We report the consequences of >330 generations of experimental evolution under larval undernutrition for male postcopulatory success and traits thought to mediate it in Drosophila melanogaster, comparing them to phenotypically plastic responses. Males from populations evolved on standard diet showed a >30% plastic reduction in the size of accessory glands (AG) when raised on a nutrient-poor larval diet. Experimental evolution on the poor diet led to a further reduction that was more pronounced in smaller individuals, changing the allometric slope between AG and wing size. Rather than the expected reduction in seminal fluid protein expression, we observed a plastic increase on poor diet in investment in Acp36DE and Acp62F at the expense of Acp26Aa and SP, and an evolutionary shift in the time course towards lower virgin and higher post-mating SFP expression. Surprisingly, neither the plastic nor evolved reduction in AG was associated with impaired performance in reproductive output from matings with excess females, the ability to induce female oviposition or performance in sperm competition ("sperm defense"), except when poor diet-adapted males were raised on standard diet. Thus, larval nutrient shortage favors reduced investment in AG development, but this is compensated for by other mechanisms, minimizing consequences for postcopulatory success.
Sexual dichromatism is generally thought to arise from sexual selection favouring elaborately coloured males, as proposed in Darwin's model. Wallace offers an expanded perspective, emphasising that the evolution of cryptic and dim female colouration to evade nest predation significantly contributed to the origin of sexual dichromatism. However, previous studies on the evolutionary forces of sexual dichromatism examining Darwin's and Wallace's models have produced mixed results. Here, we re-examined Wallace's model of female-biased selection using the largest and most ecologically diverse family of landfowl (Phasianidae), known for its wide-ranging distribution and striking colour patterns. Our results demonstrate that the level of sexual dichromatism is negatively correlated with colour complexity in females but not males, and the evolutionary rates of sexual dichromatism are positively correlated with the evolutionary rates of colour complexity in both sexes. Furthermore, we show that female colour complexity is significantly associated with nest exposure, consistent with predation-related ecological constraints on female colour evolution. Together, these findings support Wallace's model by highlighting the importance of nest predation in shaping female colouration, while further suggesting that the evolution of sexual dichromatism reflects the dynamic interplay of natural and sexual selection.
How temporal environmental variation influences evolutionary divergence and the maintenance of phenotypic diversity remains poorly understood. McCarren et al. (2026) addressed this question by examining how post-fire succession alters pollination niches in a plant with two floral phenotypes, Lapeirousia anceps. Their long-term study showed that predictable changes in pollination niches generate fluctuating selection that maintains floral polymorphism within a single population.
Species coexistence is critical to the assembly of biodiverse communities. When closely related species come into secondary contact, competitive and reproductive interactions can both influence the potential for coexistence. We tested whether ecological and/or evolutionary factors affect coexistence dynamics in a pair of closely related honeyeaters that have come into secondary contact in the Solomon Islands. Myzomela cardinalis colonized the island of Makira and established a population in sympatry with the endemic M. tristrami. These taxa are ecologically similar and hybridize. If ecological factors are promoting coexistence, we expect species to partition where they forage for resources or defend breeding territories. In addition, evolutionary pressures may facilitate coexistence if species mate assortatively, avoiding production of potentially low-fitness hybrid offspring. Using data on foraging locations, nest locations, and mate pairing, we find support for partitioning of space use, specifically in the context of nest placement. Phenotypic pairs of the recently arrived species nested significantly closer to the coastline than did endemic species pairs. We also find strong evidence of assortative mating based on phenotype. Thus, both ecological and evolutionary factors appear to contribute to the coexistence of these closely related hybridizing species.
How far can evolution push a complex structure before hitting developmental limits, and when can it break free to explore new forms? Domestic dogs (Canis familiaris) represent the most extreme skull shape diversity generated within a single species through recent artificial selection, while neotropical leaf-nosed bats (Phyllostomidae) showcase the broadest craniofacial radiation in mammals, shaped over millions of years of natural selection. Because these contrasting systems belong to the same mammalian superorder, they provide an opportunity to test whether skull evolution is constrained by the same limits operating at short timescales, or whether adaptive radiations enable access to novel morphologies at macroevolutionary scales. Using three-dimensional geometric morphometrics of 33 canids (dog breeds and wild species) and 62 phyllostomid species, we mapped cranial variation across morphospace to assess disparity and modularity, and then compared the dominant directions of cranial shape variation between clades. Despite the overlap in morphospace for some bats and dogs, we found numerous examples of evolutionary novelty in bats, supporting the emergence of macroevolutionary innovation. Both clades varied along a primary axis of snout elongation and shortening, but with distinct outcomes: the slightly more modular canine skull was largely confined to a single dominant direction of variation, whereas phyllostomids dispersed along multiple axes while occupying regions inaccessible to dogs despite exhibiting stronger cranial integration. While artificial selection in dogs has produced much more disparity per unit of evolutionary time, natural selection in bats reveals repeated evolutionary innovations rather than constrained scaling. These results demonstrate that artificial selection may inflate variation along a constrained axis, while natural selection may allow structural innovation beyond the limits of microevolutionary change.
Testis morphology may act as a determinant of sperm size, and the evolutionary pressure to produce longer sperm may impose functional constraints on testicular architecture. Postcopulatory sexual selection, particularly sperm competition, is widely recognised as a primary driver of the remarkable interspecific variation in sperm size and number across animals. Theoretical hypotheses predict that longer sperm require wider seminiferous tubules and that intense sperm competition favours an increased proportion of sperm-producing (seminiferous) tissue relative to non-spermatogenic components, independent of overall testis size. Using a phylogenetic comparative approach across 78 anuran species, we tested these two predictions. Our analyses revealed that the cross-sectional area of the seminiferous tubules was significantly and positively correlated with sperm length, thereby supporting the proposed morphogenetic constraint. Furthermore, relative testis size, which was used as a validated proxy for sperm competition intensity, was positively associated with the proportion of seminiferous tissue. In contrast, absolute testis size did not predict this tissue proportion. Our findings suggest that sperm competition shapes anuran testicular architecture through two distinct evolutionary pathways: indirect selection for wider tubules mediated by longer sperm, and direct selection for increased investment in seminiferous tissue. This tissue optimisation extends beyond simple testicular enlargement and reveals sophisticated evolutionary adaptations to postcopulatory sexual selection.
In temperate climates, dormancy enables organisms to survive winter, with timing often tied to environmental cues, such as daylength, that differ along latitudinal gradients. Adaptation to these gradients is crucial during range expansion. Clark et al. (2026) found that in the introduced northern tamarisk beetle (Diorhabda carinulata), populations from the range core were locally adapted in dormancy timing to core environments, while newly arrived edge populations showed heterogeneous responses to daylength, consistent with adaptation to seasonal timing. Genetic variation and high heritability in dormancy traits in the core populations likely drive range expansion potential through rapid evolution of dormancy traits.
Insects produce a vast diversity of sex pheromone compounds despite strong stabilizing selection on species-specific signals. In multiple insect lineages, these pheromones chemically resemble the volatiles produced by plants. Here, I propose that adaptation of insect sensory systems to plant chemistry biases the origin and diversification of insect pheromones. Repeated independent recruitment of plant-like compounds, either through sequestration or de novo biosynthesis, suggests that pheromone evolution can be shaped by ecological interactions. Receptors with dual roles in host detection and pheromone detection may be functionally constrained, further influencing signal diversification and receptor specialization. This framework links sensory biases with macroevolutionary patterns, generating testable predictions about how sensory biases shape insect pheromone evolution.
Environmental shifts often dictate Arthropod morphological evolution, yet certain lineages maintain remarkably conserved body plans across diverse habitats. Whip spiders (Amblypygi) emerged over 300 Ma and exhibit striking morphological conservatism; nonetheless, what drives this apparent conservatism remains an open question. Here, we analyzed how body size and shape evolved in response to climate and habitat by examining the contributions of bioclimatic variables and habitat type to morphology, alternative macroevolutionary models, estimating the strength of the phylogenetic signal, and testing whether evolutionary rates differ. Habitat was the strongest predictor of body size, with cave-dwelling species approximately 22% larger than surface species, while climatic gradients accounted for almost none of the shape variation. The evolution of both body size and shape was best described by a single-peak Ornstein-Uhlenbeck model, suggesting stabilizing selection toward a shared adaptive peak. Body size exhibited a moderate phylogenetic signal consistent with a Brownian motion process, whereas body shape lacked a clear phylogenetic signal, indicating stronger stabilizing selection on shape. Together, our findings suggest that the morphological conservatism of whip spiders is maintained by a "jack-of-all-trades" strategy, in which stabilizing selection favors a versatile phenotype that facilitates the occupation of diverse ecological niches without marked morphological divergence.
Environmental heterogeneity can favor phenotypic plasticity, but whether evolutionary responses in one environment predict trait expression after environmental change remains unclear for threshold traits. Polyphenisms-plasticity characterized by discrete alternative morphs-rely on developmental thresholds, and as a consequence, are often expected to permit partial evolutionary decoupling among alternative phenotypes. Using replicated experimental evolution and artificial selection of a resource polyphenism in the nematode Pristionchus exspectatus, which develops either a predatory morph or a microbivorous morph, I tested whether evolutionary changes in morph frequency are coupled across environments. Across independent lineages, I asked whether evolutionary increase in predatory morph frequency under selection predicted phenotypic change when populations were reared back in the ancestral environment. I found a strong positive cross-environment coupling: for both sexes, lineages showing larger evolutionary responses during the first 25 generations, and across the full 50-generation experiment, also showed larger increases in predatory morph expression after environmental reversal. Artificial selection on mouth-form produced the same pattern. These results show that even a discrete threshold trait can retain strong and repeatable correlated responses across environments, consistent with morph-frequency evolution being channeled by developmental architecture shared across environments.
Modularity is a pattern whereby traits evolve as interrelated systems, resulting in lower covariation between modules than within. Modularity may facilitate diversification by permitting the phenotype to independently respond to selection. The Appalachian Mountains are a biodiversity hotspot for lungless salamanders (Plethodontidae). Despite their ecological diversity, putative catalysts of their radiation in the Appalachians are poorly understood. We test whether (i) Appalachian salamanders exhibit modularity in functional, morphological characters and (ii) if the rate of evolution-a fundamental macroevolutionary signal of ecological opportunity-varies among modules and if their rates are correlated. We find that Appalachian salamanders exhibit modularity under two different models in which their head, trunk, limbs, and tail or the head, body, and limbs act as separate modules. The three major clades, Plethodon, Eurycea, and Desmognathus, vary in the presence and extent of modularity. Across Appalachian salamanders, modules exhibit non-significant differences in rates of evolution (~1.5-fold), with the trunk and tail modules exhibiting the slowest and fastest rates, respectively. We also find that the rate of modular evolution can be highly correlated, suggesting that modules may respond similarly to ecological opportunity. These results suggest that modularity underlies the morphological diversity of Appalachian salamanders by permitting localized regions of the phenotype to respond independently to selection, but not evolve at different rates.
Anglerfishes (Lophiiformes) have evolved diverse morphological solutions for life in the deep sea (>200 m). Previous studies have shown that the bathypelagic lineage (Ceratioidei) exhibits high evolutionary rates; however, the intrinsic mechanisms underlying this accelerated evolution remained unclear. Here, we examine patterns of modularity and integration, rates of morphological evolution, and evolutionary rate matrices across the skulls of 100 anglerfish species spanning shallow and deep-sea habitats using geometric morphometrics and phylogenetic comparative methods. We find that both deep-benthic and bathypelagic anglerfishes exhibit stronger patterns of integration compared to their shallow-water relatives. The iconic bathypelagic anglerfishes (Ceratioidei) differ from other Lophiiformes by exhibiting an integrated upper-lower jaw system, likely driven by selection for feeding efficiency in resource-scarce environments. Jaw evolution in Ceratioidei, Chaunacoidei, and Ogcocephaloidei exhibits novel evolutionary trajectories that differ from the dominant pattern across Lophiiformes. Our findings suggest that extreme environments promoted morphological innovation in anglerfishes through evolution in novel phenotypic directions and strengthening of evolutionary integration.
While theory has long explained how selection can favor dispersal, we know much less about the capacity for dispersal to evolve in natural populations. One challenge is to identify how heritable genetic variation in multiple dispersal traits aligns with directions of selection on these traits. Therefore, we used an experimental pedigree in the solitary ascidian Molgula occidentalis and constructed Bayesian quantitative genetic models to estimate evolvability of dispersal traits. We compared the evolvability of dispersal traits by: 1) assessing the average evolvability of the G-matrix, 2) considering specific directions of selection hypothesized from biomechanical and life history reasoning, and 3) estimating the predicted response to selection from the observed additive genetic covariance between dispersal traits and fitness components. There was greater additive genetic variation in tail length than in trunk length, as there was for maternal effect and dominance variance. As a result, selection for longer tails (faster swimming) was associated with a higher evolvability compared to selection for shorter tails and longer trunks (energetic efficiency) or shorter tails and trunks (fecundity). The predicted response to selection at hatching and settlement, based on observed additive genetic covariances, was similar for trunk and tail length. Overall, evolutionary change in dispersal distances would occur more rapidly under selection on swimming ability than on energetic economy and fecundity. Furthermore, multiple lineages within Molgulidae have repeatedly diverged along the larval morphology axis in which we measured the highest evolvability. Our results demonstrate the utility of combining estimates of evolvability with hypothesized and observed selection gradients.
Color polymorphism within populations has fascinated biologists for generations because theory predicts eventual loss of polymorphism without some form of balancing selection. However, it is challenging to distinguish a balanced polymorphism from one slowly changing via genetic drift. I used a 20-year time series to evaluate balancing selection in a population of salamanders (Plethodon ventralis) that vary in the presence of a dorsal stripe (possibly a form of disruptive coloration). Time series analysis supported balancing selection, which might be caused by negative frequency-dependent selection, heterozygote advantage, selection-migration balance, or environmental fluctuations. Polymorphism in small cryptic animals is generally thought to be stabilized by frequency-dependent predation, and I hypothesized that if selection was driven by visual predators, it would be apparent only during the active season, not when salamanders were hidden underground. Analyzing the time series by season supported that hypothesis. Dynamics over the summer inactive season were consistent with a random walk, but during the winter active season there was a strong tendency to converge toward an equilibrium of approximately 30% striped individuals. This is clear evidence of balancing selection, and I argue that frequency-dependent predation is the most likely explanation, but further work is needed to test alternative hypotheses.
Differences in ploidy levels represent a significant barrier to interspecific gene flow, but growing evidence from natural and experimental systems suggests that this barrier may not be as strong as commonly believed. In this study, we aimed to examine patterns of interploidy hybridization between two ecologically similar and partially sympatric wetland perennials in the central Apennines: diploid Cardamine apennina and allotetraploid C. amporitana. Our detailed chromosomal, flow cytometric, and genomic (RADseq) analyses revealed a strikingly high frequency of allotriploids across multiple sites, with no evidence of backcrossing, later-generation hybrids, or higher-level allopolyploids. Contrary to expectations, clonal propagation is not the primary mechanism underlying the observed abundance of triploids. Instead, a high hybridization rate with recurrent, bidirectional, and polytopic hybrid formation, together with the presumed longevity of triploid individuals, appears to explain their persistence. Our findings also raise two key issues: the possible triggers of this apparently recent burst of hybridization, and the potential long-term consequences of frequent allotriploid formation for the survival of the parental (sub)endemic species in the Apennines. From a broader perspective, this study underscores the evolutionary significance and dynamics of species contact zones.
The striking diversity of bird plumage colors has long fascinated researchers and inspired foundational evolutionary theories. Both natural and social selection can shape plumage color variation, enhancing either crypsis or conspicuousness, and thereby influencing fitness. Yet how light environments influence color function remains poorly understood. Here, we show that color variation among plumage patches aligns with their presumed functions-either attracting or diverting receiver attention-but is only weakly associated with local light conditions. Using a comprehensive dataset of side-welling irradiance, background reflectance, and hummingbird plumage spectra, we find that site-specific variation in irradiance predicts changes in crown coloration, but not in other patches. In contrast, sex was a significant predictor of coloration in all plumage patches, including those presumed to be cryptic. We found that conspicuousness across patches decreases when shifting away from the angle of peak reflectance, which suggests that iridescence and behavioral mechanisms allow hummingbirds to decrease conspicuousness at different viewing geometries. These results suggest that hummingbird coloration is not adapted to variation in specific light environments, but rather it has been tuned through time by behavior and interactions between sexes and predators to remain effective despite environmental variation.
Vertebrates have varied body size distributions. Among these, non-avian dinosaurs have uniquely large minimum and maximum sizes, but the reasons remain unknown. Energetic fitness models provide a potential explanation, by describing how physiological scaling relationships should structure body size distributions under selection to maximize the conversion of energy into offspring. We show that the body size distributions of birds, mammals, and turtles fit the expectations of energetic fitness models, but snakes, lizards, and crocodylians do not. Moreover, no reasonable combination of physiological scaling relationships explains the up-shifted minimum body size of non-avian dinosaurs or flightless birds. We propose that energetic fitness models perform well when body size distributions are governed primarily by intrinsic energetic factors. However, extrinsic constraints imposed by ecological factors such as competition, predation, or ecological niche may also be needed to explain why species body size distributions differ from the model expectations in some groups. The abrupt macroevolutionary shift from non-avian dinosaurs with minimum adult body sizes of 400 g, to birds with modal body size of 30 g cannot be explained by changes in intrinsic physiological factors, and may instead have been caused by an abrupt release of ecological constraints, potentially related to the origin of flight.
Mountains are important centers of biodiversity and studies of speciation. In particular, the north-south linear orientation of the Andes allows examination of how geographic isolation, elevation, and latitude influence divergence in closely related species such as Myioborus warblers (Parulidae), the focal taxa of this study. Traditional models of Andean speciation have emphasized allopatric divergence due to geographic barriers, but thanks to the limited sampling of hybrid zones and lack of genome-wide datasets in tropical taxa, we may have underestimated of the role of gene flow in shaping patterns of divergence in the region. Using ddRAD-seq genomic data, Céspedes Arias et al. (2026) demonstrated that geographic isolation and hybridization both contribute to the generation and maintenance of Andean warbler lineages. Their results revealed how complex patterns of genetic divergence, introgression, and isolation-by-distance contribute to the historical buildup of biodiversity in the Andes, challenging strictly isolationist models of mountain evolution.
