The fossil record allows a unique glimpse into the evolutionary history of organisms living on Earth today. We discovered a specimen of the stem group ant †Gerontoformica gracilis (Barden and Grimaldi, 2014) in Kachin amber with near-complete preservation of internal head structures, which we document employing µ-computed-tomography-based 3D reconstructions. We compare †Gerontoformica to four outgroup taxa and four extant ant species, employing parsimony and Bayesian ancestral state reconstruction to identify morphological differences and similarities between stem and crown ants and thus improve our understanding of ant evolution through the lens of head anatomy. Of 149 morphological characters, 87 are new in this study, and almost all applicable to the fossil. †Gerontoformica gracilis shares shortened dorsal tentorial arms, basally angled pedicels, and the pharyngeal gland as apomorphies with other total clade Formicidae. Retained plesiomorphies include mandible shape and features of the prepharynx. Implications of the reconstructed transitions especially for the ant groundplan are critically discussed based on our restricted taxon sampling, emphasizing the crucial information derived from internal anatomy which is applied to deep time for the first time. Based on the falcate mandible in †Gerontoformica and other Aculeata, we present hypotheses for how the shovel-shaped mandibles in crown Formicidae could have evolved. Our results support the notion of †Gerontoformica as ‘generalized’ above-ground predator missing crucial novelties of crown ants which may have helped the latter survive the end-Cretaceous extinction. Our study is an important step for anatomical research on Cretaceous insects and a glimpse into the early evolution of ant heads.

Graphical Abstract

Biogeographic patterns in the Southern Hemisphere have largely been attributed to vicariant processes, but recent studies have challenged some of the classic examples of this paradigm. The parasitoid wasp subfamily Labeninae has been hypothesized to have a Gondwanan origin, but the lack of divergence dating analysis and the discovery of a putative labenine fossil in Europe pose a challenge to that idea. Here we used a combination of phylogenomics, divergence dating and event-based biogeographical inference to test whether Gondwanan vicariance may explain the distribution patterns of Labeninae. Data from genomic ultraconserved elements were used to infer the phylogeny of Labeninae with 54 species from 9 genera and a broad selection of 99 outgroup taxa. Total-evidence divergence dating places the origin of Labeninae at around 146 mya, which is consistent with a Gondwanan origin but predates the full separation of Africa and South America. The results suggest a path for biotic exchange between South America and Australia potentially through Antarctica, until at least 49 million years ago. Total-evidence analysis places the fossil Trigonator macrocheirus Spasojevic et al. firmly inside crown-group Labeninae, suggesting that labenine distribution range at some point during the Eocene surpassed the boundaries of Gondwanaland. Biogeographic inference also indicates that North American groups represent more recent range expansions that nonetheless occurred before the formation of the Isthmus of Panama land bridge. These conclusions point to a more complex scenario than previously expected for Labeninae biogeography.

Traits that independently evolve many times are important for testing hypotheses about correlated evolution and understanding the forces shaping biodiversity. However, population genetics processes can cause hemiplasies (traits determined by genes whose topologies do not match the species tree), leading to a false impression of convergence (homoplasy) and potentially misleading inferences of correlated evolution. Discerning between homoplasies and hemiplasies can be important in cases of rapid radiations and clades with many gene tree incongruences. Here, focusing on two-clawed spiders (Dionycha) and close relatives, we evaluate if the observed distribution of characters related to a web-less lifestyle could be better explained as synapomorphies, homoplasies, or hemiplasies. We find that, although there are several convergences, hemiplasies are also sometimes probable. We discuss how these hemiplasies could affect inferences about correlation and causal relationship of traits. Understanding when and where in the tree of life hemiplasy could have happened is important, preventing false inference of convergent evolution. Furthermore, this understanding can provide alternative hypotheses that can be tested with independent data. Using traits related to the climbing ability of spiders we show that, when hemiplasy is unlikely, adequate model testing can be used to better understand correlated evolution, and propose hypotheses to be tested using controlled behavioral and mechanical experiments.

Graphical Abstract

Broadly distributed species experience divergent abiotic conditions across their ranges that may drive local adaptation. Montane systems where populations are distributed across both latitudinal and elevational gradients are especially likely to produce local adaptation due to spatial variation in multiple abiotic factors, including temperature, oxygen availability, and air density. We use whole-genome resequencing to evaluate the landscape genomics of Bombus vancouverensis Cresson (Hymenoptera: Apidae), a common montane bumble bee that is distributed throughout the western part of North America. Combined statistical approaches revealed several large windows of outlier SNPs with unusual levels of differentiation across the region and indicated that isothermality and elevation were the environmental features most strongly associated with these variants. Genes found within these regions had diverse biological functions, but included neuromuscular function, ion homeostasis, oxidative stress, and hypoxia that could be associated with tolerance of temperature, desiccation, or high elevation conditions.The whole-genome sequencing approach revealed outliers occurred in genome regions with elevated linkage disequilibrium, elevated mean F_ST, and low intrapopulation nucleotide diversity. Other kinds of structural variations were not widely associated with environmental predictors but did broadly match geographic separation. Results are consistent with other studies suggesting that regions of low recombination may harbor adaptive variation in bumble bees within as well as between species and refine our understanding of candidate genes that could be further investigated as possible targets of selection across the B. vancouverensis range.