Egg size represents a primary form of parental investment, but whether it is causally related to offspring fitness is difficult to establish. We mated Japanese quail (Coturnix japonica) from replicated selection lines for high and low maternal investment in egg size to partners from the same and alternate selection line. We combined this with detailed pedigree information to partition variation in the size and hatching success of their eggs into their underlying genetic and non-genetic components, and estimated the correlation between these traits at all levels. Variation in egg size was primarily explained by additive genetic variation among mothers (h2 = 65%) and variation in hatching success also had an additive genetic component among mothers (h2 = 16%), with little evidence for additive effects of the father on both traits. Importantly, the correlations between egg size and hatching success were positive and similar at the genetic level among mothers and the non-genetic level within mothers. Furthermore, in one of the two replicates the eggs from females selected for high maternal investment in egg size had higher hatching success. We interpret these results as being most parsimoniously explained by a positive causal effect of egg size on hatching success.

Accurate predictions of evolutionary response are required to determine whether populations can adapt to rapidly changing environments, or whether they will go extinct. The multivariate breeder's equation enables us to make these predictions using information on the genetic variance and covariance between traits (G) and selection. However, estimating G is a challenge that requires known relatedness among individuals in a population, and large sample sizes. Phenotypic correlations are easier to estimate, and past studies have argued that they can be used as a proxy, known as Cheverud's conjecture. However, multivariate patterns of variation that are important for predicting the rate and direction of evolution are rarely captured in these studies. Here, we use simulated data in which we vary heritability, sample size, pleiotropy, and the alignment of genetic and environmental effects on phenotypes, and determine the similarity of G and P with multivariate matrix comparisons. We find that the extent of pleiotropy and the relationship between genetic and environmental effects are important determinants of the similarity between G and P. While we show that P can provide a better prediction of evolutionary response when sample sizes are small, or under the limited conditions of weak pleiotropy, congruent genetic and environmental effects, and access to accurate heritability estimates, quantitative genetic studies remain crucial for making good evolutionary predictions that cannot be bypassed by using P.

Vocal amplitude is crucial for acoustic communication because it determines how far a sound travels and can convey valuable information about signal senders to signal receivers. Yet, until recently, amplitude had received limited attention in the literature mainly because of operational challenges in its measurement. As a result, fundamental questions about amplitude remain largely unanswered even in one of the most extensively studied group of singers-birds. Here, we set out to answer two such questions: to what degree do birds vary in how loud they can vocalize, and what evolutionary factors drive this variation? Using original amplitude data from 123 bird species, we show that maximum measured amplitudes vary broadly across species-from 64 dB LAeq,20ms at 1 m in a species of hummingbird to 122 dB in a species of cotinga-, with two new species emerging as candidates for the world's loudest bird (as yet identified). Regarding evolutionary drivers of amplitude, we found that species with larger body sizes and larger beak gape widths tend to achieve higher maximum amplitudes. By contrast, ecological predictors assessed-aerial mobility, habitat density, and mating system-had no unequivocal effect on maximum amplitude. By establishing connections between morphology and vocal amplitude, our results represent a step forward in uncovering what enables birds to produce the loudest acoustic signals of all terrestrial fauna.

Ocean sunfish (family Molidae) are often perceived as "low-energy" drifters composed largely of connective tissue with noticeably missing fins and even cited as "mistakes" of natural selection. Because locomotory capacity has been identified as a driver of evolution in animal mitochondrial genomes (mtDNA), ocean sunfish are expected to show signatures of relaxed selection on mtDNA compared to closely related, more energetic species. Contrary to expectations, ocean sunfish showed lower dN/dS ratios (the ratio of non-synonymous to synonymous substitution rates) in all mitochondrial-encoded genes and further analyses strongly suggest this is due to increased purifying selection on mtDNA within Molidae. Population genetic data also support more intense selection on ocean sunfish mtDNA. These results agree with recent studies recharacterizing ocean sunfish as active swimmers with a complex lifestyle including extreme vertical migrations, behavioral thermoregulation, and extreme fecundity. Finally, across 64 diverse fishes, mtDNA evolutionary rates did not correlate with swimming speed. More broadly, this study highlights how predicting selection on mitochondrial genes is not straightforward. Assuming different selective pressures on mtDNA given locomotion or different perceived "lifestyles" may be an oversimplification, analogous to regarding an unusual species as an evolutionary "mistake".

Same-sex sexual behavior (SSB) occurs across many animal taxa. Some instances of SSB likely arise from animals' mating indiscriminately and/or with imperfect assessment of their partner's sex. Selection on indiscriminate mating strategies depends on the costs and benefits of mating and the distinguishability of sexes-which in turn depend on evolvable signals of sex. Other factors like population density (which determines the rate of encountering potential mates) and investment roles (i.e., which sex invests more in reproduction) are likely relevant to the evolution of same-sex and different-sex mating, but there is no consensus as to the direction of these effects. We develop a new model of adaptive coevolution of sexual signaling and discrimination, explicitly incorporating encounter rate, mating costs, and investment roles. Coevolutionary dynamics typically lead to two alternative equilibria: one with no sexual signals and indiscriminate mating; the other with high-quality signaling and exclusively different-sex mating. Low-density conditions favor indiscriminate strategies, due to the steep opportunity costs of missing a possible mate. High density can favor either discriminate or indiscriminate strategies, depending on investment roles. By providing theoretical predictions for the effects of encounter rate and investment roles, our model bridges gaps between theoretical and empirical work on SSB.

The diversity of animal shapes and colors raises questions about the mechanisms by which species acquire complex phenotypes considering that they evolve under both natural and sexual selection. Tanagers employ a diversity of signals in social interactions which, coupled with diverse diets that include fruits and flowers, make them an excellent group for studying how diet influences the evolution of communication signals. Using Bayesian linear mixed models, plumage reflectance data for 291 tanager species (∼77% of species), and appropriate techniques for compositional diet data, we show that percentage increases in frugivory and/or nectarivory dietary components are associated with higher hue disparity scores (a proxy of plumage coloration complexity). Additionally, evolutionary decreases in the percentage of seeds and arthropods, coupled with increases in fruits, are related to higher color complexity. The presence of structural coloration in plumage was related to high hue disparity and frugivory, but carotenoid-based coloration-contrary to expectations-was not associated with either. Our results support the hypothesis that the link between diet and plumage coloration arose not from a physiological constraint but from sensory (perceptual) biases of frugivores and nectarivores for detecting colorful baits in the forest, which resulted in preferences for chromatically diverse feather patterns.

Insufficient protein intake causes malnutrition, compromising immunity and increasing the risk of pathogenic infections. In fact, under limited protein availability, organisms may experience stronger selective pressure to optimally allocate their resources between immunity vs other energy-demanding processes, such as reproduction, resulting in evolutionary trade-offs. In many species, protein deficiency may also more strongly affect female reproduction, leading to sexually-dimorphic trade-offs involving immunity and infection outcomes, though these haven't been experimentally tested. Here, we demonstrate that in Drosophila melanogaster populations evolving greater investment in early-reproduction in response to a lower-protein diet before oviposition, unmated females exhibited lower post-infection survival against Providencia rettgeri than unmated males, corroborating our prediction. However, mating eliminated these sex differences, with both sexes exhibiting nearly identical infection costs and reduced post-infection survival compared with their unselected counterparts. Overall, selected flies also had lower infection tolerance, although the effect on pathogen resistance was weak and inconsistent. Immune responses, including Toll- and IMD-mediated pathways, were either less upregulated or more downregulated in selected flies, which could explain their increased pathogen susceptibility. Downregulated metabolic pathways may also play a role. Overall, this study offers new evolutionary insights into how low-protein diets drive sex- and reproductive-status-specific variations in immunity and health.

Our understanding of how males evolve courtship discrimination and the traits they use for species identification is restricted to a few model systems. Theory predicts that males between more sympatric species should evolve stronger courtship discrimination and target female traits for reliable species identification. Thus, traits that increase divergence specifically during sexual maturity should be prime targets of species recognition. We tested this by measuring male courtship discrimination indexes (CDIs) and cuticular hydrocarbons (CHCs) across ten species pairs of Drosophila affinis subgroup. CDIs ranged from 12% to 86% of total sexual isolation, increasing between more sympatric pairs only if they had postzygotic isolation. Principle Component Analysis on CHCs revealed species-specific clustering in both sexes. CHCs between younger and older flies were not correlated and male CDIs were only correlated with older CHCs. While most CHCs showed species x age interactions, only 11(Z)-tricosene (C23:1), 10,14-pentacosadiene (C25:2), and 2-methylhexacosane (C27:0)) were significantly differentiated and became more divergent with age. We then used "rub-off" experiments and direct transfers of these CHCs to show their species-specific repellent and aphrodisiac properties. This suggests that targeting sexually mature female CHCs allows males to distinguish conspecifics from heterospecifics to avoid maladaptive hybridization or signal interference in sympatry.

Although directional selection and additive genetic variance are frequently detected in wild populations, evolutionary responses are often weak or absent, giving rise to a "paradox of stasis." To explore the relationship between selection and evolution, we used a pedigreed wild population of lizards (Anolis sagrei) to quantify natural selection, estimate additive genetic (co)variances, and track phenotypic and genetic change in adult body size while addressing several factors that could lead to evolutionary stasis. Selection via adult reproductive success consistently favored larger body size in both sexes and was stronger in males. Body size was heritable and genetically correlated with adult reproductive success in both sexes, indicating that selection acted on underlying additive genetic variance. Across five annual cohorts, mean phenotypic values and breeding values for body size increased in both sexes, exceeding expectations under genetic drift. This rapid evolutionary response was not obscured by several processes commonly invoked to explain stasis. For example, accounting for phenotypic skew strengthened expected responses, and prior viability selection on juveniles did not phenotypically or genetically oppose selection on adult body size. Our results show that rapid evolutionary change is detectable in the wild when directional selection is consistent, acts on additive genetic variance, and is not genetically opposed by early-life viability selection.

When herbivory is inevitable, plants can cope with it through tolerance, buffering its negative effects on plant fitness. Macroevolutionary patterns of tolerance to herbivory may improve our understanding about its evolution. Here, we explore the phylogenetic patterns of tolerance to herbivory and underlying functional traits, as well as the potential evolutionary constraints in the forms of fitness costs and a trade-off with resistance across the genus Datura. We performed a common garden experiment with simulated herbivory (defoliation) and analyzed data within a phylogenetic comparative explicit framework. Significant phylogenetic signal was detected only for certain tolerance underlying traits: specific leaf area and chlorophyll content, but no overall phylogenetic signal for tolerance was observed. No trade-off between tolerance and resistance was detected although a negative relationship between tolerance and plant fitness in the absence of damage was found. Our results provide evidence for an absence of correspondence between phylogeny and interspecific plant defense variation, even though some underlying traits were influenced by phylogeny. We propose that the relationship between resistance and tolerance can arise as complementary defenses to variable herbivory pressures mediated by plants' internal resource allocation while the cost of tolerance may imply a trade-off with growth or reproductive potential.

The evolution of distinct reproductive strategies within the same species can offer unique insights into adaptive divergence. Since the discovery of European flounder (Platichthys flesus) in the freshwater environment of the isolated Lake Pulmanki in the 1970s, the evolutionary origin and reproductive strategy of this euryhaline fish has remained a mystery. Using otolith chemistry and whole-genome resequencing, we tested the hypothesis that flounders in Lake Pulmanki constitute a freshwater-adapted lineage, distinct from their marine ancestors. Otoliths confirmed that most of the flounders caught in Lake Pulmanki were born and lived exclusively in freshwater, providing the first evidence for a fully freshwater confined life cycle in Platichthys. Unexpectedly, most individuals in nearby fjords were also born in freshwater. While the genome-wide differentiation among marine- and freshwater-born flounders was weak, 77 SNPs across 18 chromosomes were significantly associated with habitat-use type, mapping to 35 genes. Moreover, outlier loci show strong interchromosomal linkage disequilibrium (LD), suggesting divergent selection for allelic combinations associated with reproductive habitat use. This pattern is consistent with a genetic basis for early habitat use differences involving epistatic interactions among alleles in different chromosomes, pointing to a genetic adaptation for freshwater reproduction in Lake Pulmanki.

We discuss robust model comparison in phylogenetic path analysis and how to incorporate interactions in path analysis with d-separation. We revisit a study by Benson et al. (2025) to highlight the importance of carefully evaluating the causal models being compared to avoid erroneous conclusions. Our results show that there are biologically plausible models, not previously considered, that outperform those in the original study leading to quite different biological conclusions. We also use this work to demonstrate, using empirical data, how interactions can be incorporated in path analyses, and highlight how their presence affects the conditional independencies that must be tested for the model to be considered a plausible causal model. Our commentary highlights the importance of rigorously evaluating model space and verifying both the implied conditional independencies as well as the robustness of the assumed causal effects.

In consumer-resource species interactions, consumers can play a role in shaping the evolution of their own resources by acting as an agent of selection on the consumed species' traits. Many consumers forage on groups or patches of their resources and may change their foraging behavior within and among groups with different trait compositions. If a consumer's behavioral changes affect the fitness of their resource species, this creates the potential for consumers to impose multilevel selection on the traits of the resource species. Here, we examine how pollinator foraging behavior may create individual and multilevel selection on nectar resources in the beetle-pollinated understory wildflower, Amianthium muscaetoxicum (Fly Poison). We measured nectar traits, pollinator behavior, and reproductive outcomes of individual plants, small plant neighborhoods, and whole plant patches and examined phenotypic selection at each level. We found that nectar volume at the patch level shaped patterns of pollinator visitation, but phenotypic selection on nectar traits was most consistent with beetles engaging with plants as discrete individuals, not as groups. However, we did detect a neighbor contrast component of multilevel selection: the direction of selection on nectar volume was more negative or flat in patches with lower mean nectar production and more positive in patches with higher mean nectar production. Overall, we found strong support that selection favors individual plants with high-quantity, high-quality nectar resources across the forest.

The evolutionary trajectory of helminth parasites of the genus Crassicauda (Nematoda: Spirurida) is thought to be tightly linked with that of their cetacean hosts. However, due to limitations in sampling these giant (some > 3 meters) worms, their diversity and evolutionary relationships remain poorly understood. In this study, we (1) identify Crassicauda species infecting cetacean hosts from the Northwest Atlantic using the mitochondrial cytochrome c oxidase subunit 1, and nuclear internal transcribed spacer 2 and small subunit ribosomal RNA genes; (2) evaluate the phylogenetic relationships among our specimens, augmented with homologous sequences from GenBank, and (3) characterize host-parasite cophylogenetic structure using distance- and event-based methods. Phylogenetic analyses resolved 11 parasite species infecting 15 cetacean species, revealing cryptic and underreported diversity. Distance-based methods detected significant cophylogenetic signal between parasite and host phylogenies, while event-based analyses detected phylogenetic incongruence. This combination supports a complex evolutionary scenario in which Crassicauda lineages colonized and speciated alongside cetaceans, particularly pelagic species within the family Delphinidae, and diversified within odontocetes. Together, these results provide insight into the mosaic of evolutionary processes that shaped the genus Crassicauda and their cetacean hosts, with implications for monitoring health and disease in marine mammal populations.

Intralocus sexual conflict (IaSC) arises when selection favours different trait values in females and males despite their shared genetic architecture. In the presence of IaSC, sex-limited selection is expected to resolve this conflict by allowing one sex to adapt without constraint from selection acting on the other sex, with intersexual genetic correlations predicted to drive a decline in the fitness of the opposite sex. In a recent study, we found that male-limited (ML) selection produced the predicted decline in female fitness, but male fitness improvement was sensitive to genetic background, suggesting unintended sources of adaptation. Here we dissect these evolutionary responses using a fully factorial assay spanning control-like and selection-like conditions, explicitly accounting for the unusual genetic features of the clone-generator (CG) breeding system and the specialized (CG) females used to enforce male-limited inheritance of nuclear genetic variation. Evolved male fitness increased by more than 100% (relative to controls) when tested with all the features of the CG breeding system. Retaining the CG genetic background while replacing CG females with control females yielded an approximately 66% fitness increase. In a control-like genetic background, male fitness increased by 26% when males competed for the CG females they had potentially adapted to, but only by a marginal 6% otherwise. Despite this strong context dependence, the underlying phenotypic responses were remarkably consistent: selected male improvements were driven by enhanced precopulatory traits; greater attractiveness, reflected in higher mating success and shorter mating latency; while postcopulatory traits, including sperm offense (P2) and fecundity induced in mates, remained unchanged. Intriguingly, evolved males also inflicted substantially less mate harm on CG females, contrary to findings from an earlier ML selection experiment. Together, these findings provide clearer evidence of intralocus sexual conflict, expressed as male-benefit adaptations alongside previously established costs to females, but demonstrate that these adaptations were also shaped by the genetic manipulations and mating context of the experimental design.