American Naturalist最新文献

AbstractSpecies' interactions are shaped by their traits. Thus, we expect traits-in particular, trait (dis)similarity-to play a central role in determining whether a particular set of species coexists. Traits are, in turn, the outcome of an eco-evolutionary process summarized by a phylogenetic tree. Therefore, the phylogenetic tree associated with a set of species should carry information about the dynamics and assembly properties of the community. Many studies have highlighted the potentially complex ways in which this phylogenetic information is translated into species' ecological properties. However, much less emphasis has been placed on developing clear, quantitative expectations for community properties under a particular hypothesis. To address this gap, we couple a simple model of trait evolution on a phylogenetic tree with Lotka-Volterra community dynamics. This allows us to derive properties of a community of coexisting species as a function of the number of traits, tree topology, and the size of the species pool. Our analysis highlights how phylogenies, through traits, affect the coexistence of a set of species. Together, these results provide much-needed baseline expectations for the ways in which evolutionary history, summarized by phylogeny, is reflected in the size and structure of ecological communities.

AbstractIn multispecies mutualisms, hosts might be expected to reward only the highest-quality partner in order to maximize benefits and prevent the proliferation of cheaters. In a fluctuating environment, however, partner quality is likely to vary over time, and the maintenance of low-quality partners has been shown to be beneficial in some environmental regimes. Here, we present a model of a simple tree-fungal mutualism with two distinct environmental conditions and a host that can employ reward strategies with varying degrees of preference for higher-quality fungi. We find that in many environmental regimes, the most successful strategy for the host is one that actively maintains equal densities of the two fungal partners, in spite of their immediate differences in quality. This conservative bet-hedging strategy leads to reduced variance in the tree's carbon resources and high resilience to environmental perturbation. An alternative reward strategy, which supports only the highest-quality partner at a time, is most successful under some conditions when fluctuations in the environment are infrequent. Longer periods of environmental stasis thus increase the risk to the tree of losing fungal partner diversity. This theoretical work identifies a mechanism by which biodiversity may be actively maintained in multispecies mutualisms but that may be disrupted as environmental conditions change.

AbstractMany species of birds use shed snake skin in nest construction, but this behavior remains poorly understood. Ecological context is likely key for understanding how this unusual, but widespread, behavior evolved. We use comparative and experimental approaches to suggest that the evolution of this behavior is mediated by nest morphology and predator communities. First, we reviewed the literature and found that 78 species from 22 families have been reported to use shed snake skin in nest construction. All but one of these species are passerines and, using comparative analyses, we show that this behavior is disproportionately observed in cavity-nesting species. Second, we examined a subsample of North American species, all of which are reported to use snake skin in nest construction, to see whether the proportion of nests with snake skin differs between cavity- and open cup-nesting species. This analysis suggested that the proportion of nests with snake skin is roughly 6.5 times higher in cavity- than in open cup-nesting species. Finally, we used a series of experiments and comparisons to test four hypotheses whereby snake skin could award fitness benefits (nest predation, nest microbiotas, nest ectoparasites, social signaling) and found support for the predation hypothesis. Snake skin reduced nest predation in cavity, but not open cup, nests. These unequal fitness benefits highlight different ecological conditions between nest morphologies and likely explains why, across species, cavity-nesting birds show this behavior more frequently than open cup-nesting birds.

AbstractIn the face of ubiquitous threats from parasites, hosts can evolve strategies to resist infection or to altogether avoid parasitism, for instance by avoiding behavior that could expose them to parasites or by dispersing away from local parasite threats. At the microbial scale, bacteria frequently encounter viral parasites, bacteriophages. While bacteria are known to utilize a number of strategies to resist infection by phages and can have the capacity to avoid moving toward phage-infected cells, it is unknown whether bacteria can evolve dispersal to escape from phages. To answer this question, we combined experimental evolution and mathematical modeling. Experimental evolution of the bacterium Pseudomonas fluorescens in environments with differing spatial distributions of the phage Phi2 revealed that the host bacteria evolved resistance depending on parasite distribution but did not evolve dispersal to escape parasite infection. Simulations using parameterized mathematical models of bacterial growth and swimming motility showed that this is a general finding: while increased dispersal is adaptive in the absence of parasites, in the presence of parasites that fitness benefit disappears and resistance becomes adaptive, regardless of the spatial distribution of parasites. Together, these experiments suggest that parasites should rarely, if ever, drive the evolution of bacterial escape via dispersal.

AbstractThe parental roles of males and females differ remarkably across the tree of life, and several studies suggest that parental sex roles are associated with biased sex ratios. However, there is considerable debate on the causal relationship between sex roles and sex ratios and on the relative importance of the operational sex ratio (OSR), the adult sex ratio (ASR), and the maturation sex ratio (MSR). Here, we use individual-based evolutionary simulations to investigate the joint evolution of sex-specific parental behavior and the various sex ratios in several life history scenarios. We show that typically, but not always, the sex with lower mortality or faster maturity tends to provide most of the care. The association of parental sex roles with the various sex ratios is more intricate. At equilibrium, the OSR is typically biased toward the less caring sex, but the direction and strength of OSR biases may change considerably during evolution. When the MSR or ASR is biased, a broad spectrum of parental care patterns can evolve, although the overrepresented sex generally does most of the caring. We conclude that none of the sex ratios is a driver of parental sex roles; they rather coevolve with care biases in a subtle manner.

AbstractDespite newly formed polyploids being subjected to myriad fitness consequences, the relative prevalence of polyploidy, both contemporarily and in ancestral branches of the tree of life, suggests alternative advantages that outweigh these consequences. One proposed advantage is that polyploids may more easily colonize novel habitats, such as deglaciated areas. However, previous research conducted in diploids suggests that range expansion comes with a fitness cost, as deleterious mutations may fix rapidly on the expansion front. Here, we interrogate the potential consequences of expansion in polyploids by conducting spatially explicit forward-in-time simulations to investigate how ploidy and inheritance patterns impact the relative ability of polyploids to expand their range. We show that under realistic dominance models, autopolyploids suffer greater fitness reductions than diploids as a result of range expansion due to the fixation of increased mutational load that is masked in the range core. Alternatively, the disomic inheritance of allopolyploids provides a shield to this fixation, resulting in minimal fitness consequences. In light of this advantage provided by disomy, we investigate how range expansion may influence cytogenetic diploidization through the reversion to disomy in autotetraploids. We show that under a wide range of parameters investigated for two models of diploidization, disomy frequently evolves more rapidly on the expansion front than in the range core, and that this dynamic inheritance model has additional effects on fitness. Together our results point to a complex interaction among dominance, ploidy, inheritance, and recombination on fitness as a population spreads across a geographic range.

AbstractBiased birth sex ratios have been documented in many mammalian populations, but it is often difficult to know whether they result from biases in the sex ratio at conception and/or sex differences in prenatal mortality. It is generally admitted that there is an excess of males at conception and a higher level of mortality during gestation for males because of a positive relationship between size and vulnerability. Here, we challenge this classical prediction in a wild boar (Sus scrofa) population facing highly variable food resources (mast seeding) and in which male fetuses are heavier than females. Using long-term hunting and mast seeding data, we show that sex ratio at conception is balanced and that females suffer from higher embryonic mortality particularly in large litters, whatever the level and the type of food resources. One possible explanation is that a female embryo is ready for implantation later than an identically aged male because of slower development and is more likely to miss the implantation window. To what extent a lower survival of female embryos is a common feature in mammals remains to be carefully explored.

AbstractSpatial-temporal variation in environmental conditions is ubiquitous in nature. This variation simultaneously impacts survival, reproduction, and movement of individuals and thereby the rate at which metapopulations grow. Using the tools of stochastic demography, the metapopulation growth rate is decomposed into five components corresponding to temporal, spatial, and spatial-temporal variation in fitness and spatial and spatial-temporal covariation in dispersal and fitness. While temporal variation in fitness always reduces the metapopulation growth rate, all other sources of variation can either increase or reduce the metapopulation growth rate. Increases occur either by reducing the impacts of temporal variation or by generating a positive fitness-density covariance where individuals tend to concentrate in higher-quality patches. For example, positive autocorrelations in spatial-temporal variability in fitness generate this positive fitness-density covariance for less dispersive populations but decrease it for highly dispersive populations (e.g., migratory species). Negative autocorrelations in spatial-temporal variability have the opposite effects. Positive covariances between movement and future fitness, on short or long timescales, increase growth rates. These positive covariances can arise in unexpected ways. For example, the win-stay, lose-shift dispersal strategy in negatively autocorrelated environments can generate positive spatial covariances that exceed negative spatial-temporal covariances. This decomposition of the metapopulation growth rate provides a way to quantify the relative importance of fundamental sources of variation for metapopulation persistence.

AbstractCoevolution between two species can lead to exaggerated phenotypes that vary in a correlated manner across space. However, the conditions under which we expect such spatially varying coevolutionary patterns in polygenic traits are not well understood. We investigate the coevolutionary dynamics between two species undergoing reciprocal adaptation across space and time using simulations inspired by the Taricha newt/Thamnophis garter snake system. One striking observation from this system is that newts in some areas carry much more tetrodotoxin than in other areas, and garter snakes that live near more toxic newts tend to be more resistant to this toxin, a correlation seen across several broad geographic areas. Furthermore, snakes seem to be "winning" the coevolutionary arms race, that is, having a high level of resistance compared with local newt toxicity, despite substantial variation in both toxicity and resistance across the range. We explore how possible genetic architectures of the toxin and resistance traits would affect the coevolutionary dynamics by manipulating both mutation rate and effect size of mutations across many simulations. We find that coevolutionary dynamics alone were not sufficient in our simulations to produce the striking mosaic of levels of toxicity and resistance observed in nature, but simulations with ecological heterogeneity (in trait costliness or interaction rate) did produce such patterns. We also find that differences in polygenicity do not seem sufficient to explain the observation that snakes seem to be winning.

AbstractDespite the advantages of lasting pair bonds and the prevalence of monogamy, at least in avian species, some individuals switch mates (divorce). Divorce is generally considered to be adaptive (i.e., conferring net fitness benefits), although its causes and consequences often remain unclear, most notably regarding the genetic basis of this behavior. Using more than 30 years of data in a long-lived bird with obligate biparental care, the Alpine swift, we first described the overall patterns of mate and nest site fidelity and investigated the predictors of between-year divorce. We show that 16.6% of pairings ended in divorce, with low reproductive success and young age as predictors of divorce, and that males retained the nest site more often than females. By then studying individual repeatability and heritability of divorce, we show moderate repeatability in females and low repeatability in males and little additive genetic variance in either sex. Finally, we assessed the fitness consequences of divorce and report that an active decision to modify the pair bond (divorce) may be more beneficial than reactionary re-pairing following a partner's death. Overall, divorce may provide some reproductive benefits for Alpine swifts, but no microevolutionary potential of this behavior is evident in this population.