Ecology, sexual selection and speciation

The spectacular diversity in sexually selected traits among animal taxa has inspired the hypothesis that divergent sexual selection can drive speciation. Unfortunately, speciation biologists often consider sexual selection in isolation from natural selection, even though sexually selected traits evolve in an ecological context: both preferences and traits are often subject to natural selection. Conversely, while behavioural ecologists may address ecological effects on sexual communication, they rarely measure the consequences for population divergence. Herein, we review the empirical literature addressing the mechanisms by which natural selection and sexual selection can interact during speciation. We find that convincing evidence for any of these scenarios is thin. However, the available data strongly support various diversifying effects that emerge from interactions between sexual selection and environmental heterogeneity. We suggest that evaluating the evolutionary consequences of these effects requires a better integration of behavioural, ecological and evolutionary research.     

Operational sex ratio, sexual conflict and the intensity of sexual selection

Modern sexual selection theory indicates that reproductive costs rather than the operational sex ratio predict the intensity of sexual selection. We investigated sexual selection in the polygynandrous common lizard Lacerta vivipara . This species shows male aggression, causing high mating costs for females when adult sex ratios (ASR) are male-biased. We manipulated ASR in 12 experimental populations and quantified the intensity of sexual selection based on the relationship between reproductive success and body size. In sharp contrast to classical sexual selection theory predictions, positive directional sexual selection on male size was stronger and positive directional selection on female size weaker in female-biased populations than in male-biased populations. Thus, consistent with modern theory, directional sexual selection on male size was weaker in populations with higher female mating costs. This suggests that the costs of breeding, but not the operational sex ratio, correctly predicted the strength of sexual selection.     

Mating system, sexual dimorphism, and the opportunity for sexual selection in a territorial ungulate

In mammals, species with high sexual size dimorphism tend tohave highly polygynous mating systems associated with high variancein male lifetime reproductive success (LRS), leading to a highopportunity for sexual selection. However, little informationis available for species with weak sexual size dimorphism. Ina long-term study population, we used parentage analysis basedon 21 microsatellite markers to describe, for the first time,variance in male lifetime breeding success (LBS) of roe deer,a territorial ungulate where males weigh less than 10% morethan females. LBS ranged from 0 to 14 (mean = 4.54, variance= 15.5), and its distribution was highly skewed, with only afew males obtaining high LBS and many males failing to breedor siring only one fawn. As predicted for polygynous specieswith low sexual size dimorphism, the standardized variance inmale LBS was low (I_m = 0.75) and was only slightly higher thanthe standardized variance in female LRS (I_f = 0.53), suggestinga low opportunity for sexual selection. The I_m value reportedhere for roe deer is much lower than values reported for highlydimorphic ungulates such as red deer (I_m > 3). We suggestthat, along a continuum of opportunity for sexual selection,roe deer occupy a position closer to monogamous and monomorphicterritorial ungulates than to highly polygynous, sexually dimorphicungulates with dominance rank–based mating systems suchas harems or roving mating systems.     

Adult sex ratio,sexual dimorphism and sexual selection in a Mesozoic reptile

The evolutionary history of sexual selection in the geologic past is poorly documented based on quantification, largely because of difficulty in sexing fossil specimens. Even such essential ecological parameters as adult sex ratio (ASR) and sexual size dimorphism (SSD) are rarely quantified, despite their implications for sexual selection. To enable their estimation, we propose a method for unbiased sex identification based on sexual shape dimorphism, using size-independent principal components of phenotypic data. We applied the method to test sexual selection in Keichousaurus hui, a Middle Triassic (about 237 Ma) sauropterygian with an unusually large sample size for a fossil reptile. Keichousaurus hui exhibited SSD biased towards males, as in the majority of extant reptiles, to a minor degree (sexual dimorphism index −0.087). The ASR is about 60% females, suggesting higher mortality of males over females. Both values support sexual selection of males in this species. The method may be applied to other fossil species. We also used the Gompertz allometric equation to study the sexual shape dimorphism of K. hui and found that two sexes had largely homogeneous phenotypes at birth except in the humeral width, contrary to previous suggestions derived from the standard allometric equation.     

Developmental stability,sexual selection and speciation

Fluctuating asymmetry occurs when an individual is unable to undergo identical development of an otherwise bilaterally symmetric trait on both sides of its body. Since both sides of a bilaterally symmetric trait are the result of the actions of a single genome, fluctuating asymmetry represents an epigenetic measure of the sensitivity of development to stress. Different morphological traits may show a direct relationship between their functional importance and their degree of developmental canalization. This may explain why some characters show high degrees of fluctuating asymmetry, and why these characters more often become exaggerated secondary sexual ornaments. The degree of fluctuating asymmetry is generally larger in small marginal populations living in novel environments, and this will particularly lead to relatively large degrees of asymmetry in the least developmentally canalized traits. More stringent selection against heterozygotes in marginal populations may further break down developmental stability and linkage groups which would lead to increased genetic variance. Females may prefer to mate with males having large, but relatively symmetric morphological characters, because it is more difficult to make large traits (a good genes argument), a large trait is more easily perceived (a sensory bias preference), and because symmetry signals ability to cope with stress (a good genes argument). The low degree of developmental stability and the large amount of genetic variance in secondary sexual characters in small, marginal populations could set the scene for rapid development of divergence and speciation in marginal populations.     

Protandry, sexual selection and climate change

Protandry refers to the earlier appearance of males before females at sites of reproduction. Sexual selection has been hypothesized to give rise to sex differences in benefits and costs of early arrival, thereby selecting for earlier appearance by the sex subject to more intense sexual selection. If sexual selection is more intense, there is a greater premium on early arrival among individuals of the chosen sex because of direct selection for earlier arrival. This hypothesis leads to the prediction that changes in the costs and benefits of early arrival related to changes in environmental conditions should particularly affect the sex that arrives first and hence the degree of protandry. I tested this hypothesis using the Barn Swallow Hirundo rustica. During 1971–2003, the degree of protandry increased significantly in a Danish population because males advanced arrival date while females did not. This earlier arrival by males compared with females was correlated with a significant increase by over 1.2 standard deviations in the length of the outermost tail feathers of males, a secondary sexual character, suggesting direct selection on both protandry and the secondary sexual character. Environmental conditions during spring migration in Northern Africa, as reflected by the normalized difference vegetation index, have deteriorated since 1984, resulting in increased mortality among males during spring migration, but not among females, and this deterioration of climatic conditions was positively correlated with an increasing degree of protandry. Likewise, an increase in April temperatures at the breeding grounds during recent decades is positively correlated with increased protandry, apparently because males can arrive earlier without increasing the fitness cost of early arrival. Local population size did not predict changes in arrival date. These findings suggest that rapid changes in climate can cause a change in degree of protandry and secondary sexual characters.     

Immunocompetence, mast cells and sexual behaviour

This paper reports a provocative and novel observation. Specifically, we have shown that in Ring Doves Streptopelia roseogrisea, following a brief period of courtship, cells of the immune system–mast cells–appear in the brain. This is surprising in that it has long been thought that the blood-brain barrier prevents the movement of blood-borne cells into the parenchyma (neural tissue) of the brain. The conditions under which these cells appear, their chemical and ultrastructural features and their potential in neuroendocrine signalling functions are discussed. The significance of these cells for the expression of behaviour and their specific role in the medial habenula (the brain area to which they migrate) remain to be explored. While our research has focused on a domesticated, laboratory bird species, the relationship between sexual behaviour and immunocompetence has long been of interest in field work and in studies on the evolution of sexual behaviour. It is hoped that this work, now in its infancy, will bridge field and laboratory, as well as providing a forum for integrating mechanistic and evolutionary concepts in neuro-im-muno-endocrinology.     

Immunocompetence, mast cells and sexual behaviour

This paper reports a provocative and novel observation. Specifically, we have shown that in Ring Doves Streptopelia roseogrisea , following a brief period of courtship, cells of the immune system–mast cells–appear in the brain. This is surprising in that it has long been thought that the blood-brain barrier prevents the movement of blood-borne cells into the parenchyma (neural tissue) of the brain. The conditions under which these cells appear, their chemical and ultrastructural features and their potential in neuroendocrine signalling functions are discussed. The significance of these cells for the expression of behaviour and their specific role in the medial habenula (the brain area to which they migrate) remain to be explored. While our research has focused on a domesticated, laboratory bird species, the relationship between sexual behaviour and immunocompetence has long been of interest in field work and in studies on the evolution of sexual behaviour. It is hoped that this work, now in its infancy, will bridge field and laboratory, as well as providing a forum for integrating mechanistic and evolutionary concepts in neuro-immuno-endocrinology.     

Inbreeding,energy use and sexual signaling

Genetic quality and energy metabolism are expected to have an effect on the level of energetically costly sexual signaling. To explore this we manipulated genetic quality of male decorated crickets (Gryllodes sigillatus) by inbreeding and measured the resting metabolic rate and total energy budget of males. We also measured several aspects of the sexual signaling of males: probability to initiate calling, latency, amount of call bouts, first call bout duration, mean call bout duration and total time spent calling. Inbreeding increased the latency and lowered the first and mean call bout duration. Moreover, the resting metabolic rate had a positive effect, and body mass a negative effect on first call bout duration and mean call bout duration. Our results, suggest that sexual signals are indicative of genetic quality but are also dependent on the physical properties of individuals.     

Parasitism, host immune function, and sexual selection

Parasite-mediated sexual selection may arise as a consequence of 1) females avoiding mates with directly transmitted parasites, 2) females choosing less-parasitized males that provide parental care of superior quality, or 3) females choosing males with few parasites in order to obtain genes for parasite resistance in their offspring. Studies of specific host-parasite systems and comparative analyses have revealed both supportive and conflicting evidence for these hypotheses. A meta-analysis of the available evidence revealed a negative relationship between parasite load and the expression of male secondary sexual characters. Experimental studies yielded more strongly negative relationships than observations did, and the relationships were more strongly negative for ectoparasites than for endoparasites. There was no significant difference in the magnitude of the negative effect for species with and without male parental care, or between behavioral and morphological secondary sexual characters. There was a significant difference between studies based on host immune function and those based on parasite loads, with stronger effects for measures of immune function, suggesting that the many negative results from previous analyses of parasite-mediated sexual selection may be explained because relatively benign parasites were studied. The multivariate analyses demonstrating strong effect sizes of immune function in relation to the expression of secondary sexual characters, and for species with male parental care as compared to those without, suggest that parasite resistance may be a general determinant of parasite-mediated sexual selection.     

被子植物性系统的多样性、生态功能及分布规律

性系统是被子植物繁育系统的核心, 决定着植物种群的遗传特征、进化方向与速度, 在种群动态、群落结构及生态系统的构建与维持中具有重要意义。本文回顾了被子植物性系统的发展历程及研究方向, 总结了近30年基于性系统研究的前沿科学问题, 包括性系统的多样性和进化、与其他功能性状的生态关联、沿环境梯度的分布格局及变化规律、与群落物种共存机制和群落动态的关系及其对干扰的响应。尽管有关植物性系统的研究已经延伸到生态学领域的诸多方面, 有力地推动了各方面的发展, 但仍有很多值得关注和需要着重研究的方向和问题。本文对未来基于植物性系统的研究方向等提出了展望, 并指出, 在当前全球气候变化背景下, 性系统可作为重要的功能性状应用于指导生物多样性保护和生态系统管理政策的制定。