Repeated selection of morphometric traits in the Soay sheep on St Kilda

1. Long-term studies allow the outcomes of repeated selection events to be monitored. Here, we investigate phenotypic selection in successive winter mortality events in the Soay sheep of St Kilda, Scotland, between 1985 and 1996. Selection of three quantitative morphometric traits, body weight, hindleg length and incisor arcade breadth, was investigated in different sectors of the population.
2. Evidence from fitness differentials of positive directional selection for large size was repeatedly found in lambs and adult females. Selection in the opposing direction was only found in one year in lambs.
3. Selection gradients showed that in most years when significant selection occurred, body weight was the focus of direct selection, whereas selection of hindleg length and incisor breadth was indirect, arising from their correlation with body weight.
4. Selection was strongest in years of low over-winter survival and almost absent in years when survival was high. Intensity of selection was greatest in lambs, emphasizing the differences in selection pressure experienced by different sectors of the population, in addition to the temporal variation in selection pressure due to population density and environmental conditions.
5. Despite repeated positive selection of body weight, no evidence of a change in the population mean was found over the course of the study.     

An analysis of continent-wide patterns of sexual selection in a passerine bird

Patterns of selection are widely believed to differ geographically, causing adaptation to local environmental conditions. However, few studies have investigated patterns of phenotypic selection across large spatial scales. We quantified the intensity of selection on morphology in a monogamous passerine bird, the barn swallow Hirundo rustica, using 6495 adults from 22 populations distributed across Europe and North Africa. According to the classical Darwin-Fisher mechanism of sexual selection in monogamous species, two important components of fitness due to sexual selection are the advantages that the most attractive males acquire by starting to breed early and their high annual fecundity. We estimated directional selection differentials on tail length (a secondary sexual character) and directional selection gradients after controlling for correlated selection on wing length and tarsus length with respect to these two fitness components. Phenotype and fitness components differed significantly among populations for which estimates were available for more than a single year. Likewise, selection differentials and selection gradients differed significantly among populations for tail length, but not for the other two characters. Sexual selection differentials differed significantly from zero across populations for tail length, particularly in males. Controlling statistically for the effects of age reduced the intensity of selection by 60 to 81%, although corrected and uncorrected estimates were strongly positively correlated. Selection differentials and gradients for tail length were positively correlated between the sexes among populations for selection acting on breeding date, but not for fecundity selection. The intensity of selection with respect to breeding date and fecundity were significantly correlated for tail length across populations. Sexual size dimorphism in tail length was significantly correlated with selection differentials with respect to breeding date for tail length in male barn swallows across populations. These findings suggest that patterns of sexual selection are consistent across large geographical scales, but also that they vary among populations. In addition, geographical patterns of phenotypic selection predict current patterns of phenotypic variation among populations, suggesting that consistent patterns of selection have been present for considerable amounts of time.     

Social selection acts on behavior and body mass but does not contribute to the total selection differential in eastern chipmunks

Through social interactions, phenotypes of conspecifics can affect an individual's fitness, resulting in social selection. Social selection is assumed to represent a strong and dynamic evolutionary force that can act with or in opposition to natural selection. Few studies, however, have estimated social selection and its contribution to total selection in the wild. We estimated natural and social selection gradients on exploration, docility, and body mass, and their contribution to selection differentials, in a wild eastern chipmunk population (Tamias striatus). We applied trait-based multiple regression models derived from classical phenotypic selection analyses, which allowed us to include several social partners (i.e., neighbors). We detected social selection gradients on female docility and male body mass, indicating that female with docile neighbors and males with large neighbors had lower fitness. In both sexes, social selection gradients varied with the season. However, we found no phenotypic assortment or disassortment for the studied traits. Social selection gradients, therefore, did not contribute to total selection differentials, and natural selection alone could drive phenotypic changes. Evaluating the factors that drive the evolution of the covariance between interacting phenotypes is necessary to understand the role of social selection as an evolutionary force.     

Sex‐specific selection on energy metabolism – selection coefficients for winter survival

Selection for different fitness optima between sexes is supposed to operate on several traits. As fitness‐related traits are often energetically costly, selection should also act directly on the energetics of individuals. However, efforts to examine the relationship between fitness and components of the energy budget are surprisingly scarce. We investigated the effects of basal metabolic rate (BMR, the minimum energy required for basic life functions) and body condition on long‐term survival (8 winter months) with manipulated densities in enclosed populations of bank voles (Myodes glareolus). Here, we show that survival selection on BMR was clearly sex‐specific but density‐independent. Both the linear selection gradient and selection differential for BMR were positive in females, whereas survival did not correlate with male characteristics. Our findings emphasize the relative importance of individual physiology over ecological factors (e.g. intra‐specific competition). Most current models of the origin of endothermy underline the importance of metabolic optima in females, whose physiology evolved to fulfil demands of parental provisioning in mammals. Our novel findings of sex‐specific selection could be related to these life history differences between sexes.     

Phenotypic selection in a natural population of Impatiens pallida Nutt. (Balsaminaceae)

In the annual plant Impatiens pallida, individuals exhibit a floral heteromorphism consisting of autogamously selfing, cleistogamous (CL) flowers and partially outcrossing, chasmogamous (CH) flowers. As part of an investigation into natural selection and mating system evolution in I. pallida, we measured the magnitude and direction of phenotypic selection on nine life history characters (two traits measured on three dates, one measured on two dates and one measured once). Three of these characters were positively correlated with the ratio of CH/CL flowers produced per plant, which is an important determinant of the mating system. Values for the nine characters and three different measures of fitness (viability, fecundity, lifetime) were estimated for 500 plants in five locations over a single growing season. Based on lifetime fitness, linear selection differentials were significant for all nine characters, indicating a selective advantage to tall, leafy, highly branched plants that flowered early. However, only two of these characters had a direct effect on fitness. Selection was significant on all nine characters when based on fecundity as well as lifetime fitness; however, only three of five characters examined had significant selection based on viability fitness. For all fitness components, the frequency of significant linear and nonlinear selection coefficients was comparable (23% vs 17% of all cases, respectively), but nonlinear coefficients were generally larger. Finally, the magnitude and direction of direct linear selection was heterogeneous among locations, for all characters and all fitness components. Collectively these results suggest that selection is strong, favouring large size, high allocation to reproduction and high CH/CL flower ratios. However, any directional evolutionary changes in vegetative or reproductive characters may be constrained by strong non-linear and correlational selection.     

EXTREME SELECTION ON SIZE IN THE EARLY LIVES OF FISH

Although fitness typically increases with body size and selection gradients on size are generally positive, much of this information comes from terrestrial taxa. In the early life history of fish, there is evidence of selection both for and against larger size, leaving open the question of whether the general pattern for terrestrial taxa is valid for fish. We reviewed studies of size‐dependent survival in the early life history of fish and obtained estimates of standardized selection differentials from 40 studies. We found that 77% of estimated selection differentials favored larger size and that the strength of selection was more than five times that seen in terrestrial taxa. Selection decreased with study period duration and initial length, and disruptive selection occurred significantly more frequently than stabilizing selection. Contrary to expectations from Bergmann's rule, selection on size did not increase with latitude.     

Spatiotemporal fluctuations in natural selection acting on the gall‐parasitic aphid Tetraneura sorini

The measurement of the selection gradient is crucial for understanding the magnitude of selection acting directly on a trait and predicting the evolutionary trajectory of that trait. This study evaluated the selection gradient acting on the morphology of the gall‐parasitic aphid Tetraneura sorini during the galling process and compared the strength among populations. Gall formers (first instars) frequently fight with conspecifics or heterospecifics for usurping incipient galls using their well‐developed hind legs. First instars that successfully acquired galls were found within galls, whereas those that failed were found dead on leaf surfaces. Selection gradients were estimated using logistic stepwise regression and partial least square (PLS) regression. Calculated selection differentials indicated that first instars that secured galls were larger in body size than failed individuals through all populations. However, selection gradients on weapon traits varied largely among populations or among years in the same population. We confirmed microevolutionary changes in the relationship between traits, which accorded with the expectation from changes in the selection gradients. When gall formers were transferred onto developing buds individually, individuals that successfully induced galls had smaller body size than failed individuals. Available evidence suggests that the selection gradient on body size becomes higher with an increasing proportion of T. sorini in the Tetraneura species community. Thus, we concluded that more intense fighting with conspecifics leads to stronger selective pressure on body size, but that selective pressure for each trait is variable depending on differences in the tactics and species composition among populations.     

Sexual selection on female ornaments in the sex‐role‐reversed Gulf pipefish (Syngnathus scovelli)

Understanding how selection acts on traits individually and in combination is an important step in deciphering the mechanisms driving evolutionary change, but for most species, and especially those in which sexual selection acts more strongly on females than on males, we have no estimates of selection coefficients pertaining to the multivariate sexually selected phenotype. Here, we use a laboratory‐based mesocosm experiment to quantify pre‐ and post‐mating selection on female secondary sexual traits in the Gulf pipefish (Syngnathus scovelli), a sexually dimorphic, sex‐role‐reversed species in which ornamented females compete for access to choosy males. We calculate selection differentials and gradients on female traits, including ornament area, ornament number and body size for three episodes of selection related to female reproductive success (number of mates, number of eggs transferred and number of surviving embryos). Selection is strong on both ornament area and ornament size, and the majority of selection occurs during the premating episode of selection. Interestingly, selection on female body size, which has been detected in previous studies of Gulf pipefish, appears to be indirect, as evidenced by a multivariate analysis of selection gradients. Our results show that sexual selection favours either many bands or larger bands in female Gulf pipefish.     

Quantifying episodes of sexual selection: Insights from a transparent worm with fluorescent sperm

Sexual selection operates through consecutive episodes of selection that ultimately contribute to the observed variance in reproductive success between individuals. Understanding the relative importance of these episodes is challenging, particularly because the relevant postcopulatory fitness components are often difficult to assess. Here, we investigate different episodes of sexual selection on the male sex function, by assessing how (precopulatory) mating success, and (postcopulatory) sperm‐transfer efficiency and sperm‐fertilizing efficiency contribute to male reproductive success. Specifically, we used a transgenic line of the transparent flatworm, Macrostomum lignano, which expresses green fluorescent protein (GFP) in all cell types, including sperm cells, enabling in vivo sperm tracking and paternity analysis. We found that a large proportion of variance in male reproductive success arose from the postcopulatory episodes. Moreover, we also quantified selection differentials on 10 morphological traits. Testis size and seminal vesicle size showed significant positive selection differentials, which were mainly due to selection on sperm‐transfer efficiency. Overall, our results demonstrate that male reproductive success in M. lignano is not primarily limited by the number of matings achieved, but rather by the ability to convert matings into successful fertilizations, which is facilitated by producing many sperm.     

Subtle but ubiquitous selection on body size in a natural population of collared flycatchers over 33 years

Understanding the magnitude and long‐term patterns of selection in natural populations is of importance, for example, when analysing the evolutionary impact of climate change. We estimated univariate and multivariate directional, quadratic and correlational selection on four morphological traits (adult wing, tarsus and tail length, body mass) over a time period of 33 years (≈ 19 000 observations) in a nest‐box breeding population of collared flycatchers (Ficedula albicollis). In general, selection was weak in both males and females over the years regardless of fitness measure (fledged young, recruits and survival) with only few cases with statistically significant selection. When data were analysed in a multivariate context and as time series, a number of patterns emerged; there was a consistent, but weak, selection for longer wings in both sexes, selection was stronger on females when the number of fledged young was used as a fitness measure, there were no indications of sexually antagonistic selection, and we found a negative correlation between selection on tarsus and wing length in both sexes but using different fitness measures. Uni‐ and multivariate selection gradients were correlated only for wing length and mass. Multivariate selection gradient vectors were longer than corresponding vector of univariate gradients and had more constrained direction. Correlational selection had little importance. Overall, the fitness surface was more or less flat with few cases of significant curvature, indicating that the adaptive peak with regard to body size in this species is broader than the phenotypic distribution, which has resulted in weak estimates of selection.     

Sexual selection constrains the body mass of male but not female mice

Sexual size dimorphism results when female and male body size is influenced differently by natural and sexual selection. Typically, in polygynous species larger male body size is thought to be favored in competition for mates and constraints on maximal body size are due to countervailing natural selection on either sex; however, it has been postulated that sexual selection itself may result in stabilizing selection at an optimal mass. Here we test this hypothesis by retrospectively assessing the influence of body mass, one metric of body size, on the fitness of 113 wild‐derived house mice (Mus musculus) residing within ten replicate semi‐natural enclosures from previous studies conducted by our laboratory. Enclosures possess similar levels of sexual selection, but relaxed natural selection, relative to natural systems. Heavier females produced more offspring, while males of intermediate mass had the highest fitness. Female results suggest that some aspect of natural selection, absent from enclosures, acts to decrease their body mass, while the upper and lower boundaries of male mass are constrained by sexual selection.     

Multilevel phenotypic selection on morphological characters in a metapopulation of Silene tatarica

This study partitions selection in a natural metapopulation of a riparian plant species, Silene tatarica, into individual- and patch-level components by using contextual analysis, in which a patch refers to a spatially distinct stand of individual plants. We estimated selection gradients for two morphological characters (plant height and number of stems), their respective patch means, and plant density with respect to reproductive success in a two-year study. The approach was also extended to partition selection separately within habitats with varying degrees of exposure to river disturbances and herbivory. The selection differentials and gradients for plant height were positive at both individual and patch levels, with selection forces highest in the closed habitat with low exposure to disturbance. This pattern suggests that local groups with taller than average plants are more visible to pollinators than to groups that are shorter than average plants; and, within patches, individuals with short stature are visited less often than taller ones. Selection on the number of stems was in opposition at individual and patch levels. At the individual level the character was selected toward higher values, whereas selection at the patch-level favored smaller mean number of stems. The strength of the latter component was associated with the intensity of herbivory in different habitats, suggesting that the patch-level selection against a large number of stems might be due to high attractiveness of such patches to the main herbivore, reindeer. Consequently, direction and strength of selection in spatially structured populations may depend significantly on fitness effects arising at the group level.     

Measuring Natural Selection on Proportional Traits: Comparisons of Three Types of Selection Estimates for Resistance and Susceptibility to Herbivore Damage

To compare the strength of natural selection on different traits and in different species, evolutionary biologists typically estimate selection differentials and gradients in standardized units. Measuring selection differentials and gradients in standard deviation units or mean-standardized units facilitates such comparisons by converting estimates with potentially varied units to a common scale. In this note, I compare the performance of variance- and mean-standardized selection differentials and gradients for a unique and biologically important class of traits: proportional traits, that can only vary between zero and one, and their complements (1 minus the trait) using simple algebra and analysis of data from a field-study using morning glories. There is a systematic, mathematical relationship between unstandardized and variance-standardized selection gradients for proportional traits and their complements, but such a general relationship is lacking for mean-standardized gradients, potentially leading investigators to mistakenly conclude that a proportional change in a trait would have little effect on fitness. Despite this potential limitation, mean-standardized selection differentials and gradients represent a useful tool for studying natural selection on proportional traits, because by definition they measure how proportional changes in the mean of a trait lead to proportional changes in relative fitness.Co-ordinating editor: I. Olivieri     

SEASONAL VARIATION IN SEXUAL SELECTION IN THE MOTTLED SCULPIN

Seasonal variation in sexual and natural selection in male mottled sculpins (Cottus bairdi) can be evaluated by calculating selection differentials, which measure the magnitude of phenotypic change resulting from selection, and by calculating indices of the opportunity for selection, which indicate the potential for phenotypic selection in a given interval. Selection differentials are high at the beginning of the breeding season and decline throughout the breeding season. The magnitude and direction of selection differentials depend on when spawning occurs and are independent of the size or age of the females that spawn. Annual selection differentials due to differences in mating success (female choice) are nearly constant between years. Annual selection differentials associated with hatching success are variable. Opportunities for selection (I = fitness variance/[mean fitness]²) show clear seasonal patterns. They are highest at the beginning and at the end of the spawning season. However, this variation is dependent on the mean used to calculate I, and hence variation in I values does not indicate a significant change in the variance of male fitness.     

Maternal effects and maternal selection arising from variation in allocation of free amino acid to eggs

Maternal provisioning can have profound effects on offspring phenotypes, or maternal effects, especially early in life. One ubiquitous form of provisioning is in the makeup of egg. However, only a few studies examine the role of specific egg constituents in maternal effects, especially as they relate to maternal selection (a standardized selection gradient reflecting the covariance between maternal traits and offspring fitness). Here, we report on the evolutionary consequences of differences in maternal acquisition and allocation of amino acids to eggs. We manipulated acquisition by varying maternal diet (milkweed or sunflower) in the large milkweed bug, Oncopeltus fasciatus. Variation in allocation was detected by examining two source populations with different evolutionary histories and life‐history response to sunflower as food. We measured amino acids composition in eggs in this 2 × 2 design and found significant effects of source population and maternal diet on egg and nymph mass and of source population, maternal diet, and their interaction on amino acid composition of eggs. We measured significant linear and quadratic maternal selection on offspring mass associated with variation in amino acid allocation. Visualizing the performance surface along the major axes of nonlinear selection and plotting the mean amino acid profile of eggs from each treatment onto the surface revealed a saddle‐shaped fitness surface. While maternal selection appears to have influenced how females allocate amino acids, this maternal effect did not evolve equally in the two populations. Furthermore, none of the population means coincided with peak performance. Thus, we found that the composition of free amino acids in eggs was due to variation in both acquisition and allocation, which had significant fitness effects and created selection. However, although there can be an evolutionary response to novel food resources, females may be constrained from reaching phenotypic optima with regard to allocation of free amino acids.     

ESTIMATING AND VISUALIZING FITNESS SURFACES USING MARK–RECAPTURE DATA

Understanding how selection operates on a set of phenotypic traits is central to evolutionary biology. Often, it requires estimating survival (or other fitness‐related life‐history traits) which can be difficult to obtain for natural populations because individuals cannot be exhaustively followed. To cope with this issue of imperfect detection, we advocate the use of mark‐recapture data and we provide a general framework for both the estimation of linear and nonlinear selection gradients and the visualization of fitness surfaces. To quantify the strength of selection, the standard second‐order polynomial regression method is integrated in mark‐recapture models. To visualize the form of selection, we use splines to display selection acting on multivariate phenotypes in the most flexible way. We employ Markov chain Monte Carlo sampling in a Bayesian framework to estimate model parameters, assessing traits relevance and calculating the optimal amount of smoothing. We illustrate our approach using data from a wild population of Common blackbirds (Turdus merula) to investigate survival in relation to morphological traits, and provide evidence for correlational selection using the new methodology. Overall, the framework we propose will help in exploring the full potential of mark‐recapture data to study natural selection.     

CONDITION, GENOTYPE-BY-ENVIRONMENT INTERACTION, AND CORRELATIONAL SELECTION IN LIZARD LIFE-HISTORY MORPHS

Abstract We compared reproductive allocation and variation in condition and survivorship of two heritable female throat color morphs (orange and yellow) in a free‐living population of side‐blotched lizards (Uta stansburiana). Using path analysis and structural equation modeling, we investigated how variation in the social environment affected clutch size and egg mass and two condition traits (postlaying mass, immunological condition) and how these traits in turn affected female field survival. In the presence of many neighbors, both morphs increased their clutch sizes, although these effects were only significant in yellow females. In addition, yellow females increased their egg mass in the presence of many orange neighbors. Orange females surrounded by many orange neighbors showed sign of stress in the form of immunosuppression, whereas this effect was less pronounced in yellow females. The morphs also differed in the impact of variation in clutch size and egg mass on both condition traits. Finally, female morphotype and immune responsiveness affected fitness interactively, and hence these two traits showed signs of fitness epistasis: Selection gradients on this trait were opposite in sign in the two morphs. The correlational selection gradient (_{γthroatxantibody response}) between female throat color and antibody responsiveness was ‐0.365. Our data thus reveal important interactive effects such as genotype‐by‐environment interaction toward the social environment and morph‐specific trade‐offs as well as the occurrence of correlational selection. We discuss the use of naturally occurring and conspicuous genetic polymorphisms in field studies of selection and life‐history allocation.     

LIFE-CYCLE COMPONENTS OF SELECTION IN ERIGERON ANNUUS: I. PHENOTYPIC SELECTION

The magnitude and direction of phenotypic selection on emergence date and seedling size in Erigeron annuus was measured to determine the heterogeneity of selection among sites and the proportion of fitness variance explained by seedling size and emergence date. Three disturbance treatments (open, annual vegetation, perennial vegetation) were imposed to test the hypothesis of stronger selection on seedlings in competitive environments. Selection was most heterogeneous early in the life cycle, with significant spatial heterogeneity in the magnitude of selection on a local scale. The disturbance treatments affected only fecundity selection gradients and selection was strongest in open plots. Significant variation in the sign of selection differentials on emergence date was observed for establishment and fall viability selection episodes; at later stages selection varied in magnitude but not direction. Seedlings in the earliest cohort experienced high mortality during establishment, but increased size and fecundity later in the life cycle. Both stabilizing and disruptive selection on emergence date were observed during establishment, but in general selection was purely directional. At Stony Brook most selection on emergence date operated indirectly through seedling size, whereas at the Weld Preserve direct selection was stronger. There were persistent effects of both seedling emergence date and rosette diameter on adult fitness components, and October rosette diameter explained 18% of the total phenotypic variance in fecundity. Overall, viability fitness components were much more important than fecundity selection. Winter survivorship was the single most important episode of selection.     

Measuring sexual selection on females in sex‐role‐reversed Mormon crickets (Anabrus simplex,Orthoptera: Tettigoniidae)

Although many studies examine the form of sexual selection in males, studies characterizing this selection in females remain sparse. Sexual selection on females is predicted for sex‐role‐reversed Mormon crickets, Anabrus simplex, where males are choosy of mates and nutrient‐deprived females compete for matings and nutritious nuptial gifts. We used selection analyses to describe the strength and form of sexual selection on female morphology. There was no positive linear sexual selection on the female body size traits predicted to be associated with male preferences and female competition. Instead, we detected selection for decreasing head width and mandible length, with stabilizing selection as the dominant form of nonlinear selection. Additionally, we tested the validity of a commonly used instantaneous measure of mating success by comparing selection results with those determined using cumulative mating rate. The two fitness measures yielded similar patterns of selection, supporting the common sampling method comparing mated and unmated fractions.     

Characterizing selection in black‐throated blue warblers using a sexual network approach

Our understanding of trait evolution is built upon studies that examine the correlation between traits and fitness, most of which implicitly assume all individuals experience similar selective environments. However, accounting for differences in selective pressures, such as variation in the social environment, can advance our understanding of how selection shapes individual traits and subsequent fitness. In this study, we test whether variation in the social environment affects selection on individual phenotype. We apply a new sexual network framework to quantify each male's social environment as the mean body size of his primary competitors. We test for direct and social selection on male body size using a 10‐year data set on black‐throated blue warblers (Setophaga caerulescens), a territorial species for which body size is hypothesized to mediate competition for mates. We found that direct selection on body size was weak and nonsignificant, as was social selection via the body size of the males' competitors. Analysing both types of selection simultaneously allows us to firmly reject a role for body size in competitive interactions between males and subsequent male fitness in this population. We evaluate the application of the sexual network approach to empirical data and suggest that other phenotypic traits such as song characteristics and plumage may be more relevant than body size for male–male competition in this small passerine bird.