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

A meta-analysis of sex differences in animal personality: no evidence for the greater male variability hypothesis

The notion that men are more variable than women has become embedded into scientific thinking. For mental traits like personality, greater male variability has been partly attributed to biology, underpinned by claims that there is generally greater variation among males than females in non-human animals due to stronger sexual selection on males. However, evidence for greater male variability is limited to morphological traits, and there is little information regarding sex differences in personality-like behaviours for non-human animals. Here, we meta-analysed sex differences in means and variances for over 2100 effects (204 studies) from 220 species (covering five broad taxonomic groups) across five personality traits: boldness, aggression, activity, sociality and exploration. We also tested if sexual size dimorphism, a proxy for sex-specific sexual selection, explains variation in the magnitude of sex differences in personality. We found no significant differences in personality between the sexes. In addition, sexual size dimorphism did not explain variation in the magnitude of the observed sex differences in the mean or variance in personality for any taxonomic group. In sum, we find no evidence for widespread sex differences in variability in non-human animal personality.

The strength and pattern of natural selection on gene expression in rice

Levels of gene expression underpin organismal phenotypes^1,2, but the nature of selection that acts on gene expression and its role in adaptive evolution remain unknown^1,2. Here we assayed gene expression in rice (Oryza sativa)³, and used phenotypic selection analysis to estimate the type and strength of selection on the levels of more than 15,000 transcripts^4,5. Variation in most transcripts appears (nearly) neutral or under very weak stabilizing selection in wet paddy conditions (with median standardized selection differentials near zero), but selection is stronger under drought conditions. Overall, more transcripts are conditionally neutral (2.83%) than are antagonistically pleiotropic⁶ (0.04%), and transcripts that display lower levels of expression and stochastic noise^7-9 and higher levels of plasticity⁹ are under stronger selection. Selection strength was further weakly negatively associated with levels of cis-regulation and network connectivity⁹. Our multivariate analysis suggests that selection acts on the expression of photosynthesis genes^4,5, but that the efficacy of selection is genetically constrained under drought conditions¹⁰. Drought selected for earlier flowering^11,12 and a higher expression of OsMADS18 (Os07g0605200), which encodes a MADS-box transcription factor and is a known regulator of early flowering¹³-marking this gene as a drought-escape gene^11,12. The ability to estimate selection strengths provides insights into how selection can shape molecular traits at the core of gene action.

THE EFFECT OF A VARIABLE ENVIRONMENT ON THE GENETIC CORRELATION STRUCTURE IN A FIELD CRICKET

The evolutionary trajectory of a trait depends not only on the presence of genetic variation, but also on the pattern of genetic correlations (r_g ) among traits. Genetic correlations are most easily measured under homogeneous, controlled laboratory conditions, whereas natural populations typically experience a higher degree of environmental variability. The effect of environmental variability on genetic correlations in the cricket, Gryllus pennsylvanicus, was studied by measuring genetic correlations within and between two environments differing in levels of environmental heterogeneity. Within-environment r_g among morphological traits measured in the homogeneous laboratory environment were found to be reliable predictors of r_g measured in the experimental field environment. Laboratory measures of r_g involving life-history traits, though, were not found to reflect the same correlations measured in the heterogeneous environment. A significant negative genetic correlation between fecundity and developmental time was found in the field environment, yet was not detectable when measured in the laboratory. Phenotypic correlations may be obtained much more easily than genetic correlations, but their usefulness in evolutionary inference depends on the pattern of similarity between the two correlations. A comparison of genetic and phenotypic correlations revealed a close match between the two measures for morphological traits, but revealed only broad similarities when considering life-history traits. Male-female genetic correlations between morphological traits were high (all r_g > 0.73) and were consistently higher in the field environment than in the laboratory. The genetic correlations between the sexes in developmental time followed the same trend, but the male-female genetic correlation of gonad weights was low in both environments. Across-environment correlations were found to be strong for morphological traits and for gonad weight, whereas the genetic expression of developmental time was found to be dependent on the environment in which the crickets were raised.

Lack of support for Rensch's rule in an intraspecific test using red flour beetle (Tribolium castaneum) populations

Rensch's rule proposes a universal allometric scaling phenomenon across species where sexual size dimorphism (SSD) has evolved: in taxa with male-biased dimorphism, degree of SSD should increase with overall body size, and in taxa with female-biased dimorphism, degree of SSD should decrease with increasing average body size. Rensch's rule appears to hold widely across taxa where SSD is male-biased, but not consistently when SSD is female-biased. Furthermore, studies addressing this question within species are rare, so it remains unclear whether this rule applies at the intraspecific level. We assess body size and SSD within Tribolium castaneum (Herbst), a species where females are larger than males, using 21 populations derived from separate locations across the world, and maintained in isolated laboratory culture for at least 20 years. Body size, and hence SSD patterns, are highly susceptible to variations in temperature, diet quality and other environmental factors. Crucially, here we nullify interference of such confounds as all populations were maintained under identical conditions (similar densities, standard diet and exposed to identical temperature, relative humidity and photoperiod). We measured thirty beetles of each sex for all populations, and found body size variation across populations, and (as expected) female-biased SSD in all populations. We test whether Rensch's rule holds for our populations, but find isometry, i.e. no allometry for SSD. Our results thus show that Rensch's rule does not hold across populations within this species. Our intraspecific test matches previous interspecific studies showing that Rensch's rule fails in species with female-biased SSD.

Artificial selection on larval growth curves in Tribolium: correlated responses and constraints

Body size is often constrained from evolving. Although artificial selection on body size in insects frequently results in a sizable response, these responses usually bear fitness costs. Further, these experiments tend to select only on size at one landmark age, rather than selecting for patterns of growth over the whole larval life stage. To address whether constraints may be caused by larval growth patterns rather than final size, we implemented a function-valued (FV) trait method of selection, in which entire larval growth curves from Tribolium were artificially selected. The selection gradient function used was previously predicted to give the maximal response and was implemented using a novel selection index in the FV framework. Results indicated a significant response after one generation of selection, but no response in subsequent generations. Correlated responses included increased mortality, increased critical weight, and decreased development time (DT). The lack of response in size and development time after the first generation was likely caused by increased mortality suffered in selected lines; we demonstrated that the selection criterion caused both increased body size and increased mortality. We conclude that artificial selection on continuous traits using FV methods is very efficient and that the constraint of body size evolution is likely caused by a suite of trade-offs with other traits.

A multivariate analysis of genetic variation in the advertisement call of the gray treefrog, Hyla versicolor

Genetic variation in sexual displays is crucial for an evolutionary response to sexual selection, but can be eroded by strong selection. Identifying the magnitude and sources of additive genetic variance underlying sexually selected traits is thus an important issue in evolutionary biology. We conducted a quantitative genetics experiment with gray treefrogs (Hyla versicolor) to investigate genetic variances and covariances among features of the male advertisement call. Two energetically expensive traits showed significant genetic variation: call duration, expressed as number of pulses per call, and call rate, represented by its inverse, call period. These two properties also showed significant genetic covariance, consistent with an energetic constraint to call production. Combining the genetic variance-covariance matrix with previous estimates of directional sexual selection imposed by female preferences predicts a limited increase in call duration but no change in call rate despite significant selection on both traits. In addition to constraints imposed by the genetic covariance structure, an evolutionary response to sexual selection may also be limited by high energetic costs of long-duration calls and by preferences that act most strongly against very short-duration calls. Meanwhile, the persistence of these preferences could be explained by costs of mating with males with especially unattractive calls.

Genetic constraints underlying human reproductive timing in a premodern Swiss village

The trade-off between reproductive investment in early versus late life is central to life-history theory. Despite abundant empirical evidence supporting different versions of this trade-off, the specific trade-off between age at first reproduction (AFR) and age at last reproduction (ALR) has received little attention, especially in long-lived species with a pronounced reproductive senescence such as humans. Using genealogical data for a 19th-century Swiss village, we (i) quantify natural selection acting on reproductive timing, (ii) estimate the underlying additive genetic (co)variances, and (iii) use these to predict evolutionary responses. Selection gradients were computed using multiple linear regression, and the additive genetic variance-covariance matrix was estimated using a restricted maximum-likelihood animal model. We found strong selection for both an early AFR and a late ALR, which resulted from selection for an earlier and longer reproductive period (RP, i.e., ALR-AFR). Furthermore, postponing AFR shortened RP in both sexes, but twice as much in women. Finally, AFR and ALR were strongly and positively genetically correlated, which led to a considerable reduction in the predicted responses to selection, or even rendered them maladaptive. These results provide evidence for strong genetic constraints underlying reproductive timing in humans, which may have contributed to the evolution of menopause.

Constraints on the evolution of function-valued traits: a study of growth in Tribolium castaneum

Growth trajectories often impact individual fitness. They are continuous by nature and so are amenable to analysis using a function-valued (FV) trait framework to reveal their underlying genetic architecture. Previous studies have found high levels of standing additive genetic (co)variance for growth trajectories despite the expectation that growth should be responding to frequent strong directional selection. In this study, the FV framework is used to estimate the additive genetic covariance function for growth trajectories in larval Tribolium castaneum to address questions about standing additive genetic (co)variance and possible evolutionary constraints on growth and to predict responses to four plausible selection regimes. Results show that additive genetic (co)variance is high at the early ages, but decreases towards later ages in the larval period. A selection gradient function of the same size and in the same direction of the first eigenfunction of the G-function should give the maximal response. However, evolutionary constraints may be acting to keep this maximal response from being realized, through either conflicting effects on survivability and fecundity of larger body size, few evolutionary directions having sufficient additive variance for a response, genetic trade-offs with other traits or physiological regulatory mechanisms. More light may be shed on these constraints through the development of more sophisticated statistical approaches and implementation of additional empirical studies to explicitly test for specific types of constraints.

Species selection and the macroevolution of coral coloniality and photosymbiosis

Differences in the relative diversification rates of species with variant traits are known as species selection. Species selection can produce a macroevolutionary change in the frequencies of traits by changing the relative number of species possessing each trait over time. But species selection is not the only process that can change the frequencies of traits, phyletic microevolution of traits within species and phylogenetic trait evolution among species, the tempo and mode of microevolution can also change trait frequencies. Species selection, phylogenetic, and phyletic processes can all contribute to large-scale trends, reinforcing or canceling each other out. Even more complex interactions among macroevolutionary processes are possible when multiple covarying traits are involved. Here I present a multilevel macroevolutionary framework that is useful for understanding how macroevolutionary processes interact. It is useful for empirical studies using fossils, molecular phylogenies, or both. I illustrate the framework with the macroevolution of coloniality and photosymbiosis in scleractinian corals using a time-calibrated molecular phylogeny. I find that standing phylogenetic variation in coloniality and photosymbiosis deflects the direction of macroevolution from the vector of species selection. Variation in these traits constrains species selection and results in a 200 million year macroevolutionary equilibrium.

Direct and correlated responses to artificial selection on sexual size dimorphism in the flour beetle, Tribolium castaneum

Sexual size dimorphism (SSD) is a conspicuous yet poorly understood pattern across many organisms. Although artificial selection is an important tool for studying the evolution of SSD, previous studies have applied selection to only a single sex or to both sexes in the same direction. In nature, however, SSD likely arises through sex-specific selection on body size. Here, we use Tribolium castaneum flour beetles to investigate the evolution of SSD by subjecting males and females to sexually antagonistic selection on body size (sexes selected in opposite directions). Additionally, we examined correlated responses to body size selection in larval growth rates and development time. After seven generations, SSD remained unchanged in all selected lines; this observed lack of response to short-term selection may be attributed to evolutionary constraints arising from between-sex body size correlations. Developmental traits showed complex correlated responses under different selection treatments. These results suggest that sex-specific larval development patterns may facilitate the evolution of SSD.

Polymorphic genes of major effect: consequences for variation, selection and evolution in Arabidopsis thaliana

The importance of genes of major effect for evolutionary trajectories within and among natural populations has long been the subject of intense debate. For example, if allelic variation at a major-effect locus fundamentally alters the structure of quantitative trait variation, then fixation of a single locus can have rapid and profound effects on the rate or direction of subsequent evolutionary change. Using an Arabidopsis thaliana RIL mapping population, we compare G-matrix structure between lines possessing different alleles at ERECTA, a locus known to affect ecologically relevant variation in plant architecture. We find that the allele present at ERECTA significantly alters G-matrix structure-in particular the genetic correlations between branch number and flowering time traits-and may also modulate the strength of natural selection on these traits. Despite these differences, however, when we extend our analysis to determine how evolution might differ depending on the ERECTA allele, we find that predicted responses to selection are similar. To compare responses to selection between allele classes, we developed a resampling strategy that incorporates uncertainty in estimates of selection that can also be used for statistical comparisons of G matrices.

EVOLUTIONARY DIVERGENCE IN DIRECTIONS OF HIGH PHENOTYPIC VARIANCE IN THE OSTRACODE GENUS POSEIDONAMICUS

Trait variation and covariation are understood to influence the response of populations to natural selection on generational time scales, but their role, if any, in shaping long-term macroevolutionary divergence is still unclear. The present study uses the rich fossil record of the ostracode genus Poseidonamicus to reconstruct in great detail the evolutionary history of a set of landmark-based morphometric characters. This reconstruction included two kinds of evolutionary inferences: ancestor-descendant transitions among populations repeatedly sampled at the same location and divergence between lineages measured as independent contrasts on a phylogeny. This reconstructed history was then used to test if evolutionary changes were concentrated in directions (traits or combinations of traits) with high phenotypic variance. Two different statistics of association between evolution and variation tested the null hypothesis that evolutionary changes occur in random directions with respect to trait variability. The first of these measured the similarity between the directions of evolutionary change and the axis of maximum variance, and the second measured the degree to which evolutionary changes were concentrated in directions of high phenotypic variation. Randomization tests indicated that both kinds of evolutionary inferences (ancestor-descendant and phylogenetic contrasts) occurred preferentially in directions of high phenotypic variance (and close to the axis of maximal variation), suggesting that within-population variation can structure long-term divergence. This effect decayed after a few million years, but at least for one metric, never disappeared completely. These results are consistent with Schluter's genetic constraints model in which evolutionary trajectories on adaptive landscapes are deflected by variation within and covariation among traits.

The limits on sexual dimorphism in vegetative traits in a gynodioecious plant

Gynodioecious plants exhibit modest sexual dimorphism in vegetative and phenological traits, which stands in stark contrast to pronounced dimorphism in reproductive traits. I evaluate the roles of limited genetic variation, negative genetic covariation (within and between sex morphs), and lack of gender-differential selection in contributing to minimal sexual dimorphism for these traits in Fragaria virginiana. Major findings are as follows. First, selection was sometimes differential but rarely divergent between male and female fertility modes. Second, response to selection was constrained by low genetic variation and extensive genetic covariance. In fact, covariance between traits within sex morphs appears to represent a constraint on par with that of covariance between sex morphs. Third, these constraints combine with different modes of gamete transmission to produce very different gender-specific contributions to the mean phenotypes of the next generation. Finally, predicted responses to selection for several traits are concordant with the degree and direction of dimorphism in a closely related dioecious species. In sum, this work suggests that minimal sexual dimorphism in vegetative and phenological traits is due to similar directional selection via male and female fertility combined with the constraints of low genetic variation and extensive genetic covariance both within and between sex morphs.

EVOLUTIONARY POTENTIAL OF CHAMAECRISTA FASCICULATA IN RELATION TO CLIMATE CHANGE. II. GENETIC ARCHITECTURE OF THREE POPULATIONS RECIPROCALLY PLANTED ALONG AN ENVIRONMENTAL GRADIENT IN THE GREAT PLAINS

Climate change will test the evolutionary potential of populations. Information regarding the genetic architecture within and among populations is essential for prediction of evolutionary outcomes. However, little is known about the distribution of genetic variation for relevant traits in natural populations or alteration of genetic architecture in a changing environment. In this study, pedigreed families from three populations of the annual prairie legume Chamaecrista fasciculata were reciprocally transplanted in three environments across a broad latitudinal range in the Great Plains. The underlying premise of this work is that northern populations will in the future experience climates similar to current-day climates further south. Estimates of narrow-sense heritability ranged from 0.053 to 0.481, suggesting the potential for evolutionary change is possible for most traits. In general, the northern population harbored less genetic variation and had lower heritability for traits than the southern population. This population also experienced large reductions in fitness, as measured by estimated lifetime fecundity, when raised in either the intermediate or the southern climate, whereas the difference between the intermediate and southern population was less extreme. For fecundity, the pattern of cross-environment additive genetic correlations was antagonistic to evolutionary change in four of six cases when native and nonnative sites were compared. Six additional antagonistic positive correlations were found for the rate of phenological development and leaf thickness. Overall, the data suggest that if climate changes as predicted, the northern population will face a severe evolutionary challenge in the future because of low heritabilities, cross-environment genetic correlations antagonistic to selection, and demographic instability due to lower seed production in a hotter and drier climate.

THE QUANTITATIVE GENETICS OF FLORAL TRAIT VARIATION IN LOBELIA: POTENTIAL CONSTRAINTS ON ADAPTIVE EVOLUTION

Although pollinator-mediated natural selection has been measured on many floral traits and in many species, the extent to which selection is constrained from producing optimal floral phenotypes is less frequently studied. In particular, negative correlations between flower size and flower number are hypothesized to be a major constraint on the evolution of floral displays, yet few empirical studies have documented such a trade-off. To determine the potential for genetic constraints on the adaptive evolution of floral displays, I estimated the quantitative genetic basis of floral trait variation in two populations of Lobelia siphilitica. Restricted maximum likelihood (REML) analyses of greenhouse-grown half-sib families were used to estimate genetic variances and covariances for flower number and six measures of flower size. There was significant genetic variation for all seven floral traits in both populations. Flower number was negatively genetically correlated with four measures of flower size in one population and three measures in the other. When the genetic variance-covariance matrices were combined with field estimates of phenotypic selection gradients, the predicted multivariate evolutionary response was less than or opposite in sign to the selection gradient for flower number and five of six measures of flower size, suggesting genetic constraints on the evolution of these traits. More generally, my results indicate that the adaptive evolution of floral displays can be constrained by trade-offs between flower size and number, as has been assumed by many theoretical models of floral evolution.

The fall and rise of Dr Pangloss: adaptationism and the Spandrels paper 20 years later

Twenty years have passed since Gould and Lewontin published their critique of 'the adaptationist program' - the tendency of some evolutionary biologists to assume, rather than demonstrate, the operation of natural selection. After the 'Spandrels paper', evolutionists were more careful about producing just-so stories based on selection, and paid more attention to a panoply of other processes. Then came reactions against the excesses of the anti-adaptationist movement, which ranged from a complete dismissal of Gould and Lewontin's contribution to a positive call to overcome the problems. We now have an excellent opportunity for finally affirming a more balanced and pluralistic approach to the study of evolutionary biology.