LIFETIME REPRODUCTIVE SUCCESS, SELECTION, AND THE OPPORTUNITY FOR SELECTION IN THE WHITE-TAILED SKIMMER PLATHEMIS LYDIA (ODONATA: LIBELLULIDAE)

The contribution of extra-pair paternity to the variation in lifetime and age-specific male reproductive success in a socially monogamous species

In socially monogamous species, extra-pair paternity (EPP) is predicted to increase variance in male reproductive success (RS) beyond that resulting from genetic monogamy, thus, increasing the "opportunity for selection" (maximum strength of selection that can act on traits). This prediction is challenging to investigate in wild populations because lifetime reproduction data are often incomplete. Moreover, age-specific variances in reproduction have been rarely quantified. We analyzed 21 years of near-complete social and genetic reproduction data from an insular population of Seychelles warblers (Acrocephalus sechellensis). We quantified EPP's contribution to lifetime and age-specific opportunities for selection in males. We compared the variance in male genetic RS vs social ("apparent") RS (RS_ap ) to assess if EPP increased the opportunity for selection over that resulting from genetic monogamy. Despite not causing a statistically significant excess (19%) of the former over the latter, EPP contributed substantially (27%) to the variance in lifetime RS, similarly to within-pair paternity (WPP, 39%) and to the positive WPP-EPP covariance (34%). Partitioning the opportunity for selection into age-specific (co)variance components, showed that EPP also provided a substantial contribution at most ages, varying with age. Therefore, despite possibly not playing the main role in shaping sexual selection in Seychelles warblers, EPP provided a substantial contribution to the lifetime and age-specific opportunity for selection, which can influence evolutionary processes in age-structured populations.

Wing shape differences along a migration route of the long-distance migrant Globe Skimmer Dragonfly Pantala flavescens

Animals which migrate by flying should be subject to selection for optimal wing characteristics that maximize energy efficiency during migration. We investigated wing shape and wing area variation in the Globe Skimmer Dragonfly Pantala flavescens, which has the longest known migration of any insect. Wing shape and wing area differences between individuals in southern Peninsular India, and migrating individuals at a stop-over site on the Maldives, were compared. Results suggest that individuals which successfully reached the Maldives, on their way from India to Africa, had a broader wing base and an overall more slender wing shape than individuals in southern India. Contrary to our expectations, wing area did not differ significantly in most of our comparisons between southern India and the Maldives, suggesting that wing shape is more important than wing area for successful migration in P. flavescens. The results provide indirect evidence of natural selection on wing shape in a migrating dragonfly.

Trade-offs between larval survival and adult ornament development depend on predator regime in a territorial dragonfly

Trade-offs between juvenile survival and the development of sexually selected traits can cause ontogenetic conflict between life stages that constrains adaptive evolution. However, the potential for ecological interactions to alter the presence or strength of these trade-offs remains largely unexplored. Antagonistic selection over the accumulation and storage of resources could be one common cause of environment-specific trade-offs between life stages: higher condition may simultaneously enhance adult ornament development and increase juvenile vulnerability to predators. We tested this hypothesis in an ornamented dragonfly (Pachydiplax longipennis). Higher larval body condition indeed enhanced the initial development of its intrasexually selected wing coloration, but was opposed by viability selection in the presence of large aeshnid predators. In contrast, viability selection did not oppose larval body condition in pools when aeshnids were absent, and was not affected when we manipulated cannibalism risk. Trade-offs between larval survival and ornament development, mediated through the conflicting effects of body condition, therefore occurred only under high predation risk. We additionally characterized how body condition influences several traits associated with predator avoidance. Although body condition did not affect burst distance, it did increase larval abdomen size, potentially making larvae easier targets for aeshnid predators. As high body condition similarly increases vulnerability to predators in many other animals, predator-mediated costs of juvenile resource accumulation could be a common, environment-specific limitation on the elaboration of sexually selected traits.

FIELD MEASUREMENTS OF NATURAL AND SEXUAL SELECTION IN THE FUNGUS BEETLE, BOLITOTHERUS CORNUTUS

Selection on three phenotypic traits was estimated in a natural population of a fungus beetle, Bolitotherus cornutus. Lifetime fitness of a group of males in this population was estimated, and partitioned into five components: lifespan, attendance at the mating area, number of females courted, number of copulations attempted, and number of females inseminated. Three phenotypic characters were measured-elytral length, horn length, and weight; there were strong positive correlations among the three characters. Selection was estimated by regressing each component of fitness on the phenotypic traits. Of the three traits, only horn length was under significant direct selection. This selection was for longer horns and was due mainly to differences in lifespan and access to females. The positive selection on horn length combined with the positive correlations between horn length and the other two characters resulted in positive total selection on all three characters.

THE EVOLUTION OF SEXUAL DIMORPHISM BY SEXUAL SELECTION: THE SEPARATE EFFECTS OF INTRASEXUAL SELECTION AND INTERSEXUAL SELECTION

Libellula luctuosa, a pond dragonfly found in eastern North America, is apparently sexually dimorphic. Previous studies of the mating behavior in this species suggested that both male-male competition and female mate choice are important influences. Males compete for territories, where they attract females and where mating occurs. Female behavior influences both the copulation success and the fertilization success of males. Because of temporal and spatial separation of these episodes of sexual selection, multivariate and nonparametric statistical techniques could be used to investigate the influence of components of sexual selection on various sexually dimorphic traits. Sexual dimorphism in L. luctuosa was first quantified; then the direct effects and the form of selection were estimated. Sexually dimorphic wing size, body size, wing coloration, and body coloration are distributed either continuously or discontinuously between the sexes in L. luctuosa. These traits have apparently diverged between the sexes as a result of directional sexual selection. Body size is further influenced by stabilizing selection. Intrasexual selection (success in gaining access to a territory) and intersexual selection (success in copulation and fertilization) can influence the same or different sexually dimorphic characters. Body size is influenced by directional selection during the intrasexual phase of sexual selection and is also influenced by stabilizing selection during intersexual selection. The size of the brown wing patch is influenced by directional selection, primarily during the intersexual phase of sexual selection. There is directional selection on the white wing patch during both phases. Thus, the different proximate mechanisms of sexual selection may jointly or separately affect the evolution of sexually dimorphic characters. Further empirical and theoretical investigations into the differences in the effects of intrasexual selection and intersexual selection are needed to clarify the circumstances leading to separate consequences of these two mechanisms of sexual selection.

MEASURING THE EFFECTS OF PAIRING SUCCESS, EXTRA-PAIR COPULATIONS AND MATE QUALITY ON THE OPPORTUNITY FOR SEXUAL SELECTION

Sexual selection can act through variation in the number of social mates obtained, variation in mate quality, or variation in success at obtaining extra-pair fertilizations. Because within-pair fertilizations (WPF) and extra-pair fertilizations (EPF) are alternate routes of reproduction, they are additive, rather than multiplicative, components of fitness. We present a method for partitioning total variance in reproductive success (a measure of the opportunity for selection) when fitness components are both additive and multiplicative and use it to partition the variance into components that correspond to each mechanism of sexual selection. Computer simulations show that extra-pair fertilizations can either increase or decrease total variance, depending on the covariance between within-pair and extra-pair success. Simulations also suggest that for socially monogamous species, extra-pair fertilizations have a greater effect than variation in mate quality or pairing status on the opportunity for selection. Application of our model to data gathered for a population of red-winged blackbirds (Agelaius phoeniceus) indicates that most of the variance in male reproductive success was attributable to within-pair sources of variance. Nevertheless, extra-pair copulations increased the opportunity for selection because males varied both in the proportion of their social young that they sired and in the number of extra-pair mates that they obtained. Furthermore, large and positive covariances existed between the number of extra-pair mates a male obtained and both social pairing success and within-pair paternity, indicating that, in this population, males preferred as social mates also were preferred as extra-pair mates.

Population demography and sex ratio in a Neotropical damselfly (Odonata: Coenagrionidae) in Costa Rica

Throughout the latter part of the dry season (January-April) of 1985 and 1986, we studied the demography of mature adults in a small population of Argia chelata along a stream at Monteverde, Costa Rica. Males defended sunlit spots along the stream between 1030 h and 1400 h central standard time each day and females visiting these sunlit spots were soon mated. By individually marking all mature adults in the population we found that there was no significant difference between the sexes in daily survival rates (0.85 for males and 0.86 for females). Lifetime mating success was positively correlated with longevitiy in both males and females but the ability of males to obtain a mate appeared to be independent of age.

Although the sex ratio at emergence was 1:1, that of mature adults at the stream was strongly male-biased (87% male), perhaps as a result of a longer or more risky prereproductive period in females related to the costs of egg formation. We argue that the strongly male-biased sex ratio has important effects on the mating system of this species in that the male contact-guards the female for the entire oviposition period each day. Although this limits each male to mating with one female per day, it probably ensures that the male will fertilize most of the eggs laid by the female that day.

The strength of phenotypic selection in natural populations

How strong is phenotypic selection on quantitative traits in the wild? We reviewed the literature from 1984 through 1997 for studies that estimated the strength of linear and quadratic selection in terms of standardized selection gradients or differentials on natural variation in quantitative traits for field populations. We tabulated 63 published studies of 62 species that reported over 2,500 estimates of linear or quadratic selection. More than 80% of the estimates were for morphological traits; there is very little data for behavioral or physiological traits. Most published selection studies were unreplicated and had sample sizes below 135 individuals, resulting in low statistical power to detect selection of the magnitude typically reported for natural populations. The absolute values of linear selection gradients |beta| were exponentially distributed with an overall median of 0.16, suggesting that strong directional selection was uncommon. The values of |beta| for selection on morphological and on life-history/phenological traits were significantly different: on average, selection on morphology was stronger than selection on phenology/life history. Similarly, the values of |beta| for selection via aspects of survival, fecundity, and mating success were significantly different: on average, selection on mating success was stronger than on survival. Comparisons of estimated linear selection gradients and differentials suggest that indirect components of phenotypic selection were usually modest relative to direct components. The absolute values of quadratic selection gradients |gamma| were exponentially distributed with an overall median of only 0.10, suggesting that quadratic selection is typically quite weak. The distribution of gamma values was symmetric about 0, providing no evidence that stabilizing selection is stronger or more common than disruptive selection in nature.

Sequential conflicting selection due to multispecific interactions triggers evolutionary trade-offs in a monocarpic herb

Trade-offs are crucial in understanding phenotypic evolution of organisms. A main source of trade-offs is conflicting selection, a phenomenon very likely in complex multispecific scenarios in which many potential selective agents coexist. The main goal of this study is to investigate the selective trade-offs arising due to conflicting selection on female-fitness components in Erysimum mediohispanicum. I quantified the selection exerted on 10 plant traits by a mutualistic (pollinators) and antagonistic (gall-makers, predispersal and postdispersal seed predators, mammalian herbivores) multispecific assemblage acting sequentially throughout eight selective episodes of the plant, from floral bud to juvenile production. Variation in lifetime female fitness (quantified as number of juveniles) was related mostly to variation in number of flowers, fruit initiation, and seedling establishment. The direction of selection changed among different selective episode for many traits. Most importantly, conflicting selection was frequent in the study system, with half of the phenotypic traits experiencing opposing selection in different selective episodes. Selection at individual life-cycle stages diverged remarkably from selection based on total fitness. Consequently, the evolution of many traits is determined by the relative importance of each episode of selection, with conflicting selection inevitably yielding evolutionary compromises.

Molecular population divergence and sexual selection on morphology in the banded demoiselle (Calopteryx splendens)

The importance of sexual selection in population divergence is of much interest, mainly because it is thought to cause reproductive isolation and hence could lead to speciation. Sexually selected traits have been hypothesized to diverge faster between populations than other traits, presumably because of differences in the strength, mechanism or dynamics of selection. We investigated this by quantifying population divergence in eight morphological characters in 12 south Swedish populations of a sexually dimorphic damselfly, the banded demoiselle (Calopteryx splendens). The morphological characters included a secondary sexual character, the male melanized wing spot, which has an important function in both inter- and intrasexual selection. In addition, we investigated molecular population divergence, revealed by amplified fragment length polymorphism (AFLP) analysis. Molecular population divergence was highly significant among these Northern European populations (overall F(st)=0.054; pairwise population F(st)'s ranged from approximately 0 to 0.13). We found evidence for isolation-by-distance (r=0.70) for the molecular markers and a significant correlation between molecular and phenotypic population divergence (r=0.39). One interpretation is that population divergence for the AFLP loci are affected by genetic drift, but is also indirectly influenced by selection, due to linkage with loci for the phenotypic traits. Field estimates of sexual and natural selection from two of the populations revealed fairly strong sexual selection on wing spot length, indicating that this trait has the potential to rapidly diverge, provided that variation is heritable and the observed selection is chronic.

Condition-dependent behaviour among damselfly populations

Body condition is predicted to influence behaviours such as activity, which in turn affects energy gain and survival. In this study we investigated (i) whether populations of the damselfly Lestes sponsa (Hansemann, 1823) differ in body condition and activity among lakes, and (ii) which body condition factors affect behaviour. We estimated last instar larval behaviour (measured as activity), body condition (measured as size, body mass, muscle mass, fat content, and time to emergence), and fish presence/absence in eight lakes. Body condition of larvae differed among lakes but the presence/absence of fish in lakes had no effect on body condition. Activity did not differ among lakes and was not affected by the presence/absence of fish in lakes. Activity was negatively related to size, body mass, muscle mass, and fat content, and positively related with time to emergence, suggesting that final-instar larvae in good condition are favouring development over growth to emerge earlier. This study highlights the importance of differences in condition among populations and among individuals.

Natural selection on body size in Tribolium: possible genetic constraints on adaptive evolution

To determine whether genetic constraints on adaptive evolution were operating in a laboratory population of a flour beetle, Tribolium castaneum, we first estimated the direct selection acting on each of several body size traits. Strong selection in males for an increase in pupal weight and a decrease in the ratio of adult to pupal weight occurred. In addition, a non-significant trend for a decrease in adult width was found. No significant selection on females was detected, although there were trends toward an increase in pupal weight and a decrease in adult width. These estimates were then combined with estimates of the genetic variances and covariances of the traits to predict the multivariate response to selection, that is, the evolutionary change in the traits across one generation. These projections showed only a small predicted change in male pupal weight in spite of the strong selection on pupal weight, and a relatively large predicted increase in width in spite of the possible negative direct selection on this trait. Both of these results were due in part to the positive genetic covariance between pupal weight and width, and they therefore suggest the possibility of genetic constraints on adaptive evolution of these traits.