Lifetime reproductive success,selection on lifespan,and multiple sexual ornaments in male European barn swallows

Natural and sexual selection arise when individual fitness varies according to focal traits. Extra‐pair paternities (EPPs) can affect the intensity of selection by influencing variance in fitness among individuals. Studies of selection require that individual fitness is estimated using proxies of lifetime reproductive success (LRS). However, estimating LRS is difficult in large, open populations where EPPs cause reallocation of biological paternity. Here, we used extensive field sampling to estimate LRS in a population of barn swallows (Hirundo rustica) to estimate selection on lifespan and ornamental traits of males. We found selection on lifespan mediated both by within‐ and extra‐pair fertilization success and selection on tail length mediated by within‐ but not extra‐pair fertilization success. In addition, we found selection on tail white spots via extra‐pair fertilization success after controlling for selection on other traits. These results were not confounded by factors that hamper studies of LRS, including nonexhaustive sampling of offspring and biased sampling of males. Hence, natural and sexual selection mediated by LRS operates on lifespan, tail length, and size of the tail white spots in barn swallows.     

NATURAL SELECTION ON BODY SIZE AND LAYING DATE IN THE TREE SWALLOW

I measured natural selection on body size and laying date in a population of tree swallows (Tachycineta bicolor) from 1986 to 1988. There was little evidence of selection on body size associated with overwinter survival. Disruptive selection on tarsus length, associated with female reproductive success, was detected in one of three years. Both repeatability and mother-daughter regression suggested that laying date was heritable. I found weak evidence of selection on laying date, associated with both overwinter survival and reproduction in females. The ecological implications of both tarsus length and laying date variation in this population could not be identified. Consequently, although I was able to identify the targets of natural selection, the ecological link between trait variation and selection remains unknown.     

CORRECTING FOR SAMPLING BIAS IN QUANTITATIVE MEASURES OF SELECTION WHEN FITNESS IS DISCRETE

We show with a simulation that nonrepresentative sampling of two discrete fitness classes leads to biased estimates of selection. Systematic underestimation occurs if the selected class is overrepresented in the sample and overestimation if the unselected class is overrepresented. The bias is greater the stronger the selection intensity, the smaller the true fraction of individuals favored by selected, and the lower the sample size. We present a simple method that allows a posteriori statistical correction in cases of biased sampling given a separate estimate of the actual class representation, describe its practical implementation, and show that it works.     

Parasites favour intermediate nestling mass and brood size in cliff swallows

A challenge of life‐history theory is to explain why animal body size does not continue to increase, given various advantages of larger size. In birds, body size of nestlings and the number of nestlings produced (brood size) have occasionally been shown to be constrained by higher predation on larger nestlings and those from larger broods. Parasites also are known to have strong effects on life‐history traits in birds, but whether parasitism can be a driver for stabilizing selection on nestling body size or brood size is unknown. We studied patterns of first‐year survival in cliff swallows (Petrochelidon pyrrhonota) in western Nebraska in relation to brood size and nestling body mass in nests under natural conditions and in those in which hematophagous ectoparasites had been removed by fumigation. Birds from parasitized nests showed highest first‐year survival at the most common, intermediate brood‐size and nestling‐mass categories, but cliff swallows from nonparasitized nests had highest survival at the heaviest nestling masses and no relationship with brood size. A survival analysis suggested stabilizing selection on brood size and nestling mass in the presence (but not in the absence) of parasites. Parasites apparently favour intermediate offspring size and number in cliff swallows and produce the observed distributions of these traits, although the mechanisms are unclear. Our results emphasize the importance of parasites in life‐history evolution.     

The robustness of the weak selection approximation for the evolution of altruism against strong selection

The weak selection approximation of population genetics has made possible the analysis of social evolution under a considerable variety of biological scenarios. Despite its extensive usage, the accuracy of weak selection in predicting the emergence of altruism under limited dispersal when selection intensity increases remains unclear. Here, we derive the condition for the spread of an altruistic mutant in the infinite island model of dispersal under a Moran reproductive process and arbitrary strength of selection. The simplicity of the model allows us to compare weak and strong selection regimes analytically. Our results demonstrate that the weak selection approximation is robust to moderate increases in selection intensity and therefore provides a good approximation to understand the invasion of altruism in spatially structured population. In particular, we find that the weak selection approximation is excellent even if selection is very strong, when either migration is much stronger than selection or when patches are large. Importantly, we emphasize that the weak selection approximation provides the ideal condition for the invasion of altruism, and increasing selection intensity will impede the emergence of altruism. We discuss that this should also hold for more complicated life cycles and for culturally transmitted altruism. Using the weak selection approximation is therefore unlikely to miss out on any demographic scenario that lead to the evolution of altruism under limited dispersal.     

Selection on female behaviour fluctuates with offspring environment

Temporal variation in selection has long been proposed as a mechanism by which genetic variation could be maintained despite short‐term strong directional selection and has been invoked to explain the maintenance of consistent individual differences in behaviour. We tested the hypothesis that ecological changes through time lead to fluctuating selection, which could promote the maintenance of variation in female behavioural traits in a wild population of North American red squirrels. As predicted, linear selection gradients on female aggression and activity significantly fluctuated across years depending on the level of competition among juveniles for vacant territories. This selection acted primarily through juvenile overwinter survival rather than maternal fecundity. Incorporating uncertainty in individual measures of behaviour reduced the magnitude of annual selection gradients and increased uncertainty in these estimates, but did not affect the overall pattern of temporal fluctuations in natural selection that coincided with the intensity of competition for vacant territories. These temporal fluctuations in selection might, therefore, promote the maintenance of heritable individual differences in behaviour in this wild red squirrel population.     

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.     

Variation in viability selection among cohorts of juvenile red squirrels (Tamiasciurus hudsonicus)

Abstract.— Selection will result in observable changes in traits only if it acts consistently in space and time, but few estimates of selection in natural populations have been temporally replicated. Here we estimate viability selection on nestling growth rates for 13 cohorts (1989–2001) of red squirrels (Tamiasciurus hudsonicus) from a natural population located in southwestern Yukon, Canada. Directional selection on nestling growth rates varied in magnitude and direction from one cohort to the next. The magnitude of directional selection was relatively weak in most years (median (β= 0.24), but there were episodes of very strong viability selection ((β > 0.5) in some cohorts. We found no evidence of significant stabilizing or disruptive selection on this trait. Examination of viability selection episodes over shorter time periods suggested that the strength of selection on juveniles in this population was positively related to the time scale over which selection was measured. Viability selection from birth to emergence from the natal nest (50 days of age) and from emergence to successful recruitment (100 days of age) were positively correlated, but were both independent of selection on nestling growth rates from recruitment to potential breeding age (one year). The strength of directional selection on growth rates prior to recruitment was negatively correlated with spring temperature whereas selection from recruitment to breeding was positively correlated with the abundance of spruce cones produced in the previous fall. Episodes of strong directional selection from birth to breeding age appear to be due to potentially rare combinations of environmental conditions. As a result, predicting the occurrence of very strong episodes of selection will be extremely difficult, but predicting the microevolutionary responses to observed selection on individual cohorts remains feasible.     

Population variation in sexual selection and its effect on size allometry in two dung fly species with contrasting sexual size dimorphism

Body size is one of the most important quantitative traits under evolutionary scrutiny. Sexual size dimorphism (SSD) in a given species is expected to result if opposing selection forces equilibrate differently in both sexes. We document variation in the intensity of sexual and fecundity selection, male and female body size, and thus SSD among 31 and 27 populations of the two dung fly species, Scathophaga stercoraria and Sepsis cynipsea, across Switzerland. Whereas in S. cynipsea females are larger, the SSD is reversed in S. stercoraria. We comprehensively evaluated Fairbairn and Preziosi's (1994) general, three-tiered scenario, hypothesizing that sexual selection for large male size is the major driving force of SSD allometry within these two species. Sexual selection intensity on male size in the yellow dung fly, S. stercoraria, was overall positive, greater, and more variable among populations than fecundity selection on females. Also, sexual selection intensity in a given population correlated positively with mean male body size of that population for both the field-caught fathers and their laboratory-reared sons, indicating a response to selection. In S. cvnipsea, sexual selection intensity on males was lower overall and significantly positive, about equal in magnitude, but more variable than fecundity selection on females. However, there was no correlation between the intensity of sexual selection and mean male body size among populations. In both species, the laboratory-reared offspring indicate genetic differentiation among populations in body size. Despite fulfillment of all key prerequisites, at least in S. stercoraria, we did not find hypoallometry for SSD (Rensch's rule, i.e., greater evolutionary divergence in male size than female size) for the field-caught parents or the laboratory-reared offspring: Female size was isometric to male size in both species. We conclude that S. cynipsea does not fit some major requirements of Fairbairn and Preziosi's (1994) scenario, whereas for S. stercoraria we found partial support for it. Failure to support Rensch's rule within the latter species may be due to phylogenetic or other constraints, power limitations, erroneous estimates of sexual selection, insufficient genetic isolation of populations, or sex differences in viability selection against large size.     

Repeatability of extra-pair mating in tree swallows

Models of sexual selection assume that female mating preferences are heritable and, thus, repeatable for individual females across multiple mating episodes. Previous studies of the repeatability of female preference have examined individuals in captivity and focused presumably on social mate choice. However, extra-pair mating is widespread and can also influence sexual selection. We examined the repeatability of extra-pair mating in a wild population of tree swallows (Tachycineta bicolor) by experimentally inducing females to lay two clutches in rapid succession within the same season. We found that the proportion of extra-pair young and the number of extra-pair sires were highly repeatable for individual females. However, the repeatability of specific extra-pair sires was low. We suggest that this unusual pattern of mating may be due to females maximizing the heterozygosity of their offspring.     

A Pragmatic Approach for Determining Otter Distribution from Disparate Occurrence Records

Opportunistic records of animal occurrence may be problematic for inferring species distribution and habitat requirements because of unknown and uncontrolled sources of sampling variance. In this study, we used occurrence records for river otters (Lontra canadensis) derived from sign surveys, road kills, trapper bycatch, and opportunistic sightings (n = 185 records collected 2001–2012) to assess the potential distribution and habitat relationships of otters across central and western New York, USA. To mitigate for obvious observation biases, we standardized observation intensity across regions a priori and restricted inference to readily accessible areas (i.e., ≤700 m from the nearest road). Model selection, and the direction of covariate effects, proved robust to these sampling biases although effect sizes varied −7.1% to +48.0% after bias correction, with the coefficient for the proportion of available shoreline being the most unstable. Ultimately, the top bias-corrected model proved a reliable index for otter probability of occurrence given a strong, positive, and linear relationship with a withheld set of standardized survey records for otters collected in winter 2016–2017 (n = 57; R² = 0.90). This model indicated that approximately 20% of the study area represented high probability of otter occurrence. We demonstrated that reliable inference on wildlife habitat requirements can be obtained from disparate records of animal occurrence provided that data biases are known and effectively mitigated. © 2020 The Wildlife Society.     

Estimating major gene effects with partial information using Gibbs sampling

A method for estimating major gene effects using Gibbs sampling to infer genotype of individuals with unknown values, was compared with a standard mixed-model analysis. The purpose of this study was to evaluate the effect of including information of individuals with unknown genotypes on the estimates and their error variances (Ve) of the single-gene effects. When genotypes were known for all the individuals, results using the Gibbs method (GS) were similar to those obtained with the mixed model (MM). In the absence of selection, when information from individuals with unknown genotypes was included, GS yielded unbiased estimates of the major gene effects while reducing the Ve associated with them. This reduction in Ve depended on the gene frequency and mode of action of the major locus. For the additive effect, the reduction in Ve ranged from 29 to 69% of the total reduction which would have been obtained if all individuals had had a known genotype. Similarly the reduction in Ve found for the dominance effect ranged from 12 to 58%. Estimates using GS generally had small detectable biases when the polygenic heritability used in the analysis was inflated or estimated simultaneously. However, the benefit of using information from individuals with unknown genotypes was still maintained when comparing the mean square error of the estimates using either GS or MM when genotypes are only known for a subset of the population. When the population has been under selection, the use of Gibbs sampling to incorporate information of individuals without genotypes reduced substantially the bias and mean square error found for MM analysis on partial data. Nevertheless, there was some bias detected using Gibbs sampling. The gene frequency of the major gene in the base population was also well estimated despite its change over generations due to selection.     

QUANTITATIVE GENETICS OF SEXUAL DIMORPHISM IN HUMAN BODY SIZE

A classical data set is used to predict the effect of selection on sexual dimorphism and on the population means of three characters—stature, span, and cubit—in humans. Given selection of equal intensity, the population means of stature and of cubit should respond more than 60 times as fast as dimorphism in these characters. The population mean of span should also respond far more rapidly than dimorphism, but no numerical estimate of the ratio of these rates was possible. These results imply that sexual dimorphism in these characters can evolve only very slowly. Consequently, hypotheses about the causes of sexual dimorphism cannot be tested by comparing the dimorphism of different human societies. It has been suggested that primate sexual dimorphism may be an allometric response to selection for larger body size. We show that such selection can indeed generate sexual dimorphism, but that this effect is too weak to account for the observed relationship between dimorphism and body size in primates.     

Impacts of environmental heterogeneity on natural selection in a wild bird population*

Natural selection has been studied for several decades, resulting in the computation of thousands of selection estimates. Although the importance of environmental conditions on selection has often been suggested, published estimates rarely take into account the effects of environmental heterogeneity on selection patterns. Here, we estimated linear and nonlinear viability selection gradients on morphological traits of 12-day old nestlings in a wild population of tree swallows (Tachycineta bicolor) across a large-scale heterogeneous study system in southern Québec, Canada. We assessed the environmental drivers of nestling survival and evaluated their effects on strength and direction of selection gradients. Separate analyses of environmental variables showed that high temperatures and heavy rainfall caused stronger positive linear selection on morphological traits. Weaker linear selection was also measured in more extensively cultivated areas. Both strength and shape of nonlinear quadratic and correlational components of selection were modified by environmental variables. Considering all environmental variables revealed that precipitation since hatching affected patterns of linear selection on traits, while temperatures since hatching shaped nonlinear selection patterns. Our study underlines the importance of quantifying linear and nonlinear natural selection under various environmental conditions and how the evolutionary response of traits may be affected by ongoing human-induced environmental changes.     

Sexual signal loss in field crickets maintained despite strong sexual selection favoring singing males

Evolutionary biologists commonly seek explanations for how selection drives the emergence of novel traits. Although trait loss is also predicted to occur frequently, few contemporary examples exist. In Hawaii, the Pacific field cricket (Teleogryllus oceanicus) is undergoing adaptive sexual signal loss due to natural selection imposed by eavesdropping parasitoids. Mutant male crickets (“flatwings”) cannot sing. We measured the intensity of sexual selection on wing phenotype in a wild population. First, we surveyed the relative abundance of flatwings and “normal‐wings” (nonmutants) on Oahu. Then, we bred wild‐mated females’ offspring to determine both female genotype with respect to the flatwing mutation and the proportion of flatwing males that sired their offspring. We found evidence of strong sexual selection favoring the production of song: females were predominantly homozygous normal‐wing, their offspring were sired disproportionately by singing males, and at the population level, flatwing males became less common following a single sexual selection event. We report a selection coefficient describing the total (pre‐ and postcopulatory) sexual selection favoring normal‐wing males in nature. Given the maintenance of the flatwing phenotype in Hawaii in recent years, this substantial sexual selection additionally suggests an approximate strength of opposing natural selection that favors silent males.     

Geographical delimitation of a partial selective sweep in African Drosophila melanogaster

Positive selection leaves characteristic footprints on DNA variation but detecting such patterns is challenging as the age, the intensity and the mode of selection as well as demography and evolutionary parameters (mutation and recombination rates) all play roles and these are difficult to disentangle. We recorded nucleotide variation in a sample of isogenic chromosomes from a western African population of Drosophila melanogaster at a locus (Fbp2) for which a partial selective sweep had previously been reported. We compared this locus to four other genes from the same chromosomes and from a European and an East African population. Then, we assessed Fbp2 variation in a sample of 370 chromosomes covering a comprehensive geographic sampling of 16 African localities. The signature of selection was tested while accounting for the demographic history of the populations. We found a significant signal of selection in two West African localities including Ivory Coast. Variation at Fpb2 would thus represent a case of an ongoing selective sweep in the range of this species. A weaker, nonsignificant, signal of selection was, however, apparent in some other populations, thus leaving open several possibilities: (i) the selective sweep originated in Ivory Coast and has spread to the rest of the continent; (ii) several African populations report the signature of a selective event having occurred in an ancestral population; (iii) this genome region is subject to independent selective events in African populations; and (iv) A neutral scenario with population subdivision and local bottleneck cannot be fully excluded to explain the molecular patterns observed in some populations.     

The conflict between natural and artificial selection in finite populations

Summary A single locus model of the interaction between natural selection and artificial selection for a quantitative character in a finite population, assuming heterozygote superiority in natural fitness but additive action on the character, has been studied using transition probability matrices.If natural selection is strong enough to create a selection plateau in which genetic variance declines relatively slowly, then the total response to artificial selection prior to the plateau will be much less than that expected in the absence of natural selection, and the half-life of response will be shorter. Such a plateau is likely to have a large proportion, if not all, of the original genetic variance still present. In selection programmes using laboratory animals, it seems likely that the homozygote favoured by artificial selection must be very unfit before such a plateau will occur. A significant decrease in population fitness as a result of artificial selection does not necessarily imply that the metric character is an important adaptive character.These implications of this model of natural selection are very similar to those derived by James (1962) for the optimum model of natural selection. In fact, there seems to be no aspect of the observable response to artificial selection that would enable anyone to distinguish between these two models of natural selection.     

Mating density and the strength of sexual selection against deleterious alleles in Drosophila melanogaster

Deleterious alleles constantly enter populations via mutation. Their presence reduces mean fitness and may threaten population persistence. It has been suggested that sexual selection may be an efficient way by which deleterious alleles are removed from populations but there is little direct experimental evidence. Because of its potential role in mutational meltdowns, there is particular interest in whether the strength of sexual selection changes with density. For each of eight visible markers in Drosophila melanogaster we have compared the strength of sexual selection at two densities. We find evidence of strong sexual selection against most but not all of these alleles. There is no evidence that sexual selection tends to be stronger (or weaker) at high density relative to low density. In addition, we also measure the effects of these mutations on two key parameters relevant to population productivity--juvenile viability and female fecundity. In most cases, sexual selection is as strong or stronger than these other forms of selection.     

Sex differences in survival selection in the serin,Serinus serinus

Natural selection is demonstrated in most natural populations which suggests that populations are dispatched from their adaptive peaks as a result of selection on correlated characters, or conflicting selection between the sexes. We analysed patterns of survival selection in a population of serins (Serinus serinus) outside Barcelona over a period of 13 years. There was directional selection for increased wing length in males and females accompanied by strong disruptive selection on both tail and wing length in males and a selection against a positive correlation between the two characters in males. In females there was directional selection for increased bill width but decreased bill depth, which should be contrasted to the stabilizing selection acting on bill depth in males. There were conflicting selection on the characters within a sex and conflicting selection of the same characters between sexes, which constrain the rate of access to the nearest adaptive peak.