Out of the temperate zone: A phylogenomic test of the biogeographical conservatism hypothesis in a contrarian clade of ants

Abstract

Aim The standard latitudinal diversity gradient (LDG), in which species richness decreases from equator to pole, is a pervasive pattern observed in most organisms. Some lineages, however, exhibit inverse LDGs. Seemingly problematic, documenting and studying contrarian groups can advance understanding of LDGs generally. Here, we identify one such contrarian clade and use a historical approach to evaluate alternative hypotheses that might explain the group's atypical diversity pattern. We focus on the biogeographical conservatism hypothesis (BCH) and the diversification rate hypothesis (DRH). Location Global. Taxon Ants (Hymenoptera: Formicidae: Stenammini). Methods We examined the shape of the LDG in Stenammini by plotting latitudinal midpoints for all extant, described species. We inferred a robust genome‐scale phylogeny using UCE data. We estimated divergence dates using beast2 and tested several biogeographical models in BioGeoBEARS . To examine diversification rates and test for a correlation between rate and latitude, we used the programs BAMM and STRAPP, respectively. Results Stenammini has a skewed inverse LDG with a richness peak in the northern temperate zone. Phylogenomic analyses revealed five major clades and several instances of non‐monophyly among genera ( Goniomma , Aphaenogaster ). Stenammini and all its major lineages arose in the northern temperate zone. The tribe originated ~51 Ma during a climatic optimum and then diversified and dispersed southward as global climate cooled. Stenammini invaded the tropics at least seven times, but these events occurred more recently and were not linked with increased diversification. There is evidence for a diversification rate increase in Holarctic Aphaenogaster + Messor , but we found no significant correlation between latitude and diversification rate generally. Main Conclusions Our results largely support the BCH as an explanation for the inverse latitudinal gradient in Stenammini. The clade originated in the Holarctic and likely became more diverse there due to center‐of‐origin, time‐for‐speciation and niche conservatism effects, rather than latitudinal differences in diversification rate.

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