American Naturalist最新文献

AbstractNatural selection is widely considered responsible for the fit between organisms and their environment. Lizard limb length variation is a paradigmatic example: studies have shown that limb length differences tightly correlate with habitat use among species, while small differences in limb length between individuals can affect biomechanical function, fitness, and survival within populations. It has therefore been surprising for many field biologists to find otherwise-healthy wild lizards with damaged or missing limbs, appearing to challenge associated expectations of substantial fitness costs. We document limb loss (from a foot to an entire limb) in 58 lizard species, with all cases showing healed limbs and many lizards appearing robust and healthy. Data indicate that limb-deficient lizards typically comprise less than 1% of populations and often exhibit body condition, sprint speed performance, and survival comparable to limb-intact individuals. We discuss the implications of these findings for how evolutionary adaptation is studied and understood in natural populations and provide a perspective on conventional assumptions about the strength and ubiquity of selection pressures on seemingly critical traits. Is natural selection always as omnipresent as Darwin envisioned it to be?

AbstractEukaryogenesis is the prototypical example of an egalitarian evolutionary transition in individuality, and endosymbiosis, more generally, is central to the origins of many complex biological systems. Why do only some symbioses undergo such a transition, and how does the host-symbiont relationship change during this process? Here, we characterize endosymbiosis by two emergent collective-level properties: host and symbiont survival as a collective ("mutual dependence") and the level of synchronized reproduction ("reproductive cohesion"). Using adaptive dynamics, we study the evolution of the traits underlying these properties. First, by adding a carrying capacity for the collective population-a realism omitted in previous models-we find novel reasons why complete dependence or cohesion might not evolve, thus providing further theoretical support for the rarity of transitions in individuality. Second, our model suggests that asymmetries in evolutionary outcomes of hosts and symbionts can be explained by a difference in their population growth parameters, coupled with their shared fate when in a collective. Last, we show that during the early stages of an endosymbiosis, even if investments in dependence and cohesion are uncorrelated, mutual dependence arises faster than reproductive cohesion. Our results hence shed light on three aspects of endosymbiosis: coevolution between the host and symbiont, coevolution between dependence and cohesion, and ultimately the opportunity to undergo an evolutionary transition. Connecting to ecological factors, this work uncovers fundamental properties of endosymbioses, providing a clear way forward for theoretical and empirical investigations.

AbstractGradient variation evolves when environmental and genotypic effects on a phenotype covary positively (cogradient variation) or negatively (countergradient variation) across locations, whereas gene-by-environment interactions (G × E) reflect nonadditive genetic and environmental influences on phenotypes. Spatial covariance in environmental and genotypic effects (Cov_GE) shapes variation in quantitative traits, facilitates local adaptation, and provides insights into eco-evolutionary dynamics. Yet several debates regarding gradient variation remain unresolved, including whether qualitative patterns of reaction norms accurately reflect Cov_GE, whether cogradient or countergradient variation occurs more frequently than G × E, and whether general patterns emerge according to taxonomic groups, forms of environmental gradient, or trait types. We conducted a quantitative survey of 556 phenotypes and measured Cov_GE and G × E across various phenotypes, taxa, and environmental gradients. We found that the qualitative assessment of reaction norms was unreliable for identifying Cov_GE and that Cov_GE occurred as frequently as G × E. No distinct patterns in Cov_GE emerged across environmental, taxonomic, or trait-based groups. Our results challenge prevailing views regarding Cov_GE and suggest that gradient variation can evolve under any environmental condition, taxonomic grouping, or trait type. We suggest that broader application of quantitative methods for Cov_GE across diverse systems will enhance our understanding of Cov_GE in nature.

AbstractSince the late 1890s up until today, how phenotypic plasticity interacts with genetic adaptation has been a debated issue. Proponents of a positive causal role of phenotypic plasticity-James M. Baldwin in the first place-supported the view that in altered environmental conditions, phenotypic plasticity is a key factor allowing a population to avoid extinction and then genetic evolution to catch up ("original Baldwin effect" [OBE]). Opponents, such as Ernst Mayr, regularly pointed out that phenotypic plasticity, by masking genetic variation, slows gene-level evolution ("Mayr effect" [ME]). For decades, this opposition remained only verbal and qualitative. To resolve it, we propose here a stochastic model that, following Baldwin's intuitive take, combines the minimal number of ingredients to account for extinction, selection, mutation, and plasticity. We study evolutionary rescue of the population (arrival and invasion of an adaptive genetic mutant) in the altered environment for different values of phenotypic plasticity, here quantified as the probability p that the maladapted genotype develops into the adapted phenotype. Our claim is that OBE can be a genuine evolutionary mechanism, depending on the level of phenotypic plasticity with respect to a threshold value p^⋆. When $p<p^{\star }$, increasing p promotes evolutionary rescue by delaying extinction ("strong" OBE); when $p>p^{\star }$, plasticity sustains population survival and increasing p has two antagonistic effects: to accelerate adaptation by increasing the supply of adaptive mutants ("weak" OBE, intermediate values of p) and to slow down adaptation by decreasing their fitness advantage (ME, high values of p).

AbstractMany ecological models treat exploitative competition in isolation from interference competition. Corresponding theory centers around the R* rule, according to which consumers that share a single limiting resource cannot coexist. Here we model motile consumers that directly interfere while handling resources, mechanistically capturing both exploitative and interference competition. Our analytical coexistence conditions show that interference competition readily promotes coexistence. In contrast to previous theory, coexistence does not require intraspecific interference propensities to exceed interspecific interference propensities or for interference behaviors to carry a direct (rather than merely an opportunity) cost. The underlying mechanism of coexistence can resemble the hawk-dove game, the dominance-discovery trade-off (akin to the competition-colonization trade-off), or a novel trade-off we call the "dove-discovery trade-off," depending on parameter values. Competitive exclusion via the R* rule occurs only when differences in exploitative abilities swamp other differences between species, and it occurs more easily when differences in R* reflect different search speeds than when they reflect different handling times. Our model provides a mathematically tractable framework that integrates exploitative and interference competition and synthesizes previous disparate models.

AbstractAnimals employ various mechanisms for camouflage, including color change, that may facilitate habitat use. However, the extent to which these mechanisms operate under nocturnal conditions is unclear. To investigate this, we combined a background-induced color change experiment with visual modeling to test whether altering backgrounds for a tropical tree frog (Pithecopus hypochondrialis) could induce short-term color change under nocturnal conditions to match the viewing background, as perceived by three predator classes: snakes, mammals, and birds. We demonstrated that frogs can change color multiple times from green to brown and back across grass and leaf litter backgrounds in dim conditions. Frog visual contrast varied by predator and background. Brown frogs matched against leaf litter across all predators, whereas green frogs were more variable and comparatively less well matched against grass. Notably, frogs achieved near-optimal color matching against both backgrounds for avian predators, with green frogs matching into grass and brown frogs matching into leaf litter. Our study provides evidence that P. hypochondrialis undergoes rapid background-induced color changes at night maintaining effective camouflage, at least against avian predators. We emphasize the need to assess rapid color change against visually guided predators in natural conditions and the importance of understanding viewing conditions for illuminating the ecology and evolution of camouflage.

AbstractWarming induces metabolic changes in microbial organisms, including increased respiration. Empirical studies have shown that evolution can compensate for thermal sensitivity and reduce respiration rate at high temperatures. Evolutionary adaptation may mitigate the effects of warming, but it remains unclear to what extent organisms can overcome thermodynamic constraints through evolution. Furthermore, evolutionary adaptations are modulated by interactions with plastic changes to respiration and other metabolic traits. We develop a mechanistic model including both evolution and metabolic plasticity to explore how adaptation to temperature affects variability in metabolic traits in mixotrophic marine microorganisms under thermal stress. By combining modeling with empirical data, we show that variability in metabolic activity between mixotrophs with different temperature histories can be explained by changes to the carbon budget facilitated by evolved reductions in respiration. The model suggests that evolution enhances thermal resilience over evolutionary timescales. Evolving mixotrophs exhibit less metabolic variability in response to temperature changes. In contrast, over shorter timescales plastic responses dominate over evolutionary adaptations, producing transient changes to metabolic activity following a temperature change. These results highlight the interplay between different biological adaptive mechanisms and provide a modeling framework for representing variability in microbial metabolism in the context of climate change.

AbstractEcologists differ in the degree to which they consider the linear type I functional response to be an unrealistic versus sufficient representation of predator feeding rates. Empiricists tend to consider it unsuitably nonmechanistic, and theoreticians tend to consider it necessarily simple. Holling's original rectilinear type I response is dismissed by satisfying neither desire, with most compromising on the smoothly saturating type II response for which searching and handling are assumed to be mutually exclusive activities. We derive a "multiple-prey-at-a-time" response and a generalization that includes the type III to reflect predators that can continue to search when handling an arbitrary number of already-captured prey. The multiprey model clarifies the empirical relevance of the linear and rectilinear models and the conditions under which linearity can be a mechanistically reasoned description of predator feeding rates, even when handling times are long. We find evidence for the presence of linearity in 35% of 2,591 compiled empirical datasets and support for the hypothesis that larger predator-prey body mass ratios permit predators to search while handling greater numbers of prey. Incorporating the multiprey response into the Rosenzweig-MacArthur population dynamic model reveals that a nonexclusivity of searching and handling can lead to coexistence states and dynamics that are not anticipated by theory built on the linear type I, type II, and type III models. In particular, it can lead to bistable fixed point and limit cycle dynamics with long-term crawl-by transients between them under conditions where abundance ratios reflect top-heavy food webs and the functional response is linear despite having an inherent upper limit. We conclude that functional response linearity should not be considered empirically unrealistic but also that more cautious inferences should be drawn in theory presuming the linear type I to be appropriate.

AbstractWarming climate has led to significant phenological advances in many plant and animal populations. Whether these advances represent evolutionary responses or phenotypic plasticity remain typically unknown. Using a 53-year-long time series of individually marked Great Tits (Parus major) and Willow Tits (Poecile montanus), we investigated whether the significant breeding time advances in these species could be explained as resulting from evolutionary responses, phenotypic plasticity, or both. In the case of both species, we did not find any evidence for changes in breeding values for timing of breeding, suggesting that the observed changes do not have a genetic and, hence, evolutionary basis. In contrast, we found that annually fluctuating environmental effects explained most of the variation in first egg-laying dates, suggesting that advances in breeding time were attributable to phenotypic plasticity. We further inferred that phenotypic plasticity in response to spring temperatures can fully explain the observed advancement of Great Tit phenology over time, whereas Willow Tits have advanced their phenology much beyond what would be expected from phenotypic plasticity in response to spring temperatures. The latter observation suggests that some other yet unidentified environmental factor, uncorrelated with spring temperatures, likely explains about half of the advancement in their breeding time.

AbstractThe evolution of sexual dimorphism is predicted to resolve conflict that can arise from divergent evolutionary interests between sexes, enabling each sex to reach its fitness optimum. However, most of the genome is shared between sexes, which can lead to a genetic constraint for dimorphism evolution. Most studies of intersexual genetic constraints have focused on the effect of genetic correlations, r_mf, for single traits. However, multivariate studies of the B matrix of intersexual genetic covariances suggest that sexual dimorphism may be more evolvable than inferred from r_mf because of the potential for indirect responses to selection from correlated traits. To comprehensively address this question, we collected and reanalyzed published estimates of B using a recently developed approach to quantify the evolvability of sexual monomorphism and dimorphism. We find that across the traits and species we study, the evolvability of dimorphism is lower than that of monomorphism, but also that sexually concordant and antagonistic selection are almost equally capable of producing dimorphism. We also find that asymmetry in B would affect the response to selection more in females than in males. Our results show that sexual dimorphism is more evolvable than studies of r_mf suggest and underscore that sexually antagonistic selection is not required for the evolution of sexual dimorphism.