Determining the Drivers of Anti-Tropical Distributions Across the Fish Tree of Life

Abstract

Anti-tropical distributions are those where populations of a single species, or multiple closely related taxa, are distributed outside of, and on opposing sides of, the tropics. These latitudinally disjunct distributions have been noted for over a century. Despite this long history of interest, little has been concluded regarding the actual mechanisms that drive this pattern, with several prominent hypotheses competing with one another in the literature. Here I review the proposed drivers of anti-tropicality, and subsequently test them using fishes with a variety of life history and taxonomic differences. This includes (1) a temperately restricted family with anti-tropical distributions – Cheilodactylidae, (2) a tropical reef fish family with a single temperate anti-tropical genus – Prionurus, and (3) a variety of fishes from across the fish tree of life that have populations split by the tropics. Using complete taxonomic sampling, and phylogenomic approaches coupled with fossil calibration points, I find evidence for recent equatorial divergence events in the Pleistocene and Pliocene, as well as divergence events dating to the Miocene for both Cheilodactylidae and Prionurus. Furthermore, taxonomic issues were detected, and explored within both of these groups. To disentangle the multiple hypotheses that can explain recent transitions, I used ecological niche models coupled with extant distributional data for a variety of species across the fish tree of life that exhibit intra-specific anti-tropicality. These data reveal distinct support for both glacial dispersal, and biotic exclusion from the tropics. These results are then interpreted in a comprehensive framework to determine what drives anti-tropical distributions in marine systems. Overall, multiple mechanisms seem responsible that act in concert over time to produce these distributions. Certain equatorial divergence events are recovered in time periods currently not associated with any anti-tropical hypotheses. It seems likely that stochastic crossing events may be important in the initial colonization of a new hemisphere