The potential role of haplodiploid sex determination in promoting the evolution of altruism and eusociality has been the subject of intense debate for over 50 y. Different theoretical models have suggested that haplodiploidy influences relatedness in a way that either does or does not make it easier for altruism to evolve. This debate over the “haplodiploidy hypothesis” can only be resolved with a decisive empirical test that controls for potential phylogenetic bias. Here we critically examine the current state of evidence for an adaptive link between haplodiploidy and eusociality, applying phylogenetically informed methods to ensure that statistical tests reflect independent evolutionary transitions. Using data from 5,678 species, across all major insect orders, we find no evidence that haplodiploidy favors an increased rate of eusocial evolution. We show that this result is robust to: a) different analytical approaches; b) alternative ways of defining both eusociality and haplodiploidy; and c) uncertainty in eusociality assignments. Our analyses suggest that previously reported associations between haplodiploidy and eusociality are likely to have been artifacts, false-positive results primarily driven by a high transition rate to eusociality within the Hymenoptera. This high transition rate could be explained by any factor associated with that group, such as parental care, monogamy, or the possession of a powerful sting.

Explaining variation in the extent of division of labour remains a major problem for our understanding of how complex life evolved. Ants show remarkable variation in their extent of reproductive division of labour, from workers who can reproduce sexually and are approximately the same size as queens, to workers that are completely sterile and 300x smaller than their queens. Examining data from 546 species of ant, we found that: (i) the ancestral ant worker likely had full reproductive potential, though was effectively sterile in the presence of a queen; (ii) the loss of worker reproductive potential generally followed a sequential step-by-step process, via reduced capacity for sexual reproduction, then the production of males only, and finally complete sterility; (iii) the independent evolution of complete sterility has occurred approximately 17 times, with only 42% of ant species having sterile workers; (iv) reproductive size dimorphism has increased to higher levels around 9 times. Exploring potential causality, we found support for the size-complexity hypothesis, that increased colony size has favoured increased division of labour between queens and workers, examining both queen-worker size dimorphism and the loss of reproductive capacity in workers.

Arbuscular mycorrhizal fungi form symbioses with ~70% of plant species, building hyphal networks that exchange nutrients for host-derived carbon. These tubular networks move ~1 billion metric tons of carbon per year into Earth's soils. However, we have no quantitative understanding of the hyphal infrastructure required to carry out this resource transfer. We assembled data from 322 studies representing more than 16,000 soil cores across nine biomes and developed machine-learning models to predict hyphal densities globally. With robotic imaging of more than 300,000 hyphae, we calibrated a biomass model from our spatial predictions. We estimate that global topsoils contain 1.10 × 1017 ± 0.13 × 1017 SD kilometers of living hyphae, weighing ~300 ± 60 SD megatons, ~4- to 6-fold the biomass of humans. Our uncertainty analyses identified undersampled ecosystems that require additional empirical attention.

Kissing can be observed across the animal kingdom. This presents an evolutionary puzzle, since the fitness benefits of kissing are unclear. We use a non-anthropocentric approach to define kissing as a non-agonistic interaction involving directed, intraspecific, oral-oral contact with some movement of the lips/mouthparts and no food transfer. Using this definition we collate basic observational data across the Afro-Eurasian primates and employ Bayesian phylogenetic methods to reconstruct the evolutionary history of kissing. We find that kissing occurs in most extant large apes, and likely also occurred in Neanderthals (Homo neanderthalensis), first evolving in the ancestor to this group ~21.5-16.9 mya. Additionally, we highlight various life history variables that correlate reasonably, but not perfectly, with kissing across the apes (multi-male mating systems, non-folivorous diets, and premastication). With a major caveat about the quantity of available data at present, we hope that our results provide a useful starting point for further research into the adaptive function of kissing that highlights hypothesis generation and testing within a phylogenetic framework.

Mycorrhizal fungi are ecosystem engineers that sustain plant life and help regulate Earth's biogeochemical cycles1-3. However, in contrast to plants and animals, the global distribution of mycorrhizal fungal biodiversity is largely unknown, which limits our ability to monitor and protect key underground ecosystems4,5. Here we trained machine-learning algorithms on a global dataset of 25,000 geolocated soil samples comprising >2.8 billion fungal DNA sequences. We predicted arbuscular mycorrhizal and ectomycorrhizal fungal richness and rarity across terrestrial ecosystems. On the basis of these predictions, we generated high-resolution, global-scale maps and identified key reservoirs of highly diverse and endemic mycorrhizal communities. Intersecting protected areas with mycorrhizal hotspots indicated that less than 10% of predicted mycorrhizal richness hotspots currently exist in protected areas. Our results describe a largely hidden component of Earth's underground ecosystems and can help identify conservation priorities, set monitoring benchmarks and create specific restoration plans and land-management strategies.

Baumard and André (2025) have suggested that cultural dynamics can be studied as a form of ecology. This provides a simpler unified approach to explaining cultural evolution, within the context of human behavioiur being shaped by natural selection. We briefly expand on two points: (1) why inclusive fitness represents our most general answer to what organisms are selected to maximise; (2) the potential for using existing eco-evolutionary theory methods to model cultural evolution.

Symbiotic nutrient exchange between arbuscular mycorrhizal (AM) fungi and their host plants varies widely depending on their physical, chemical, and biological environment. Yet dissecting this context dependency remains challenging because we lack methods for tracking nutrients such as carbon (C) and phosphorus (P). Here, we developed a new approach to quantitatively estimate C and P fluxes in the AM symbiosis from comprehensive network morphology quantification, achieved by robotic imaging and machine learning based on roughly 100 million hyphal shape measurements. We found that rates of C transfer from the plant and P transfer from the fungus were, on average, related proportionally to one another. This ratio was nearly invariant across AM fungal strains despite contrasting growth phenotypes, but was strongly affected by plant host genotype. Fungal phenotype distributions were bounded by a Pareto front with a shape favoring specialization in an exploration-exploitation trade-off. This means AM fungi can be fast range expanders or fast resource extractors, but not both. Manipulating the C/P exchange rate by swapping the plant host genotype shifted this Pareto front, indicating that the exchange rate constrains possible AM fungal growth strategies. We show by mathematical modeling how AM fungal growth at fixed exchange rate leads to qualitatively different symbiotic outcomes depending on fungal traits and nutrient availability. ### Competing Interest Statement The authors have declared no competing interest. International Human Frontier Science Program Organization, https://ror.org/02ebx7v45, 0029 European Research Council, https://ror.org/0472cxd90, 834164, 101076062 Grantham Foundation, https://ror.org/04mm88136 Schmidt Family Foundation, https://ror.org/014bj2y47 Paul G. Allen Family Foundation, https://ror.org/01degd278 Ammodo Foundation Hefner Foundation Quadrature climate Foundation Bezos earth fund Dutch Research Council, 202.012, SPI.2023.2, 024.004.014

Birds differ in their parent-offspring interactions, and these differences may be caused by environmental variation. When food is plentiful, chicks that are begging more are fed more. When food is scarce, parents instead feed larger offspring. This change could be due to offspring adjusting their behavior, or to confounding factors not directly related to current food availability, such as brood size. Alternatively, it could be due to parents responding to signals differently based on their experience of food availability in the recent past. We tested these competing explanations experimentally by manipulating food availability in wild great tits, Parus major. We then standardized food availability, and manipulated offspring size and behavior by creating mixed cross-fostered broods just before filming. This isolated the effect of parental strategies while holding food availability and offspring begging and size constant across treatments. We found that when parents received supplemented food prior to filming, they were: (1) more likely to preferentially feed chicks that were begging more; and (2) less likely to preferentially feed larger chicks. Chicks, conversely, did not differ in their begging in relation to prior environmental conditions, but instead begged in relation to their immediate feeding history and their nestmates' begging intensity. Overall, our results suggest that parents have more control over food distribution than suggested by scramble competition models, and that parents can flexibly adjust how they respond to offspring signals and cues in response to food availability. Consequently, different signaling systems and parental plasticity are favored depending on environmental conditions. Parents control food distribution in a flexible manner, even when it seems like offspring run the show. New experimental research in great tits reveals widespread plasticity in parental response to offspring signals, and that who and what parents pay attention to depends on environmental quality.

Bacterial death is critical in nutrient recycling. However, the underlying mechanisms that permit macromolecule recycling after bacterial death are largely unknown. We demonstrate that bacteria encode post-mortem protein catabolism via Lon protease released from the dead bacteria. Growth assays reveal that the lysate of Lon protease-null bacteria does not provide a growth benefit to wild type cells. This deficiency is reversed with exogenous recombinant Lon protease, confirming its post-mortem role and is independent of Lon ATPase activity. Biochemistry, growth assays and metabolomics demonstrate that Lon protease facilitates peptide nutrient release, benefitting living cells and acting as a cooperative public good. We also show that the production of Lon protease cannot be explained by a personal benefit to living cells. Although Lon protease can also provide a benefit to living cells under stressful conditions by helping control protein quality, this private benefit does not outweigh the cost under the conditions examined. These results suggest that Lon protease represents a post-mortem adaptation that can potentially be explained by considering the post-mortem indirect benefit to other cells (kin selection). This discovery highlights an unexpected post-mortem biochemistry, reshaping our understanding of nutrient recycling.

Comparative genomics, whereby the genomes of different species are compared, has the potential to address broad and fundamental questions at the intersection of genetics and evolution. However, species, genomes and genes cannot be considered as independent data points within statistical tests. Closely related species tend to be similar because they share genes by common descent, which must be accounted for in analyses. This problem of non-independence may be exacerbated when examining genomes or genes but can be addressed by applying phylogeny-based methods to comparative genomic analyses. Here, we review how controlling for phylogeny can change the conclusions of comparative genomics studies. We address common questions on how to apply these methods and illustrate how they can be used to test causal hypotheses. The combination of rapidly expanding genomic datasets and phylogenetic comparative methods is set to revolutionize the biological insights possible from comparative genomic studies. Controlling for phylogeny is essential in comparative genomics studies, because species, genomes and genes are not independent data points within statistical tests. The authors review the application of phylogeny-based comparative methods to genomic data to control for non-independence and how to test for causal hypotheses.

Experiments have shown that when one plant is attacked by a pathogen or herbivore, this can lead to other plants connected to the same mycorrhizal network up-regulating their defense mechanisms. It has been hypothesized that this represents signaling, with attacked plants producing a signal to warn other plants of impending harm. We examined the evolutionary plausibility of this and other hypotheses theoretically. We found that the evolution of plant signaling about an attack requires restrictive conditions, and so will rarely be evolutionarily stable. The problem is that signaling about an attack provides a benefit to competing neighbors, even if they are kin, and so reduces the relative fitness of signaling plants. Indeed, selection is often more likely to push plant behavior in the opposite direction—with plants signaling dishonestly about an attack that has not occurred, or suppressing a cue that they have been attacked. Instead, we show that there are two viable alternatives that could explain the empirical data: 1) the process of being attacked leads to a cue (information about the attack) which is too costly for the attacked plant to fully suppress; 2) mycorrhizal fungi monitor their host plants, detect when they are attacked, and then the fungi signal this information to warn other plants in their network. Our results suggest the empirical work that would be required to distinguish between these possibilities.

The size-complexity hypothesis is a leading explanation for the evolution of complex life on earth. It predicts that in lineages that have undergone a major transition in organismality, larger numbers of lower-level subunits select for increased division of labour. Current data from multicellular organisms and social insects support a positive correlation between the number of cells and number of cell types and between colony size and the number of castes. However, the implication of these results is unclear, because colony size and number of cells are correlated with other variables which may also influence selection for division of labour, and causality could be in either direction. Here, to resolve this problem, we tested multiple causal hypotheses using data from 794 ant species. We found that larger colony sizes favoured the evolution of increased division of labour, resulting in more worker castes and greater variation in worker size. By contrast, our results did not provide consistent support for alternative hypotheses regarding either queen mating frequency or number of queens per colony explaining variation in division of labour. Overall, our results provide strong support for the size-complexity hypothesis.

Cooperation is prevalent across bacteria, but risks being exploited by non-cooperative cheats. Horizontal gene transfer, particularly via plasmids, has been suggested as a mechanism to stabilize cooperation. A key prediction of this hypothesis is that genes which are more likely to be transferred, such as those on plasmids, should be more likely to code for cooperative traits. Testing this prediction requires identifying all genes for cooperation in bacterial genomes. However, previous studies used a method which likely misses some of these genes for cooperation. To solve this, we used a new genomics tool, SOCfinder, which uses three distinct modules to identify all kinds of genes for cooperation. We compared where these genes were located across 4648 genomes from 146 bacterial species. In contrast to the prediction of the hypothesis, we found no evidence that plasmid genes are more likely to code for cooperative traits. Instead, we found the opposite-that genes for cooperation were more likely to be carried on chromosomes. Overall, the vast majority of genes for cooperation are not located on plasmids, suggesting that the more general mechanism of kin selection is sufficient to explain the prevalence of cooperation across bacteria.

Pangenomes vary across bacteria. Some species have fluid pangenomes, with a high proportion of genes varying between individual genomes. Other species have less fluid pangenomes, with different genomes tending to contain the same genes. Two main hypotheses have been suggested to explain this variation: differences in species’ bacterial lifestyle and effective population size. However, previous studies have not been able to test between these hypotheses because the different features of lifestyle and effective population size are highly correlated with each other, and phylogenetically conserved, making it hard to disentangle their relative importance. We used phylogeny-based analyses, across 126 bacterial species, to tease apart the causal role of different factors. We found that pangenome fluidity was lower in i) host-associated compared with free-living species and ii) host-associated species that are obligately dependent on a host, live inside cells, and are more pathogenic and less motile. In contrast, we found no support for the competing hypothesis that larger effective population sizes lead to more fluid pangenomes. Effective population size appears to correlate with pangenome variation because it is also driven by bacterial lifestyle, rather than because of a causal relationship.

Bacteria exhibit varying niche breadths, with generalists thriving in diverse environments and specialists confined to specific habitats. This variability reflects the adaptability of bacteria to their environment and may influence their speciation and extinction rates. We used phylogenetic causal inference and diversification analysis techniques to investigate the influence of cooperation on bacterial niche breadth evolution and patterns of diversification across 25,785 species. We found: (1) a positive correlation between the proportion of genes for cooperation and niche breadth; (2) a decreased proportion of genes for cooperation promotes niche contraction; and (3) species with a higher proportion of genes for cooperation show increased speciation and extinction rates when their niche breadths are narrower. Our study highlights the role of genes for cooperation in shaping both niche breadth and diversification of bacteria, underscoring their critical function in maintaining the ecological versatility and diversity of bacteria. ### Competing Interest Statement The authors have declared no competing interest.

Cooperation is commonly believed to be favourable in spatially structured environments, as these systems promote genetic relatedness that reduces the likelihood of exploitation by cheaters. Here we show that a Pseudomonas aeruginosa population that exhibited cooperative swarming was invaded by cheaters when subjected to experimental evolution through cycles of range expansion on solid media, but not in well-mixed liquid cultures. Our results suggest that cooperation is disfavoured in a more structured environment, which is the opposite of the prevailing view. We show that spatial expansion of the population prolongs cooperative swarming, which was vulnerable to cheating. Our findings reveal a mechanism by which spatial structures can suppress cooperation through modulation of the quantitative traits of cooperation, a process that leads to population divergence towards distinct colonization strategies.

The growth and success of many bacteria appear to rely on a stunning range of cooperative behaviours. But what is cooperation and how is it studied?

Hamilton’s rule provides the cornerstone for our understanding of the evolution of all forms of social behavior, from altruism to spite, across all organisms, from viruses to humans. In contrast to the standard prediction from Hamilton’s rule, recent studies have suggested that altruistic helping can be favored even if it does not benefit relatives, as long as it decreases the environmentally induced variance of their reproductive success (“altruistic bet-hedging”). However, previous predictions both rely on an approximation and focus on variance-reducing helping behaviors. We derived a version of Hamilton’s rule that fully captures environmental variability. This shows that decreasing (or increasing) the variance in the absolute reproductive success of relatives does not have a consistent effect—it can either favor or disfavor the evolution of helping. We then empirically quantified the effect of helping on the variance in reproductive success across 15 species of cooperatively breeding birds. We found that a) helping did not consistently decrease the variance of reproductive success and often increased it, and b) the mean benefits of helping across environments consistently outweighed other variability components of reproductive success. Altogether, our theoretical and empirical results suggest that the effects of helping on the variability components of reproductive success have not played a consistent or strong role in favoring helping.