Lifetime selection on adult body size and components of body size in a waterstrider: opposing selection and maintenance of sexual size dimorphism

Abstract.— Sexual size dimorphism (SSD), the difference in body size between males and females, is common in almost all taxa of animals and is generally assumed to be adaptive. Although sexual selection and fecundity selection alone have often been invoked to explain the evolution of SSD, more recent views indicate that the sexes must experience different lifetime selection pressures for SSD to evolve and be maintained. We estimated selection acting on male and female adult body size (total length) and components of body size in the waterstrider Aquarius remigis during three phases of life history. Opposing selection pressures for overall body size occurred in separate episodes of fitness for females in both years and for males in one year. Specific components of body size were often the targets of the selection on overall body size. When net adult fitness was estimated by combining each individual's fitnesses from all episodes, we found stabilizing selection in both sexes. In addition, the net optimum overall body size of males was smaller than that of females. However, even when components of body size had experienced opposing selection pressures in individual episodes, no components appeared to be under lifetime stabilizing selection. This is the first evidence that contemporary selection in a natural population acts to maintain female size larger than male size, the most common pattern of SSD in nature.     

Directional and anteroposterior asymmetry of common white markings in the legs of the Arabian horse: response to selection

Arabian bay horses manifest, on the average, more common white markings in their hind legs than their forelegs (anteroposterior asymmetry) and more common white markings in their left legs than their right legs (directional asymmetry). To determine if genetic variation exists for these types of asymmetry, the phenotypic response was studied in bay foals when their dams and sires were selected for the directions of fore-hind and left-right differences. In the fore-hind studies, the quantitative shifts in the bay foals were in the direction specified by the selection scheme and the observed deviations were all statistically significant. The shifts were also consistently in the direction favored by selection in the left-right studies, but only two of six observed deviations were statistically significant using a one-tailed test of significance. Thus, only marginal statistical evidence is available to support the observed consistent responses to selection in the left-right studies. These differential responses are reflected in the magnitudes of the heritability estimates. Based on the overall results, it is concluded that both types of asymmetry have a genetic basis in the Arabian horse, but much more genetic variation is present for anteroposterior asymmetry than for directional asymmetry. This revised version was published online in July 2006 with corrections to the Cover Date.     

Interactions between sexual and natural selection on the evolution of a plumage badge

The evolutionary stability of signals varies due to interactions between sexual and natural selection. A tidal-marsh sparrow, Melospiza georgiana nigrescens, possesses darker pigmentation than an inland-marsh sparrow, M. g. georgiana. Studies of feather-degrading bacteria and convergent evolution among salt-marsh vertebrates suggest this dark coloration is due to environmental selection. Sexually dichromatic swamp sparrow crowns, however, may be additionally under sexual selection. We investigated ties between two plumage patches (rusty cap and black forehead) and two behaviors (male-male aggression and parental care) in the coastal and inland subspecies to test the effect of sexual versus natural selection on badge evolution. Across both subspecies the extent of rusty feathers in the cap patch was correlated positively with parental care and negatively with aggression, and the extent of black feathers in the forehead patch was correlated positively with aggression. Males with larger forehead patches produced more offspring along the coast, while males with larger cap patches did so inland. The date of the first nesting attempt for both subspecies correlated with cap patch extent, suggesting a similar role for female choice. Natural selection likely accounts for darker coastal females. Coastal male head color, however, is darker due to increased selection for larger forehead patches via intrasexual competition, yet it remains largely rusty due to female choice for larger cap patches. Increased sexual dichromatism among coastal plain swamp sparrows thus provides a clear example of the interplay between sexual and natural selection in subspecies divergence.     

Artificial selection on male longevity influences age-dependent reproductive effort in the black field cricket Teleogryllus commodus

Although the trade-off between reproductive effort and longevity is central to both sexual selection and evolutionary theories of aging, there has been little synthesis between these fields. Here, we selected directly on adult longevity of male field crickets Teleogryllus commodus and measured the correlated responses of age-dependent male reproductive effort, female lifetime fecundity, and several other life-history traits. Male longevity responded significantly to five generations of divergent selection. Males from downward-selected lines commenced calling sooner and reached their peak calling effort at a younger age. They called more per night and, despite living less than half as long, called more overall than males selected for increased longevity. Females from the downward-selected lines lived significantly shorter lives than females from the upward-selected lines but still produced the same number of offspring. Nymph survival, development time, and body size and weight at eclosion did not show significant correlated response to selection on male longevity, despite evidence for substantial genetic variation in each of these traits. Collectively, our findings directly support the antagonistic pleiotropy model of aging and suggest an important role for sexual selection in the aging process.     

Morpho morphometrics: Shared ancestry and selection drive the evolution of wing size and shape in Morpho butterflies

Butterfly wings harbor highly diverse phenotypes and are involved in many functions. Wing size and shape result from interactions between adaptive processes, phylogenetic history, and developmental constraints, which are complex to disentangle. Here, we focus on the genus Morpho (Nymphalidae: Satyrinae, 30 species), which presents a high diversity of sizes, shapes, and color patterns. First, we generate a comprehensive molecular phylogeny of these 30 species. Next, using 911 collection specimens, we quantify the variation of wing size and shape across species, to assess the importance of shared ancestry, microhabitat use, and sexual selection in the evolution of the wings. While accounting for phylogenetic and allometric effects, we detect a significant difference in wing shape but not size among microhabitats. Fore and hindwings covary at the individual and species levels, and the covariation differs among microhabitats. However, the microhabitat structure in covariation disappears when phylogenetic relationships are taken into account. Our results demonstrate that microhabitat has driven wing shape evolution, although it has not strongly affected forewing and hindwing integration. We also found that sexual dimorphism of forewing shape and color pattern are coupled, suggesting a common selective force.     

Sexual selection favors female-biased sex ratios: the balance between the opposing forces of sex-ratio selection and sexual selection

In a verbal model, Trivers and Willard proposed that, whenever there is sexual selection among males, natural selection should favor mothers that produce sons when in good condition but daughters when in poor condition. The predictions of this model have been the subject of recent debate. We present an explicit population genetic model for the evolution of a maternal-effect gene that biases offspring sex ratio. We show that, like local mate competition, sexual selection favors female-biased sex ratios whenever maternal condition affects the reproductive competitive ability of sons. However, Fisherian sex-ratio selection, which favors a balanced sex ratio, is an opposing force. We show that the evolution of maternal sex-ratio biasing by these opposing selection forces requires a positive covariance across environments between the sex-ratio bias toward sons (b) and the mating success of sons (r). This covariance alone is not a sufficient condition for the evolution of maternal sex-ratio biasing; it must be sufficiently positive to outweigh the opposing sex-ratio selection. To identify the necessary and sufficient conditions, we partition total evolutionary change into three components: (1) maternal sex-ratio bias, (2) sexual selection on sons, and (3) sex-ratio selection. Because the magnitude of the first component asymmetrically affects the strength of the second, biasing broods toward females in a poor environment evolves faster than the same degree of bias toward males in a good environment. Consequently, female-biased sex ratios, rather than male-biased sex ratios, are more likely to evolve. We discuss our findings in the context of the primary sex-ratio biases observed in strongly sexually selected species and indicate how this perspective can assist the experimental study of sex ratio evolution.     

The effects of latitude,body size,and sexual selection on wing shape in a damselfly

Under natural selection, wing shape is expected to evolve to optimize flight performance. However, other selective factors besides flight performance may influence wing shape. One such factor could be sexual selection in wing sexual ornaments, which may lead to alternative variations in wing shape that are not necessarily related to flight performance. In the present study, we investigated wing shape variations in a calopterygid damselfly along a latitudinal gradient using geometric morphometrics. Both sexes show wing pigmentation, which is a known signal trait at intra‐ and interspecific levels. Wing shape differed between sexes and, within the same sex, the shape of the hind wing differed from the front wing. Latitude and body size explained a high percentage of the variation in wing shape for female front and hind wings, and male front wings. In male hind wings, wing pigmentation explained a high amount of the variation in wing shape. On the other hand, the variation in shape explained by pigmentation was very low in females. We suggest that the conservative morphology of front wings is maintained by natural selection operating on flight performance, whereas the sex‐specific differences in hind wings most likely could be explained by sexual selection. The observed sexual dimorphism in wing shape is likely a result of different sex‐specific behaviours. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 102 , 263–274.     

Optimal marker-assisted selection to increase the effective size of small populations

An approach to the optimal utilization of marker and pedigree information in minimizing the rates of inbreeding and genetic drift at the average locus of the genome (not just the marked loci) in a small diploid population is proposed, and its efficiency is investigated by stochastic simulations. The approach is based on estimating the expected pedigree of each chromosome by using marker and individual pedigree information and minimizing the average coancestry of selected chromosomes by quadratic integer programming. It is shown that the approach is much more effective and much less computer demanding in implementation than previous ones. For pigs with 10 offspring per mother genotyped for two markers (each with four alleles at equal initial frequency) per chromosome of 100 cM, the approach can increase the average effective size for the whole genome by approximately 40 and 55% if mating ratios (the number of females mated with a male) are 3 and 12, respectively, compared with the corresponding values obtained by optimizing between-family selection using pedigree information only. The efficiency of the marker-assisted selection method increases with increasing amount of marker information (number of markers per chromosome, heterozygosity per marker) and family size, but decreases with increasing genome size. For less prolific species, the approach is still effective if the mating ratio is large so that a high marker-assisted selection pressure on the rarer sex can be maintained.     

Sexual dichromatism,size dimorphism,and microscale anatomy of white wing stripe in blue tits

Achromatic patches are a common element of plumage patterns in many bird species and there is growing body of evidence that in many avian taxa they can play a signaling role in mate choice. Although the blue tit Cyanistes caeruleus is a well-established model species in the studies on coloration, its white wing patch has never been examined in the context of sex-specific trait expression. In this exploratory study, we examined sexual size dimorphism and dichromatism of greater covert’s dots creating white wing patch and analyzed its correlations with current body condition and crown coloration—a trait with established role in sexual selection. Further, we qualitatively analyzed microstructural barb morphology underlying covert’s coloration. We found significant sexual dimorphism in the dot size independent of covert size and sexual dichromatism in both white dot and blue outer covert’s vane spectral characteristics. Internal structure of covert barbs within the white dot was similar to the one found in barbs from the blue part that is, with a medullary area consisting of dead keratinocytes containing channel-type ß-keratin spongy nanostructure and centrally located air cavities. However, it lacked melanosomes which was the main observed difference. Importantly, UV chroma of covert’s blue vane was positively correlated with crown UV chroma and current condition (the latter only in males), which should be a premise for further research on the signal function of the wing stripe.     

A comparative perspective on longevity: the effect of body size dominates over ecology in moths

Both physiologically and ecologically based explanations have been proposed to account for among‐species differences in lifespan, but they remain poorly tested. Phylogenetically explicit comparative analyses are still scarce and those that exist are biased towards homoeothermic vertebrates. Insect studies can significantly contribute as lifespan can feasibly be measured in a high number of species, and the selective forces that have shaped it may differ largely between species and from those acting on larger animals. We recorded adult lifespan in 98 species of geometrid moths. Phylogenetic comparative analyses were applied to study variation in species‐specific values of lifespan and to reveal its ecological and life‐history correlates. Among‐species and between‐gender differences in lifespan were found to be notably limited; there was also no evidence of phylogenetic signal in this trait. Larger moth species were found to live longer, with this result supporting a physiological rather than ecological explanation of this relationship. Species‐specific lifespan values could not be explained by traits such as reproductive season and larval diet breadth, strengthening the evidence for the dominance of physiological determinants of longevity over ecological ones.     

Genetic analysis of white facial and leg markings in the Swiss Franches-Montagnes Horse Breed

White markings and spotting patterns in animal species are thought to be a result of the domestication process. They often serve for the identification of individuals but sometimes are accompanied by complex pathological syndromes. In the Swiss Franches-Montagnes horse population, white markings increased vastly in size and occurrence during the past 30 years, although the breeding goal demands a horse with as little depigmented areas as possible. In order to improve selection and avoid more excessive depigmentation on the population level, we estimated population parameters and breeding values for white head and anterior and posterior leg markings. Heritabilities and genetic correlations for the traits were high (h(2) > 0.5). A strong positive correlation was found between the chestnut allele at the melanocortin-1-receptor gene locus and the extent of white markings. Segregation analysis revealed that our data fit best to a model including a polygenic effect and a biallelic locus with a dominant-recessive mode of inheritance. The recessive allele was found to be the white trait-increasing allele. Multilocus linkage disequilibrium analysis allowed the mapping of the putative major locus to a chromosomal region on ECA3q harboring the KIT gene.     

Digest: A synergistic approach explains the evolutionary connection between brain size and longevity*

The cognitive buffer hypothesis poses that brain size evolves to buffer individuals from environmental changes, increasing survival. Jiménez-Ortega et al. (2020) explored this hypothesis using a phylogenetic path analysis and showed that there is a direct causal link between brain size and longevity in birds, even when allometric effects are taken into account. Furthermore, a synergistic model was better supported than models that included independent effects of brain size and body size.     

Variability of a consistent trait: The size of the white wing patch in European Stonechats (Saxicola rubicola rubicola)

Numerous bird species exhibit striking white patches on their plumages that have been reported as signals of individual quality in the context of sexual selection. Whereas differences in white plumage traits between individuals have been well studied, phenotypic variation and the factors affecting their consistency within the individual have received less attention. Here, we studied the consistency in the size of the white wing patch and its components of variation in a Spanish population of European Stonechats Saxicola rubicola rubicola monitored over 6 years (2007–2012). The wing patch was larger in males than in females for all age-classes, and first-year individuals exhibited smaller wing patches than older birds, particularly males. This trait also varied within the plumage-year at both population and individual level, with slight changes from the moulting period to the breeding season and a sharp decrease afterwards. In addition, patch size varied both between and within individuals across years. The daily mean temperature experienced in the month immediately prior to the moult (i.e. May) had a positive effect on male wing patch size. In females, however, no variable was found to explain wing patch variation satisfactorily. Despite being a dynamic trait throughout the moults, baseline and adjusted repeatabilities indicated a moderately high consistency for white wing patch size in both sexes. Therefore, the white wing patch in Stonechats is a dynamic trait that changes throughout individuals' lives, but is also a consistent trait at the individual level. These results indicate that this depigmented patch could convey information about both the long-term quality and the current state of individuals, harmonizing the existence of intra-individual changes in the size of sexual traits with its potential role as a genetic quality indicator.     

Trade‐off between fecundity and survival generates stabilizing selection on gall size

Complex interactions within multitrophic communities are fundamental to the evolution of individual species that reside within them. One common outcome of species interactions are fitness trade‐offs, where traits adaptive in some circumstances are maladaptive in others. Here, we identify a fitness trade‐off between fecundity and survival in the cynipid wasp Callirhytis quercusbatatoides that induces multichambered galls on the stem of its host plant Quercus virginiana. We first quantified this trade‐off in natural populations by documenting two relationships: a positive association between the trait gall size and fecundity, as larger galls contain more offspring, and a negative association between gall size and survival, as larger galls are attacked by birds at a higher rate. Next, we performed a field‐based experimental evolution study where birds were excluded from the entire canopy of 11 large host trees for five years. As a result of the five‐year release from avian predators, we observed a significant shift to larger galls per tree. Overall, our study demonstrates how two opposing forces of selection can generate stabilizing selection on a critical phenotypic trait in wild populations, and how traits can evolve rapidly in the predicted direction when conditions change.     

The evolution of competition and policing: opposing selection within and among groups

Background  

Although selection favors exploitative competition within groups, a group of hypercompetitive individuals may be less productive than a cooperative group. When competition is costly for group fitness, among-group selection can favor groups with 'policing' individuals who reduce within-group competition at a cost to their own fitness, or groups of individuals who restrain their competitive intensity ('self policing'). We examine these possibilities in a series of explicit population-genetic models.     

N- and C-terminal residues of eIF1A have opposing effects on the fidelity of start codon selection

Translation initiation factor eIF1A stimulates preinitiation complex (PIC) assembly and scanning, but the molecular mechanisms of its functions are not understood. We show that the F131A,F133A mutation in the C-terminal tail (CTT) of eIF1A impairs recruitment of the eIF2-GTP-Met-tRNA(i)(Met) ternary complex to 40S subunits, eliminating functional coupling with eIF1. Mutating residues 17-21 in the N-terminal tail (NTT) of eIF1A also reduces PIC assembly, but in a manner rescued by eIF1. Interestingly, the 131,133 CTT mutation enhances initiation at UUG codons (Sui(-) phenotype) and decreases leaky scanning at AUG, while the NTT mutation 17-21 suppresses the Sui(-) phenotypes of eIF5 and eIF2beta mutations and increases leaky scanning. These findings and the opposite effects of the mutations on eIF1A binding to reconstituted PICs suggest that the NTT mutations promote an open, scanning-conducive conformation of the PIC, whereas the CTT mutations 131,133 have the reverse effect. We conclude that tight binding of eIF1A to the PIC is an important determinant of AUG selection and is modulated in opposite directions by residues in the NTT and CTT of eIF1A.     

Antagonistic selection between adult thorax and wing size in field released Drosophila melanogaster independent of thermal conditions

Attempts to explain size variation in Drosophila and other small insects often focus on the larval stage and association between development time and size, but patterns are also influenced by direct selection on size-related traits in the adults. Here we use multiple field releases of Drosophila melanogaster to test the association between size and one component of field fitness, the ability of Drosophila to locate resources for feeding and breeding. We find antagonistic selection between wing length and thorax length in both males and females, such that capture at baits is higher for flies with relatively larger thorax lengths and smaller wings. However flies with large wings relative to thoraces disperse further as reflected in the longer distances moved to baits. These patterns did not depend strongly on weather conditions, suggesting that selection on adult size is at least partly independent of temperature. Antagonistic selection between size traits can generate changes in size along gradients if the distribution of resources in the environment varies and selects for different dispersal patterns, particularly as dispersal is relatively higher under warmer conditions.