American Naturalist最新文献

AbstractMajor ongoing theoretical and empirical challenges are to predict impacts of immigration on extinction probabilities of remaining populations within fragmented habitats. Comprehensive prediction requires considering multiple genetic effects on demography, including inbreeding and resulting inbreeding depression, additive genetic variance in fitness and resulting adaptive microevolution, and local adaptation and resulting migration load. However, all such effects have not been quantified or modeled simultaneously, especially for small wild populations experiencing regular natural immigration. We used quantitative genetic individual-based simulations parameterized using long-term data from song sparrows (Melospiza melodia) to show that contrary to broad expectation, increasing immigration could slightly increase short-term extinction probability. This outcome arose because while immigration reduced inbreeding and resulting expression of inbreeding depression, migration load stemming from apparent local adaptation was substantial and counteracted local adaptive microevolution, especially given heterosis-enhanced introgression. However, alternative self-reinforcing outcomes of rapid extinction due to an inbreeding-induced extinction vortex or migrational meltdown, as well as persistence due to microevolution of increased population growth, commonly arose. These results imply that altering dispersal rates among populations will not necessarily predictably affect local population persistence over short eco-evolutionary timeframes and highlight how remaining populations can lie on a knife-edge between persistence and alternative routes to genetically induced extinction.

AbstractIn simultaneous hermaphrodites, resource availability and the temporal distribution of mates determine male and female fitness and optimal sex allocation. In insect-pollinated plants, we expect individuals to allocate more to their female function when they are large and more to their male function when other individuals have many ovules available to be fertilized. Here, we studied the dependence of sex allocation and male and female components of reproductive success on both the size and the timing of reproduction in the plant Pulsatilla alpina (Ranunculaceae), accounting for inbreeding depression and variation in the mating system. Female reproductive success depended positively on size, whereas male reproductive success depended on mate availability and the timing of flowering, as predicted. Moreover, male reproductive success trended to a saturating function of allocation to stamens, whereas female reproductive success was a slightly accelerating function of pistil production. These results provide new insights into the reproductive strategies of perennial plants and help to explain the joint strategy in P. alpina of andromonoecy (the production of both male and bisexual flowers by individuals over the course of their lives) and gender diphasy (a shift between a male and a hermaphrodite phase among seasons).

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.