Which evolutionary processes influence natural genetic variation for phenotypic traits?

Although many studies provide examples of evolutionary processes such as adaptive evolution, balancing selection, deleterious variation and genetic drift, the relative importance of these selective and stochastic processes for phenotypic variation within and among populations is unclear. Theoretical and empirical studies from humans as well as natural animal and plant populations have made progress in examining the role of these evolutionary forces within species. Tentative generalizations about evolutionary processes across species are beginning to emerge, as well as contrasting patterns that characterize different groups of organisms. Furthermore, recent technical advances now allow the combination of ecological measurements of selection in natural environments with population genetic analysis of cloned QTLs, promising advances in identifying the evolutionary processes that influence natural genetic variation.     

Phenotypic selection in natural populations: what limits directional selection?

Studies of phenotypic selection document directional selection in many natural populations. What factors reduce total directional selection and the cumulative evolutionary responses to selection? We combine two data sets for phenotypic selection, representing more than 4,600 distinct estimates of selection from 143 studies, to evaluate the potential roles of fitness trade-offs, indirect (correlated) selection, temporally varying selection, and stabilizing selection for reducing net directional selection and cumulative responses to selection. We detected little evidence that trade-offs among different fitness components reduced total directional selection in most study systems. Comparisons of selection gradients and selection differentials suggest that correlated selection frequently reduced total selection on size but not on other types of traits. The direction of selection on a trait often changes over time in many temporally replicated studies, but these fluctuations have limited impact in reducing cumulative directional selection in most study systems. Analyses of quadratic selection gradients indicated stabilizing selection on body size in at least some studies but provided little evidence that stabilizing selection is more common than disruptive selection for most traits or study systems. Our analyses provide little evidence that fitness trade-offs, correlated selection, or stabilizing selection strongly constrains the directional selection reported for most quantitative traits.     

Does spatial variation in predation pressure modulate selection for aposematism?

It is widely believed that aposematic signals should be conspicuous, but in nature, they vary from highly conspicuous to near cryptic. Current theory, including the honest signal or trade‐off hypotheses of the toxicity–conspicuousness relationship, cannot explain why adequately toxic species vary substantially in their conspicuousness. Through a study of similarly toxic Danainae (Nymphalidae) butterflies and their mimics that vary remarkably in their conspicuousness, we show that the benefits of conspicuousness vary along a gradient of predation pressure. Highly conspicuous butterflies experienced lower avian attack rates when background predation pressure was low, but attack rates increased rapidly as background predation pressure increased. Conversely, the least conspicuous butterflies experienced higher attack rates at low predation pressures, but at high predation pressures, they appeared to benefit from crypsis. Attack rates of intermediately conspicuous butterflies remained moderate and constant along the predation pressure gradient. Mimics had a similar pattern but higher attack rates than their models and mimics tended to imitate the signal of less attacked model species along the predation pressure gradient. Predation pressure modulated signal fitness provides a possible mechanism for the maintenance of variation in conspicuousness of aposematic signals, as well as the initial survival of conspicuous signals in cryptic populations in the process of aposematic signal evolution, and an alternative explanation for the evolutionary gain and loss of mimicry.     

Is there a relationship between early genotoxicity and life-history traits in Vicia faba exposed to cadmium-spiked soils?

The Vicia faba-micronucleus test is usually performed to assess the genotoxic potential of pure substances, effluents or water extracts from soil. It is also a relevant, early biological tool to detect mutagenic substances in crude soils. Nevertheless, the physiological meaning of such DNA damage for the plant in the long term remains to be elucidated. To know more about this, two experiments were carried out with the plant model V. faba. In a preliminary short-term experiment, seeds were exposed for five days to a soil spiked with different concentrations of CdCl(2) in order to identify the concentration inducing the highest number of micronuclei without affecting plant growth. Thereafter, a long-term experiment was performed in the soil spiked with such a concentration (i.e. 510μmol CdCl(2) per kg dry soil), in which V. faba seeds were directly sowed and allowed to grow during 151 days. As a result, Cd-spiked soil did not affect seed-emergence time nor the growth-rate of the plants for the first two months. The first signs of toxicity appeared after the 70th day of exposure. Interestingly, exposed plants produced their first flower earlier and had a longer flowering period than did control ones. Nevertheless, total flower production was less abundant in exposed plants than in control plants. Moreover, fruits appeared in control plants whereas no fruit was formed in exposed plants. At last, exposed plants showed a reduced life time. Our results suggest that the micronucleus assay can provide a predictive biomarker of long-term deleterious effects in plants.     

Cyclic variation in women’s preferences for masculine traits

Women’s preferences for several male traits, including voices, change over the menstrual cycle, but the proximate causes of these changes are unknown. This paper explores relationships between levels of estradiol, progesterone, luteinizing hormone, follicle stimulating hormone, prolactin, and testosterone (estimated using menstrual cycle information) and women’s preferences for male vocal masculinity in normally cycling and hormonally contracepting heterosexual females. Preferences for vocal masculinity decreased with predicted progesterone levels and increased with predicted prolactin levels in normally cycling—but not hormonally contracepting—women. Adaptive explanations for menstrual variation in women’s preferences for masculine traits are discussed and evaluated in light of these findings.     

Are animal personality traits linked to life-history productivity?

Animal personality traits such as boldness, activity and aggressiveness have been described for many animal species. However, why some individuals are consistently bolder or more active than others, for example, is currently obscure. Given that life-history tradeoffs are common and known to promote inter-individual differences in behavior, we suggest that consistent individual differences in animal personality traits can be favored when those traits contribute to consistent individual differences in productivity (growth and/or fecundity). A survey of empirical studies indicates that boldness, activity and/or aggressiveness are positively related to food intake rates, productivity and other life-history traits in a wide range of taxa. Our conceptual framework sets the stage for a closer look at relationships between personality traits and life-history traits in animals.     

Phenotypic shifts caused by predation: selection or life-history shifts?

Predators can impose both selection and life-history shifts in prey populations. Because both processes may affect phenotypic distributions, the estimates of selection differentials may be biased. We carried out two field experiments to disentangle these separate effects. We studied whether dragonfly predation by Aeshna cyanea changes the distributions in body size and lamellae morphology in the damselfly Lestes sponsa. Damselflies have caudal lamellae which are used in escapes by swimming. In a first experiment, we manipulated predator presence (No Aeshna, Encaged Aeshna or Free-ranging Aeshna) and stopped the experiment when all larvae had moulted once. In a second experiment, larvae were confronted with a Free-ranging Aeshna but collected before moulting, and survivors were compared with a control sample taken at the start of the experiment. The presence of Aeshna largely reduced the survival probabilities of the Lestes larvae at a very similar rate in both experiments. Daily survival probabilities did not differ between the No Aeshna and Encaged Aeshna treatments. In the Free-ranging Aeshna treatment of the first experiment, size was reduced compared to the other two treatments, creating a significant apparent selection differential. This was probably mainly due to predator-induced reduced growth because in the second experiment, where growth effects were excluded, size of the survivors did not differ from the control sample. In both experiments there was a significant selection pressure for larger lamellae. Standardized directional selection differentials were similar in both experiments (0.57 and 0.28 phenotypic standard deviation units). No survival selection on lamellae shape was detected. These results are in agreement with previous findings that lamellae size, but not lamellae shape, enhances swimming performance and thereby predator escape in this species. This revised version was published online in July 2006 with corrections to the Cover Date.     

Is seasonal variation in leaf traits adaptive for the annual plant Dicerandra linearifolia?

Empirical studies of phenotypic plasticity have often relied on the plausibility that a plastic response to the environment would increase fitness in order to diagnose the response as adaptive. I conducted a test of the hypothesis that seasonal variation in leaf traits is an adaptive response to seasonal variation in environmental conditions faced by the annual plant Dicerandralinearifolia. This species exhibits variation in leaf morphology and anatomy in response to temperature that is consistent with the expectations for adaptive plasticity. I examined variation in the size, thickness and density of stomata of leaves that develop in summer and winter and used analysis of phenotypic selection during winter and summer seasons to test the hypothesis that seasonal variation in these traits is adaptive. Regression analyses of estimated dry mass (as a proxy for fitness) on leaf traits revealed no evidence supporting the adaptive hypothesis. Selection favoured individuals with large and thick leaves in both winter and summer, and density of stomata had little or no effect on estimated relative fitness in any season. Correspondence between seasonal variation in leaf thickness and density of stomata and expectations for adaptive plasticity appears to be purely fortuitous. Seasonal variation in leaf traits may persist simply because there is no selection against individuals in which these traits vary. My results underscore the importance of definitive tests of the hypothesis of adaptation to distinguish adaptive plasticity from neutral or nonadaptive phenotypic plasticity.     

Can spatial variation alone lead to selection for dispersal?

The stability of models of age-dependent predation in continuous time with predators exhibiting a functional response are analyzed. A number of new features of biological importance emerge that are not present in simpler models. These include limits to the length of juvenile periods (both upper and lower) for stability, and the possibility that increases or decreases in any of the model parameters can be stabilizing or destabilizing. Hence, increased delays are not necessarily destabilizing. The variance in the length of the juvenile period is shown to be an important factor determining stability. Additionally, the relative stability of predation only on juveniles or only on adults is compared.     

Impacts of fever on locust life-history traits: costs or benefits?

Fever, like other mechanisms for defence against pathogens, may have positive and negative consequences for host fitness. In ectotherms, fever can be attained through modified behavioural thermoregulation. Here we examine potential costs of behavioural fever by holding adult, gregarious desert locusts at elevated temperatures simulating a range of fever intensities. We found no effect of fever temperatures on primary fitness correlates of survival and fecundity. However, flight capacity and mate competition were reduced, although there was no relation between time spent at fever temperatures and magnitude of the response. While these effects could indicate a direct cost of fever, they are also consistent with a shift towards the solitaria phase state that, in a field context, could be considered an adaptive life-history response to limit the impact of disease. These conflicting interpretations highlight the importance of considering complex defence mechanisms and trade-offs in an appropriate ecological context.     

Designing an early selection morphological linear traits index for dressage in the Pura Raza Español horse

Making a morphological pre-selection of Pura Raza Español horses (PRE) for dressage is a challenging task within its current breeding program. The aim of our research was to design an early genetic selection morphological linear traits index to improve dressage performance, using 26 morphological linear traits and six dressage traits (walk, trot, canter, submission, general impression – partial scores – and total score) as selection criteria. The data set included morphological linear traits of 10 127 PRE (4159 males and 5968 females) collected between 2008 and 2013 (one record per horse) and 19 095 dressage traits of 1545 PRE (1476 males and 69 females; 12.4 records of average) collected between 2004 and 2014. A univariate animal model was applied to predict the breeding values (PBV). A partial least squares regression analysis was used to select the most predictive morphological linear traits PBV on the dressage traits PBV. According to the Wold Criterion, the 13 morphological linear traits (width of head, head–neck junction, upper neck line, neck–body junction, width of chest, angle of shoulder, lateral angle of knee, frontal angle of knee, cannon bone perimeter, length of croup, angle of croup, ischium–stifle distance and lateral hock angle) most closely related to total score PBV, partial scores PBV and gait scores PBV (walk, trot and canter) were selected. A multivariate genetic analysis was performed among the 13 morphological linear traits selected and the six dressage traits to estimate the genetic parameters. After it, the selection index theory was used to compute the expected genetic response using different strategies. The expected genetic response of total score PBV (0.76), partial scores PBV (0.04) and gait scores PBV (0.03) as selection objectives using morphological linear traits PBV as criteria selection were positive, but lower than that obtained using dressage traits PBV (1.80, 0.16 and 0.14 for total score PBV, partial scores PBV and gait scores PBV) or dressage traits PBV and morphological linear traits PBV (2.97, 0.16 and 0.15 for total score PBV, partial scores PBV and gait scores PBV), as selection criteria. This suggests that it is possible to preselect the PRE without dressage traits PBV using as selection criteria the morphological linear traits PBV, but the expected genetic response will be lower.     

What can natural selection explain?

One approach to assess the explanatory power of natural selection is to ask what type of facts it can explain. The standard list of explananda includes facts like trait frequencies or the survival of particular organisms. Here, I argue that this list is incomplete: natural selection can also explain a specific kind of individual-level fact that involves traits. The ability of selection to explain this sort of fact (‘trait facts’) vindicates the explanatory commitments of empirical studies on microevolution. Trait facts must be distinguished from a closely related kind of fact, that is, the fact that a particular individual x has one trait rather than another. Whether or not selection can explain the latter type of fact is highly controversial. According to the so-called ‘Negative View’ it cannot be explained by selection. I defend the Negative View against Nanay’s (2005) objection.     

How strong is natural selection?

The strength of selection in nature has long been a controversial subject, partly because there were few quantitative measurements of phenotypic selection available until recently. In a new paper, Kingsolver and colleagues reviewed 63 studies and found that the median standardized directional selection gradient (a measure of the strength of phenotypic selection) was 0.16. Whether this means selection in nature is strong or weak depends both on one's point of view and on the error in selection estimates.     

Sexual dimorphism in snake tail length: sexual selection,natural selection,or morphological constraint?

Male snakes typically have longer tails relative to body length than females, but the extent of this dimorphism varies among species. Three hypotheses have been suggested to explain tail dimorphism. The Morphological Constraint Hypothesis proposes that males have relatively longer tails to accommodate hemipenes and retractor muscles. The Female Reproductive Output Hypothesis proposes that females have relatively shorter tails as a secondary result of natural selection for increased reproductive capacity. The Male Mating Ability Hypothesis proposes that sexual selection favours relatively longer tails in males during courtship. These hypotheses make different predictions about the relationships among tail length, body size, male reproductive morphology, female reproductive output, mode of reproduction, and male mating behaviour among and within taxa. Predictions were tested using published data for 56 genera in the family Colubridae and original data for the water snake, Nerodia sipedon. Tail length dimorphism was more male-biased in tam having relatively short tails (r=–0.52, P < 0.001), hemipenes and retractor muscles occupied a greater proportion of the tail in taxa having relatively short tails (r=– 0.71, P < 0.00l and r=– 0.66, P = 0.001, respectively), and tail length dimorphism was more male-biased in taxa in which body size dimorphism was more female-biased (r=– 0.60, P < 0.001). These results support both the Morphological Constraint Hypotheses and the Female Reproductive Output Hypothesis. However, tests of other predictions, including those regarding patterns within N. sipedon , failed to support any of the three hypotheses. Comparisons among taxa suggest several species in which further tests of these hypotheses would be especially appropriate.     

Dissection of complex traits in forest trees — opportunities for marker-assisted selection

Due to their long reproductive cycles and the time to expression of mature traits, marker-assisted selection is particularly attractive for tree breeding. In this review, we discuss different approaches used for developing markers and propose a method for application of markers in low linkage disequilibrium (LD) populations. Identification of useful markers for application in tree breeding is mainly based on two approaches, quantitative trait locus (QTL) mapping and association genetic studies. While several studies have identified significant markers, effect of the individual markers is low making it difficult to utilize them in breeding programs. Recently, genomic selection (GS) was proposed for overcoming some of these difficulties. In GS, high density markers are used for predicting phenotypes from genotypes. Currently small effective populations with high LD are being tested for GS in tree breeding. For wider application, GS needs to be applied in low LD populations which are found in many tree breeding programs. Here we propose an approach in which the significant markers from association studies may be used for developing prediction models in low LD populations using the same methods as in GS. Preliminary analyses indicate that a modest numbers of markers may be sufficient for developing prediction models in low LD populations. GS based on large numbers of random markers or small numbers of associated markers is poised to make marker-assisted selection a reality in forest tree breeding.     

Is biotic resistance enhanced by natural variation in diversity?

Theories linking diversity to ecosystem function have been challenged by the widespread observation of more exotic species in more diverse native communities. Few studies have addressed the underlying processes by dissecting how biotic resistance to new invaders may be shaped by the same environmental influences that determine diversity and other community properties. In grasslands with heterogeneous soils, we added invaders and removed competitors to analyze the causes of invasion resistance. Abiotic resistance was measured using invader success in the absence of the resident community. Biotic resistance was measured as the reduction in invader success in the presence of the resident community. Invaders were most successful where biotic resistance was lowest and abiotic resistance was highest, confirming the dominant role of biotic resistance. Contrary to theory, though, biotic resistance was highest where both species richness and functional diversity were lowest. In the multivariate framework of a structural equation model, biotic resistance was independent of community diversity, and was highest where fertile soils led to high community biomass. Seed predation slightly augmented biotic resistance without qualitatively changing the results. Soil‐related genotypic variation in the invader also did not affect the results. We conclude that in natural systems, diversity may be correlated with invasibility and yet have little effect on biotic resistance to invasion. More generally, the environmental causes of variation in diversity should be considered when examining the potential functional consequences of diversity.