Out of the temperate zone: a phylogenomic test of the biogeographical conservatism hypothesis in a contrarian clade of ants (Hymenoptera: Formicidae: Stenammini)
Branstetter, Michael G. et al. (2022), Out of the temperate zone: a phylogenomic test of the biogeographical conservatism hypothesis in a contrarian clade of ants (Hymenoptera: Formicidae: Stenammini), Dryad, Dataset, https://doi.org/10.5061/dryad.2280gb5tv
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 the ant tribe Stenammini as a 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).
Methods: We examined the shape of the LDG in Stenammini by plotting latitudinal midpoints for all described, extant species. We then inferred a genome-scale phylogeny and used the tree to estimate divergence dates and ancestral areas. We also examined diversification rate heterogeneity across the tree and tested for a correlation between rate and latitude.
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 the combined effects of center-of-origin, time-for-speciation, and niche conservatism, rather than latitudinal differences in diversification rate.
National Science Foundation, Award: DEB-1354739
National Science Foundation, Award: DEB-1932405