The latitudinal diversity gradient in New World swallowtail butterflies is caused by contrasting patterns of out-of- and into-the-tropics dispersal

Aim: The aim was to determine processes driving the latitudinal diversity gradient (LDG) in New World swallowtail butterfly diversity. We tested three mechanisms commonly invoked to explain the LDG: ecological opportunity, evolutionary rates and biogeographical history. Location: New World and Eurasia. Time period: Oligocene–Present. Major taxa studied: New World swallowtail butterfly clade (Papilio: Agehana, Alexanoria, Chilasa, Heraclides and Pterourus). Methods: We integrated data from the most complete current phylogeny of this clade with geographical distributions of each species inferred from ecological niche models (ENMs). We tested for correlation between breadth of available abiotic ecological niche space, latitude and differential rates of diversification between tropical and non-tropical lineages. The clade's history of climatic and geographical occupancy was also reconstructed using both continuous ancestral character reconstructions and biogeographical history inferred under a dispersal–extinction–cladogenesis model. We considered both latitudinal and climatic definitions of tropicality in our reconstructions. Results: There was no strong support for ecological opportunity or macroevolutionary processes as latitudinal diversity gradient drivers. Instead, we recovered discordant patterns in phylogenetic reconstructions of latitudinal geographical range and suitable abiotic climate conditions. Heraclides are likely to have originated and diversified in climatically and latitudinally tropical environments before some lineages dispersed to temperate habitats. The Alexanoria + Chilasa + Pterourus clade is likely to have originated in climatically and latitudinally temperate habitat before dispersing and diversifying; some lineages are likely to have dispersed into the latitudinal tropics via highland temperate-analogue environments. Main conclusions: The LDG in New World swallowtails results from complex interactions between ecological niche evolution and biogeographical history; both out-of-the-tropics and into-the-tropics processes have contributed to the LDG. Our results present an example where temperate zones appear to be a source, instead of a sink, for biodiversity. Our results emphasize the need to consider biogeographical history not only from the perspective of shifts in geographical space, but also in terms of constraints enforced by ecological niche conservatism.