Aim Abiotic environmental conditions shape ecological and evolutionary processes, yet quantifying their influence on organisms remains challenging due to variation among metrics and their intercorrelations. This study evaluates the utility of temporal environmental predictability measures and assesses their explanatory power in phylogenetic comparative analyses. Innovation We systematically compare widely used metrics of predictability and explore their correlations with environmental means and variances in a global meteorological dataset. Using cooperative breeding birds as a case study, we assess the impact of including predictability metrics in phylogenetic comparative analyses. We demonstrate the consequences of choosing specific metrics and the trade-offs between increased data inclusion and model interpretability. Main conclusions Predictability metrics, though intuitively meaningful, have been conceptualised and quantified with diverse approaches. We found that different measures of predictability can exhibit contrasting global patterns and strong correlations with other environmental quantities. Therefore, our findings caution against overloading statistical analyses with correlated predictors, highlighting the need for a thoughtful selection of environmental metrics to avoid spurious interpretations in ecological and evolutionary studies.

Obligately multicellular organisms, where cells can only reproduce as part of the group, have evolved multiple times across the tree of life. Obligate multicellularity has only evolved when clonal groups form by cell division, rather than by cells aggregating, as clonality prevents internal conflict. Yet obligately multicellular organisms still vary greatly in ‘multicellular complexity’ (the number of cells and cell types): some comprise few cells and cell-types, and others billions of cells and thousands of types. Here, we test if variation in multicellular complexity is explained by conflict-suppressing mechanisms, namely a single cell bottleneck at the start of development, and a strict separation of germline and somatic cells. Applying a phylogenetic comparative analysis to the life-cycles 138 lineages of plants, animals, fungi and algae, we show that an early segregation of the germline stem-cell lineage is key to the evolution of more cell types. In contrast, the presence of a strict single cell bottleneck was not related to either the number of cells or cell types but was associated with early germline segregation. Our results suggest that segregating the germline earlier in development enabled greater evolutionary innovation, possibly through conflict suppression or via greater developmental flexibility.

Organisms inhabiting extreme thermal environments, such as desert birds, have evolved spectacular adaptations to thermoregulate during hot and cold conditions. However, our knowledge of selection for thermoregulation and the potential for evolutionary responses is limited, particularly for large organisms experiencing extreme temperature fluctuations. Here we use thermal imaging to quantify selection and genetic variation in thermoregulation in ostriches (Struthio camelus), the world’s largest bird species that is experiencing increasingly volatile temperatures. We found that females that are better at regulating their head temperatures (‘thermoregulatory capacity’) had higher egg-laying rates under hotter conditions. Thermoregulatory capacity was both heritable and showed signatures of local adaptation: females originating from more unpredictable climates were better at regulating their head temperatures in response to temperature fluctuations. Together these results reveal that past and present evolutionary processes have shaped genetic variation in thermoregulatory capacity, which appears to protect critical organs, such as the brain, from extreme temperatures during reproduction.

Sex chromosomes have evolved repeatedly across the tree of life and often exhibit extreme size dimorphism due to genetic degeneration of the sex-limited chromosome (e.g. the W chromosome of some birds and Y chromosome of mammals). However, in some lineages, ancient sex-limited chromosomes have escaped degeneration. Here, we study the evolutionary maintenance of sex chromosomes in the ostrich (Struthio camelus), where the W remains 65% the size of the Z chromosome, despite being more than 100 million years old. Using genome-wide resequencing data, we show that the population-scaled recombination rate of the pseudoautosomal region (PAR) is higher than similar-sized autosomes and is correlated with pedigree-based recombination rate in the heterogametic females, but not homogametic males. Genetic variation within the sex-linked region (SLR) (π = 0.001) was significantly lower than in the PAR, consistent with recombination cessation. Conversely, genetic variation across the PAR (π = 0.0016) was similar to that of autosomes and dependent on local recombination rates, GC content, and to a lesser extent, gene density. In particular, the region close to the SLR was as genetically diverse as autosomes, likely due to high recombination rates around the PAR boundary restricting genetic linkage with the SLR to only ~50Kb. The potential for alleles with antagonistic fitness effects in males and females to drive chromosome degeneration is therefore limited. While some regions of the PAR had divergent male-female allele frequencies, suggestive of sexually antagonistic alleles, coalescent simulations showed this was broadly consistent with neutral genetic processes. Our results indicate that the degeneration of the large and ancient sex chromosomes of the ostrich may have been slowed by high recombination in the female PAR, reducing the scope for the accumulation of sexually antagonistic variation to generate selection for recombination cessation.

Group-living species show a diversity of social organisation, from simple mated pairs to complex communities of inter-dependent individuals performing specialized tasks. The advantages of living in cooperative groups are well understood, but why some species breed in small aggregations while others evolve large, complex groups with clearly divided roles is unclear. We address this problem by reconstructing the evolutionary pathways to cooperative breeding across 4730 bird species. We show that differences in the way groups form at the origin of cooperative breeding predicts the level of group complexity that emerges. Groups that originate through the retention of offspring have a clear reproductive divide with distinct breeder and helper roles. This was associated with reproductive specialization, where breeders invest more in fecundity and less in care. In contrast, groups formed through the aggregation of unrelated adults are smaller and lack specialization. These results help explain why some species have not transitioned beyond simple groups, while others have taken the path to increased group complexity.

In birds that breed cooperatively in family groups, adult offspring often delay dispersal to assist the breeding pair in raising their young. Kin selection is thought to play an important role in the evolution of this breeding system. However, evidence supporting the underlying assumption that helpers increase the reproductive success of breeders is inconsistent. In 10/19 species where the effect of helpers on breeder reproductive success has been estimated while controlling for the effects of breeder and territory quality, no benefits of help were detected. Here we use phylogenetic meta-analysis to show that the inconsistent evidence for helper benefits across species is explained by study design. After accounting for low sample sizes and the different study designs used to control for breeder and territory quality, we found that helpers consistently enhanced the reproductive success of breeders. Therefore, the assumption that helpers increase breeder reproductive success is supported by evidence across cooperatively breeding birds.