In the last few million years, tropical Africa has experienced pronounced climatic
shifts with progressive aridification. Such changes must have had a great impact on
freshwater biota, such as Odonata. With about forty species, Trithemis dominates dragonfly
communities across Africa, from rain-pools to streams, deserts to rainforests, and
lowlands to highlands. Red-bodied species tend to favor exposed, standing and often
temporary waters, have strong dispersal capacities, and some of the largest geographic
ranges in the genus. Those in cooler habitats, like forest streams, are generally
dark-bodied and more sedentary. We combined molecular analyses of ND1, 16S, and ITS
(ITSI, 5.8S, and ITSII) with morphological, ecological, and geographical data for
81% of known Trithemis species, including three Asian and two Madagascan endemics.
Using molecular clock analyses, the genus's origin was estimated 6-9Mya, with multiple
lineages arising suddenly around 4Mya. Open stagnant habitats were inferred to be
ancestral and the rise of Trithemis may have coincided with savannah-expansion in
the late Miocene. The adaptation of red species to more ephemeral conditions leads
to large ranges and limited radiation within those lineages. By contrast, three clades
of dark species radiated in the Plio-Pleistocene, each within distinct ecological
confines: (1) lowland streams, (2) highland streams, and (3) swampy habitats on alternating
sides of the Congo-Zambezi watershed divide; together giving rise to the majority
of species diversity in the genus. During Trithemis evolution, multiple shifts from
open to more forested habitats and from standing to running waters occurred. Allopatry
by habitat fragmentation may be the dominant force in speciation, but possibly genetic
divergence across habitat gradients was also involved. The study demonstrates the
importance of combining ecological and phylogenetic data to understand the origin
of biological diversity under great environmental change.
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