American Naturalist最新文献

AbstractThis article develops mathematical theory for the population dynamics of microbiomes with their hosts and for holobiont evolution caused by holobiont selection. The objective is to account for the formation of microbiome-host integration. Microbial population dynamic parameters must mesh with the host's for coexistence. A horizontally transmitted microbiome is a genetic system with "collective inheritance." The microbial source pool in the environment corresponds to the gamete pool for nuclear genes. Poisson sampling of the microbial source pool corresponds to binomial sampling of the gamete pool. However, holobiont selection on the microbiome does not lead to a counterpart of the Hardy-Weinberg law or to directional selection that always fixes microbial genes conferring the highest holobiont fitness. A microbe might strike an optimal fitness balance between lowering its within-host fitness while increasing holobiont fitness. Such microbes are replaced by otherwise identical microbes that contribute nothing to holobiont fitness. This replacement can be reversed by hosts that initiate immune responses to nonhelpful microbes. This discrimination leads to microbial species sorting. Host-orchestrated species sorting followed by microbial competition, rather than coevolution or multilevel selection, is predicted to be the cause of microbiome-host integration.

AbstractEnvironmental temperatures potentially influence reproductive performance and sexual selection by restricting opportunities for activity. However, explicit tests of the behavioral mechanisms linking thermal variation to mating and reproductive performance are rare. We address this gap in a temperate lizard by combining social network analysis with molecular pedigree reconstruction in a large-scale thermal manipulation experiment. Populations exposed to cool thermal regimes presented fewer high-activity days compared with populations exposed to a warmer regime. While plasticity in thermal activity responses in males masked overall differences in activity levels, prolonged restriction nevertheless affected the timing and consistency of male-female interactions. Females were less capable than males of compensating for lost activity time under cold stress, and less active females in this group were significantly less likely to reproduce. While sex-biased activity suppression appeared to limit male mating rates, this did not correspond to a heightened intensity of sexual selection or shifts in the targets of sexual selection. In many populations facing thermal activity restriction, sexual selection on males may play a limited role relative to other thermal performance traits in facilitating adaptation.

AbstractEnvironmentally induced reductions in fitness components (survival, fecundity) are generally considered as passive, maladaptive responses to stress. However, there is also mounting evidence for active, programmed forms of environmentally induced cell death in unicellular organisms. While conceptual work has questioned how such programmed cell death (PCD) might be maintained by natural selection, few experimental studies have investigated how PCD influences genetic differences in longer-term fitness across environments. Here, we tracked the population dynamics of two closely related strains of the halotolerant microalga Dunaliella salina following transfers across salinities. We showed that after a salinity increase, only one of these strains displayed a massive population decline (-69% in 1 h), largely attenuated by exposure to a PCD inhibitor. However, this decline was followed by a rapid demographic rebound, characterized by faster growth than the nondeclining strain, such that sharper decline was correlated with faster subsequent growth across experiments and conditions. Strikingly, the decline was more pronounced in conditions more favorable to growth (more light, more nutrients, less competition), further suggesting that it was not simply passive. We explored several hypotheses that could explain this decline-rebound pattern, which suggests that successive stresses could select for higher environmentally induced death in this system.

AbstractThe basic tenets of the evolutionary theories of senescence are well supported. However, there has been little progress in determining the relative influences of mutation accumulation and life history optimization. The causes of the well-established inverse relationship between life span and body size across dog breeds are used here to test these two classes of theories. The life span-body size relationship is confirmed for the first time after controlling for breed phylogeny. The life span-body size relationship cannot be explained by evolutionary responses to differences in extrinsic mortality either of contemporary breeds or of breeds at their establishment. The development of breeds larger and smaller than ancestral gray wolves has occurred through changes in early growth rate. This may explain the increase in the minimum age-dependent mortality rate with breed body size and thus higher age-dependent mortality throughout adult life. The main cause of this mortality is cancer. These patterns are consistent with the optimization of life history as described by the disposable soma theory of the evolution of aging. The dog breed life span-body size relationship may be the result of the evolution of greater defense against cancer lagging behind the rapid increase in body size during recent breed establishment.

AbstractThe social interactions that an individual experiences are a key component of its environment and can have important consequences for reproductive success. The dear enemy effect posits that having familiar neighbors at a territory boundary can reduce the need for territory defense and competition and potentially increase cooperation. Although fitness benefits of reproducing among familiar individuals are documented in many species, it remains unclear to what extent these relationships are driven by direct benefits of familiarity itself versus other socioecological covariates of familiarity. We use 58 years of great tit (Parus major) breeding data to disentangle the relationship between neighbor familiarity, partner familiarity, and reproductive success while simultaneously considering individual and spatiotemporal effects. We find that neighbor familiarity was positively associated with reproductive success for females but not males, while an individual's familiarity with their breeding partner was associated with fitness benefits for both sexes. There was strong spatial heterogeneity in all investigated fitness components, but our findings were robust and significant over and above these effects. Our analyses are consistent with direct effects of familiarity on individuals' fitness outcomes. These results suggest that social familiarity can yield direct fitness benefits, potentially driving the maintenance of long-term bonds and evolution of stable social systems.

AbstractThe global rise in anthropogenic reactive nitrogen and the negative impacts of N deposition on terrestrial plant diversity are well documented. The R* theory of resource competition predicts reversible decreases in plant diversity in response to N loading. However, empirical evidence for the reversibility of N-induced biodiversity loss is mixed. In a long-term N-enrichment experiment in Minnesota, a low-diversity state that emerged during N addition has persisted for decades after additions ceased. Hypothesized mechanisms preventing recovery of biodiversity include nutrient recycling, insufficient external seed supply, and litter inhibition of plant growth. Here, we present an ordinary differential equation model that unifies these mechanisms, produces bistability at intermediate N inputs, and qualitatively matches the observed hysteresis at Cedar Creek. Key features of the model, including native species' growth advantage in low-N conditions and limitation by litter accumulation, generalize from Cedar Creek to North American grasslands. Our results suggest that effective biodiversity restoration in these systems may require management beyond reducing N inputs, such as burning, grazing, haying, and seed additions. By coupling resource competition with an additional interspecific inhibitory process, the model also illustrates a general mechanism for bistability and hysteresis that may occur in multiple ecosystem types.

AbstractIn multispecies disease systems, pathogen spillover from a "reservoir community" can maintain disease in a "sink community" where it would otherwise die out. We develop and analyze models for spillover and disease spread in sink communities, focusing on questions of control: which species or transmission links are the most important to target to reduce the disease impact on a species of concern? Our analysis focuses on steady-state disease prevalence, assuming that the timescale of interest is long compared with that of disease introduction and establishment in the sink community. We identify three regimes as the sink community R₀ scales from 0 to 1. Up to $R_0\approx 0.3$, overall infection patterns are dominated by direct exogenous infections and one-step subsequent transmission. For $R_0\approx 1$, infection patterns are characterized by dominant eigenvectors of a force-of-infection matrix. In between, additional network details can be important; we derive and apply general sensitivity formulas that identify particularly important links and species.

AbstractOffspring desertion by parents generally occurs at an early stage of parental care, which is thought to minimize the costs of parental care prior to desertion. This study investigated the effects of endocrinological constraints on early total filial cannibalism by male Rhabdoblennius nitidus in the field, a paternal brooding blennid fish with androgen-dependent brood cycling. In brood reduction experiments, cannibal males showed low levels of plasma 11-ketotestosterone (11-KT) relative to noncannibals and also similar levels of 11-KT to males in the parental care phase. Since 11-KT regulates male courtship intensity, males with decreased courtship activity would exhibit total filial cannibalism. However, there is a possibility that a transient increase in 11-KT levels at the early stage of parental care delays total filial cannibalism. In contrast, total filial cannibalism could occur before a decline to the lowest 11-KT levels, at which point males might still be able to exhibit courtships, possibly to reduce the costs of parental care. To understand how much and when caregiving males exhibit mating and parental care behaviors, it is important to consider not only the presence of endocrinological constraints but also its intensity and flexibility.