Tropical origin, global diversification and dispersal in the pond damselflies (Coenagrionoidea) revealed by a new molecular phylogeny

The processes responsible for the formation of Earth’s most conspicuous
diversity pattern, the latitudinal diversity gradient (LDG), remain
unexplored for many clades in the Tree of Life. Here, we present a
densely-sampled and dated molecular phylogeny for the most speciose clade
of damselflies worldwide (Odonata: Coenagrionoidea), and investigate the
role of time, macroevolutionary processes and biome-shift dynamics in
shaping the LDG in this ancient insect superfamily. We used process-based
biogeographic models to jointly infer ancestral ranges and speciation
times, and to characterise within-biome dispersal and biome-shift dynamics
across the cosmopolitan distribution of Coenagrionoidea. We also
investigated temporal and biome-dependent variation in diversification
rates. Our results uncover a tropical origin of pond damselflies and
featherlegs ~ 105 Ma, while highligthing uncertainty of ancestral ranges
within the tropics in deep time. Even though diversification rates have
declined since the origin of this clade, global climate change and
biome-shifts have slowly increased diversity in warm- and cold-temperate
areas, where lineage turnover rates have been relatively higher. This
study underscores the importance of biogeographic origin and time to
diversify as important drivers of the LDG in pond damselflies and their
relatives, while diversification dynamics have instead resulted in the
formation of ephemeral species in temperate regions. Biome-shifts,
although limited by tropical niche conservatism, have been the main factor
reducing the steepness of the LDG in the last 30 Myr. With ongoing climate
change and increasing northward range expansions of many damselfly taxa,
the LDG may become less pronounced. Our results support recent calls to
unify biogeographic and macroevolutionary approaches to increase our
understanding of how latitudinal diversity gradients are formed and why
they vary across time and among taxa.