群落水平食物网能流季节演替特征

稳定碳、氮同位素比值分析技术是研究生态系统中物质循环与能量流动的有效技术.δ13C可以用来判断食物网中不同生物的能量来源;δ15N主要用于确定生物在食物网中所处的营养位置.通常用δ13C—δ15N图来表征某一特定时间或空间的食物网结构,但是这种方法在比较不同时间和空间食物网结构中功效较差.同时这种定性描述食物网结构也无法满足食物网复杂变化下的假说验证.应用环形统计方法,以太湖梅梁湾鱼类群落为例,定量评价了群落水平食物网能流季节演替特征.结果表明太湖梅梁湾鱼类营养生态位移动的季节特征明显.进一步物种水平分析结果表明,各种鱼类角度和幅度随季节均有变化.Rayleigh检验结果表明,群落中不同种类在秋冬、冬春和夏秋均有显著的一致的方向变化;而春夏期间不显著.Watson-William检验结果表明,群落水平的鱼类营养生态位移动在秋冬和冬春季节转换中有显著差异.引起鱼类群落水平营养生态位在食物网空间中季节性移动的主要因素为可利用资源稳定同位素的季节变化和不同鱼类种群自身的食性季节性转变.同时,由于梅梁湾食物网鱼类群落杂食性水平高,季节性浮游初级生产力成为食物网能量流动的重要驱动作用.因此,在富营养化生态系统中,食物网群落水平营养生态位季节波动也暗示了系统稳定性的下降.定量评价食物网变化有助于认识和理解食物网结构与功能在生态学和生态系统管理等方面的重要.     

海拔对青海湖流域群落水平植物功能性状的影响

海拔变化会引起气压、温度、降水、土壤湿度和风速等环境因子发生急剧变化, 植物功能性状-海拔的相互关系对于预测全球变化背景下山地植物的适应方式具有重要意义。该研究在青海湖流域海拔3 400-4 200 m范围内布设了5个样地(海拔间隔约200 m), 通过植物群落调查, 测定植物功能性状和土壤理化性质, 结合气象数据, 探讨了海拔对青海湖流域群落水平植物功能性状的影响。结果如下: (1)群落加权平均植株高度(H)、叶片干物质含量(LDMC)、叶片碳氮比(C:N)和叶片氮磷比(N:P)随海拔升高显著降低, 比根表面积(SRA)随海拔升高波动下降, 比叶面积(SLA)、叶片氮含量(LNC)和叶片磷含量(LPC)随海拔升高显著升高, 叶片碳含量(LCC)、比根长(SRL)和根组织密度(RTD)随海拔未发生显著变化。(2)所有性状的变异来源以物种组成变化为主, N:P和LPC的种内性状变异与物种组成变化呈现正的协变效应, 其余性状为负的协变效应。(3)降水和0- 10 cm土层土壤养分含量对SLA变化的解释率较高, 温度和10-20 cm土层土壤养分含量对其余性状随海拔变化的解释率较高。以上结果表明青海湖流域植物群落主要通过物种更替来适应随海拔升高而剧烈变化的环境, 且各群落中的非优势种倾向于占据与优势种相反的性状空间来提高资源利用率, 随海拔变化的热量和深层土壤养分含量是群落水平植物功能性状变化的主要影响因子。     

How to fail in advertising: The potential of marketing theory to predict the community-level selection of defended prey

Economics and ecology both present us with a key challenge: scaling up from individual behaviour to community-level effects. As a result, biologists have frequently utilized theories and frameworks from economics in their attempt to better understand animal behaviour. In the study of predator–prey interactions, we face a particularly difficult task—understanding how predator choices and strategies will impact the ecology and evolution not just of individual prey species, but whole communities. However, a similar challenge has been encountered, and largely solved, in Marketing, which has created frameworks that successfully predict human consumer behaviour at the community level. We argue that by applying these frameworks to non-human consumers, we can leverage this predictive power to understand the behaviour of these key ecological actors in shaping the communities they act upon. We here use predator–prey interactions, as a case study, to demonstrate and discuss the potential of marketing and human-consumer theory in helping us bridge the gap from laboratory experiments to complex community dynamics.     

Facilitation by nurse plants contributes to vegetation recovery in human-disturbed desert ecosystems

Aims Facilitation by nurse plants is a common interaction in harsh environments and this positive plant–plant interaction may promote vegetation recovery in ecosystems affected by human activities. Determining the relevance of this process, however, requires assessing how nurse plants influence the establishment of other species, as well as the proportion of species in the regional species pool that would benefit from the presence of nurse plants in human-disturbed areas. Further, since vegetation recovery is a time-dependent process, the community-level consequences of facilitation are likely to vary among landscapes with different disturbance history. Thus, an integrative perspective of the relevance of nurse plants for vegetation recovery could be obtained by measuring their effects across different human-disturbed landscapes of the target region. This study focuses on these issues and uses a regional-scale approach to assess the community-level effects of a widespread nurse plant of American deserts, the creosotebush (Larrea tridentata).Methods This study was conducted in the southernmost portion of Chihuahuan Desert because most floodplain valleys of this region have been affected by human activities during the past centuries. For this study, we selected 10 floodplain valleys differing in their age (i.e. the time elapsed after human activities were ceased). At each landscape, we measured the cover of creosotebushes and the proportion of plant species positively associated with them, as well as the density of seeds in the soil beneath creosotebush canopies. All these data were regressed against the age of the landscapes. Further, to assess whether positive association patterns were due to facilitation or other processes, we conducted field experiments and measured the ecophysiological performance of plant species established beneath and outside creosotebush canopies.Important findings Most plant species from the target region were positively associated to creosotebushes, and our field experiments and ecophysiological measures indicated that these distribution patterns can be attributed to facilitative interactions. In most landscapes, the density of seeds was higher beneath creosotebushes than in the surrounding habitats, suggesting that these shrubs may also act as seed traps. The community-level effects of creosotebushes increased with landscape age and creosotebush cover, indicating that magnitude of these effects depends on the disturbance history of each site. These results highlight the relevance of performing large-scale assessments for identifying the consequences of facilitation on vegetation recovery across space and time. We then propose that this kind of large-scale approach should be taken into account in the development of conservation programs aimed at the recovery and preservation of plant biodiversity in harsh environments.     

Directionality of contact networks suppresses selection pressure in evolutionary dynamics

Individuals of different types, may it be genetic, cultural, or else, with different levels of fitness often compete for reproduction and survival. A fitter type generally has higher chances of disseminating their copies to other individuals. The fixation probability of a single mutant type introduced in a population of wild-type individuals quantifies how likely the mutant type spreads. How much the excess fitness of the mutant type increases its fixation probability, namely, the selection pressure, is important in assessing the impact of the introduced mutant. Previous studies mostly based on undirected and unweighted contact networks of individuals showed that the selection pressure depends on the structure of networks and the rule of reproduction. Real networks underlying ecological and social interactions are usually directed or weighted. Here we examine how the selection pressure is modulated by directionality of interactions under several update rules. Our conclusions are twofold. First, directionality discounts the selection pressure for different networks and update rules. Second, given a network, the update rules in which death events precede reproduction events significantly decrease the selection pressure than the other rules.     

Kin selection and ethnic group selection

Ethnicity looks something like kinship on a larger scale. The same math can be used to measure genetic similarity within ethnic/racial groups and relatedness within families. For example, members of the same continental race are about as related (r = 0.18–0.26) as half-siblings (r = 0.25). However (contrary to some claims) the theory of kin selection does not apply straightforwardly to ethnicity, because inclusive fitness calculations based on Hamilton's rule break down when there are complicated social interactions within groups, and/or groups are large and long-lasting. A more promising approach is a theory of ethnic group selection, a special case of cultural group selection. An elementary model shows that the genetic assimilation of a socially enforced cultural regime can promote group solidarity and lead to the regulation of recruitment to groups, and to altruism between groups, based on genetic similarity – in short, to ethnic nepotism. Several lines of evidence, from historical population genetics and political psychology, are relevant here.     

Fertility selection,genetic selection and evolution

The relationship between fertility selection as measured by the correlation in progeny number between parents and offspring, and selection at individual loci is investigated in humans. Estimates for the magnitude of fertility selection (0.1) and the rate of gene substitution (0.5 gene substitutions per generation per genome) are used in various mathematical models for selection. It is found that the observed magnitude of fertility selection cannot be explained by non‐epistatic directional selection at individual loci. A symmetric quantitative directional selection model is consistent with the observed data. But it is possible that fertility selection does not have a genetic basis.     

Directionality in the evolution of influenza A haemagglutinin

The evolution of haemagglutinin (HA), an important influenza virus antigen, has been the subject of intensive research for more than two decades. Many characteristics of HA's sequence evolution are captured by standard Markov chain substitution models. Such models assign equal fitness to all accessible amino acids at a site. We show, however, that such models strongly underestimate the number of homoplastic amino acid substitutions during the course of HA's evolution, i.e. substitutions that repeatedly give rise to the same amino acid at a site. We develop statistics to detect individual homoplastic events and find that they preferentially occur at positively selected epitopic sites. Our results suggest that the evolution of the influenza A HA, including evolution by positive selection, is strongly affected by the long-term site-specific preferences for individual amino acids.     

Directionality,convergence, and rate of change during early succession in the Inland Pampa,Argentina

Abstract. We tested the following hypotheses forthe first five years of a grassland succession: (a) community changes are mainly directional and related to time after disturbance ratherthanto environmental fluctuations; (b) rates of succession decrease over time, and (c) plant communities in different plots converge on a similar composition within five years of succession. We tested those hypotheses using a Principal Components Analysis applied to data from four successional plots established in successive y ears in a large cropland in the Inland Pampa, Argentina. Community changes were correlated to the age of the plots, and unrelated to rainfall variability, a major environmental variable in grasslands. Successional rates were constant over the five years, probably because of the continued dominance of different annuals; we conclude that successional rates depend on the life history of the dominant species rather than on any emergent community property. We found no evidence of convergence ordivergence; we concluded that the results of successional studies may depend on the temporal and spatial scale of observation.     

Why male butterflies are non-mimetic: natural selection,sexual selection,group selection,modification and sieving*

Thomas Belt suggested that the frequent limitation of mimicry in butterflies to the female resulted from sexual selection. Because female butterflies store sperm they can be fully fertile after only one mating; the reproductive success of a male is proportional to the number of times he mates. Sexual selection is therefore much stronger in males than females, with selection coefficients being greater by a small multiple of the number of times a female is courted during her life (long-lived species) or of the reciprocal of the female mortality rate between courtships (short-lived species). As butterflies of both sexes respond to colour when courting, sexual selection resists colour changes especially strongly in males. As a result, genes conferring new mimetic colour patterns can often become established in a butterfly population much more readily if their expression is initially limited to females; when the population size of a Batesian mimic, its model, and its predator fluctuates, such sex-limited genes have an enhanced probability of ultimate fixation in the population, and a reduced chance of loss; this effect is accentuated by the selection of modifiers which improve the mimicry. When the establishment of unimodal mimicry (expressed in both sexes) is favoured in a Batesian mimic, the gene tends to rise to an equilibrium frequency at which modifiers suppressing the expression of the mimicry only in males and'modifiers enhancing the mimicry only in females are favoured. The outcome is female-limited mimicry, or unimodal mimicry with better mimicry in the females, the males either retaining some of their sexual colour or the selective behaviour of the females becoming altered. In a Muellerian mimic there is no such equilibrium and selection ultimately favours expression of mimicry in both sexes and an appropriate alteration in the courtship responses. Hence Muellerian mimicry is seldom female-limited. Exceptional cases appear to result from the sexes flying in separate habitats. The genetical evidence in Papilio and Heliconius favours initial limitation of expression over subsequent modification as the usual basis for female-limited mimicry. Other explanations of female-limited mimicry can be found wanting in various ways; a higher predation rate on females could produce sex-limitation, but is probably not a strong factor. But the greater variability of the female in Lepidoptera may indicate lesser developmental stability, which could result in greater penetrance of mutants in the female, and hence account for the initial female-limitation. At very high densities of a mimetic species which has no non-mimetic form, mimicry tends to deteriorate more rapidly in a unimodal than in an otherwise identical sex-limited species. Although by itself this would equally favour male-limitation, and hence cannot explain the predominance of female-limitation, this effect may over evolutionary time be causing a slight increase in the proportion of sex-limited species among mimics. The stability of some mimetic polymorphisms is investigated by linear approximation: in some instances a stable equilibrium can be changed into an oscillating equilibrium by changes in the population size.     

动物的性选择

有性生殖为个体之间设置了一个冲突和竞争的环境,因为雌雄个体都希望最大限度地把自己的基因传递给后代。性选择保证了动物的繁殖成功。性选择可以分为性别内选择和性别间选择。性别内选择促使动物格斗器官或其他有利于在格斗中获胜的一些特征得到发展;性别间选择促使用来吸引异性的一些特征得到发展。雌性动物的配偶选择或与所选择的特征协同进化,或可以从选择中得到直接的利益。     

The interpretation of selection coefficients

Evolutionary biologists have an array of powerful theoretical techniques that can accurately predict changes in the genetic composition of populations. Changes in gene frequencies and genetic associations between loci can be tracked as they respond to a wide variety of evolutionary forces. However, it is often less clear how to decompose these various forces into components that accurately reflect the underlying biology. Here, we present several issues that arise in the definition and interpretation of selection and selection coefficients, focusing on insights gained through the examination of selection coefficients in multilocus notation. Using this notation, we discuss how its flexibility—which allows different biological units to be identified as targets of selection—is reflected in the interpretation of the coefficients that the notation generates. In many situations, it can be difficult to agree on whether loci can be considered to be under “direct” versus “indirect” selection, or to quantify this selection. We present arguments for what the terms direct and indirect selection might best encompass, considering a range of issues, from viability and sexual selection to kin selection. We show how multilocus notation can discriminate between direct and indirect selection, and describe when it can do so.     

Marker assisted selection with optimised contributions of the candidates to selection

The benefits of marker assisted selection (MAS) are evaluated under realistic assumptions in schemes where the genetic contributions of the candidates to selection are optimised for maximising the rate of genetic progress while restricting the accumulation of inbreeding. MAS schemes were compared with schemes where selection is directly on the QTL (GAS or gene assisted selection) and with schemes where genotype information is not considered (PHE or phenotypic selection). A methodology for including prior information on the QTL effect in the genetic evaluation is presented and the benefits from MAS were investigated when prior information was used. The optimisation of the genetic contributions has a great impact on genetic response but the use of markers leads to only moderate extra short-term gains. Optimised PHE did as well as standard truncation GAS (i.e. with fixed contributions) in the short-term and better in the long-term. The maximum accumulated benefit from MAS over PHE was, at the most, half of the maximum benefit achieved from GAS, even with very low recombination rates between the markers and the QTL. However, the use of prior information about the QTL effects can substantially increase genetic gain, and, when the accuracy of the priors is high enough, the responses from MAS are practically as high as those obtained with direct selection on the QTL.     

Differential selection between the sexes and selection for sex

Anisogamy is known to generate an important cost for sexual reproduction (the famous "twofold cost of sex"). However, male-female differences may have other consequences on the evolution of sex, due to the fact that selective pressures may differ among the sexes. On the one hand, intralocus sexual conflict should favor asexual females, which can fix female-beneficial, male-detrimental alleles. On the other hand, it has been suggested repeatedly that sexual selection among males may help to purge the mutation load, providing an advantage to sexual females. However, no analytical model has computed the strength of selection acting on a modifier gene affecting the frequency of sexual reproduction when selection differs between the sexes. In this article, we analyze a two-locus model using two approaches: a quasi-linkage-equilibrium (QLE) analysis and a local stability analysis, whose predictions are verified using a multilocus simulation. We find that costly sex can be maintained when selection is stronger in males than in females, but acts in the same direction in both. Complete asexuality, however, evolves under any other form of selection. Finally, we discuss how experimental measurements of fitness variances and covariances between sexes could be used to determine the overall direction and strength on selection for sex arising from differences in selection between males and females.     

The conditional ancestral selection graph with strong balancing selection

Using a heuristic separation-of-time-scales argument, we describe the behavior of the conditional ancestral selection graph with very strong balancing selection between a pair of alleles. In the limit as the strength of selection tends to infinity, we find that the ancestral process converges to a neutral structured coalescent, with two subpopulations representing the two alleles and mutation playing the role of migration. This agrees with a previous result of Kaplan et al., obtained using a different approach. We present the results of computer simulations to support our heuristic mathematical results. We also present a more rigorous demonstration that the neutral conditional ancestral process converges to the Kingman coalescent in the limit as the mutation rate tends to infinity.     

Individual and group selection with competition

Summary The response of a randomly mating population which is expected to follow selection of phenotypic units, comprising individuals or groups whose members have an arbitrary degree of relatedness, was formulated using a model which included additive and dominance competition effects. The derivation involved three steps. Twenty-two quadratic components were defined, six describing individual (direct) and neighbor (associate) effects, and 16 describing direct by associate interactions for different loci, for single loci with different alleles, and for identical alleles. Six covariances between pairs of individual phenotypes and three of individuals with their offspring were defined according to whether or not their direct or associate genotypes are common, and expressed in terms of the quadratic components. Finally, variances of selection units of different types and their covariance with their offspring were expressed as compounds of these individual covariances. Explicit formulations for mass, clonal and full-sib selection show that without constraints on the quadratic components, and hence on the magnitude and type of competition operative, no predictions as to the relative efficiencies of these three methods can be made.     

Sexual selection is stabilizing selection in pupfish [Cyprinodon pecosensis)

One of the predictions of the 'good genes' model of sexual selection is that reproductively successful males with well-developed indicator traits should show smaller variances for non-indicator traits, that are not directly associated with mating success, when compared to non-breeding males and females. Thus sexual selection should reinforce stabilizing natural selection in reducing the variance in quantitative traits. This prediction is tested by analysing variation in eight morphological traits of breeding males, non-breeding males, and females of pupfish (Cyprinodon pecosensis). Breeding males tended to be less variable than non-breeding males for all principal component factors, and for all morphological traits except for depth, although these differences were statistically significant only for PC2, and PC5 and for pelvic fin length, number of pelvic fin rays and number of preopercular and preorbital pores. Similarly, breeding males tended to be less variable than females for all principal component factors and for all morphological traits except for number of preopercular pores. These differences were statistically significant for PC2, and for depth, pelvic fin length, number of preorbital pores and pectoral fin rays. The overall pattern of reduced variability in independent traits of breeding males revealed by principal component analysis is very consistent and highly significant (P<10⁵). These results support the prediction of the 'good genes' model and show that reproductively active males are subject to more severe stabilizing selection for several quantitative traits than non-breeding males and females. Thus sexual selection, through male-male competition, female choice, or an interaction of both selective processes, results in stabilizing selection on quantitative morphological traits.     

Alternative formulations of multilevel selection

Hierarchical expansions of the theory of natural selection exist in two distinct bodies of thought in evolutionary biology, the group selection and the species selection traditions. Both traditions share the point of view that the principles of natural selection apply at levels of biological organization above the level of the individual organism. This leads them both to considermultilevel selection situations, where selection is occurring simultaneously at more than one level. Impeding unification of the theoretical approaches of the multilevel selection traditions are the different goals of investigators in the different subdisciplines and the different types of data potentially available for analysis. We identify two alternative approaches to multilevel situations, which we termmultilevel selection [1] andmultilevel selection [2]. Of interest in the former case are the effects of group membership onindividual fitnesses, and in the latter the tendencies for the groups themselves to go extinct or to found new groups (i.e., group fitnesses). We argue that: neither represents the entire multilevel selection process; both are aspects of any multilevel selection situation; and both are legitimate approaches, suitable for answering different questions. Using this formalism, we show that: multilevel selection [2] does not require emergent group properties in order to provide an explanatory mechanism of evolutionary change; multilevel selection [1] is usually more appropriate for neontological group selection studies; and species selection is most fruitfully considered from the point of view of multilevel selection [2]. Finally we argue that the effect hypothesis of macroevolution, requiring, in selection among species, both the absence of group effects on organismic fitness (multilevel selection [1]), and the direct determination of species fitnesses by those of organisms, is untestable with paleontological data. Furthermore, the conditions for the effect hypothesis to hold are extremely restrictive and unlikely to apply to the vast majority of situations encountered in nature.     

Group selection in plant populations

Summary Several mechanisms have been proposed for group selection, to account for the evolution of altruistic traits. One type, Neighbourhood models, suggests that individuals react with those immediately around them, but with no recognition mechanism. The organization of plant populations seems especially favorable for this type of selection. The possibility of Neighbourhood selection was investigated by simulating a plant population. It was possible for an altruistic trait to evolve, though only under restricted conditions. The main requirement was gene flow only by very restricted pollen dispersal, and a high benefit : cost ratio in the altruistic relationship. Under conditions favourable for such evolution, the starting frequency of the allele, the initial pattern, and the population size, had little effect. Inbreeding tended to prevent the increase of the altruism allele, though this depended on the mechanism of selfing. Known ecological features of plants are discussed that could be considered altruistic and hence require some form of group selection for their evolution, and whether the benefit : cost requirements are likely to be met. Neighbourhood models of group selection are a possibility in plant populations, and we therefore cannot exclude the possibility of altruism in plants. However, Neighbourhood selection is weak force, unlikely to be effective in the face of opposing individual selection. It may be more important as reinforcement of individual selection.