We tested this hypothesis using a meta-analysis of transplant experiments from >125 studies (mostly of plants). Read More

Am Nat 2020 Mar 27;195(3):485-503. Epub 2020 Jan 27.

Organisms encounter a wide range of toxic compounds in their environments, from chemicals that serve anticonsumption or anticompetition functions to pollutants and pesticides. Although we understand many detoxification mechanisms that allow organisms to consume toxins typical of their diet, we know little about why organisms vary in their ability to tolerate entirely novel toxins. We tested whether variation in generalized stress responses, such as antioxidant pathways, may underlie variation in reactions to novel toxins and, if so, their associated costs. Read More

Am Nat 2020 Mar 15;195(3):534-546. Epub 2020 Jan 15.

The ontogeny of seed plants usually involves a dormant dehydrated state and the breaking of dormancy and germination, which distinguishes it from that of most organisms. Seed germination and seedling establishment are critical ontogenetic stages in the plant life cycle, and both are fueled by respiratory metabolism. However, the scaling of metabolic rate with respect to individual traits remains poorly understood. Read More

Am Nat 2020 Mar 17;195(3):547-560. Epub 2020 Jan 17.

Marsupials show a smaller range of forelimb ecomorphologies than placental mammals, and it is hypothesized that this results from macroevolutionary constraints imposed by the specialized reproductive biology of marsupials. Specifically, the accelerated development of the marsupial forelimb allows neonates to crawl to the mother's pouch but may constrain adult morphology. This hypothesis makes three main predictions: (i) that marsupial forelimbs should show less interspecific disparity than their hindlimbs, (ii) that morphological integration within the marsupial forelimb is stronger than integration between limbs, and (iii) that these patterns should be strongest in diprotodontians, which undergo the most rigorous crawls as neonates. Read More

Am Nat 2020 02 27;195(2):275-283. Epub 2019 Dec 27.

Orchids are globally distributed, a feature often attributed to their tiny dustlike seeds. They were ancestrally terrestrial but in the Eocene expanded into tree canopies, with some lineages later returning to the ground, providing an evolutionarily replicated system. Because seeds are released closer to the ground in terrestrial species than in epiphytic ones, seed traits in terrestrials may have been under selective pressure to increase seed dispersal efficiency. Read More

Am Nat 2020 02 23;195(2):231-246. Epub 2019 Dec 23.

Spreading populations are subject to evolutionary processes acting on dispersal and reproduction that can increase invasion speed and variability. It is typically assumed that dispersal and demography traits evolve independently, but abundant evidence points to correlations between them that may be positive or negative and genetic, maternal, or environmental. We sought to understand how demography-dispersal correlations modify the eco-evolutionary dynamics of range expansion. Read More

Am Nat 2020 02 13;195(2):181-191. Epub 2020 Jan 13.

Insect flight is made possible by different morphological structures: wings produce the lift, the thorax drives the wings' movements, and the abdomen serves as a secondary control device. As such, the covariation of these structures could reflect functional constraints related to flight performances. This study examines evolutionary convergences in wasp body shapes to provide the first evidence for morphological integration among insect wings, thorax, and abdomen. Read More

Am Nat 2020 02 19;195(2):129-144. Epub 2019 Dec 19.

Evolutionary biologists have long sought to understand the full complexity in pattern and process that shapes organismal diversity. Although phylogenetic comparative methods are often used to reconstruct complex evolutionary dynamics, they are typically limited to a single phenotypic trait. Extensions that accommodate multiple traits lack the ability to partition multidimensional data sets into a set of mosaic suites of evolutionarily linked characters. Read More

Am Nat 2020 02 18;195(2):216-230. Epub 2019 Dec 18.

Many species show synchronous fluctuations in population size over large geographical areas, which are likely to increase their regional extinction risk. Here we examine how the degree of spatial synchrony in population dynamics is affected by trophic interactions using a two-species predator-prey model with spatially correlated environmental noise. We show that the predator has a larger spatial scale of population synchrony than the prey if the population fluctuations of both species are mainly determined by the direct effect of stochastic environmental variations in the prey. Read More

Am Nat 2020 02 18;195(2):284-289. Epub 2019 Dec 18.

Theoretical models often have fundamentally different goals than do empirical studies of the same topic. Models can test the logic of existing hypotheses, explore the plausibility of new hypotheses, provide expectations that can be tested with data, and address aspects of topics that are currently inaccessible empirically. Theoretical models are common in ecology and evolution and are generally well cited, but I show that many citations appearing in nontheoretical studies are general to topic and that a substantial proportion are incorrect. Read More