Habitat Opening Fostered Diversity: Impact of Dispersal and
Habitat-shifts in the Diversification of a Speciose Afrotropical Insect
Group
Abstract
The opening of habitats associated with the emergence of C4 grasslands
during the Neogene has had a massive influence on the evolution of plant
and animal communities. Strikingly, the impacts of grassland expansion
on species diversification in Africa, where the largest surface of
grasslands and savannas in the world is located, are not well
understood. To explore the impact of habitat opening, we investigate the
evolution of noctuid stemborers, a group of moths mostly associated with
open habitats, and whose diversity is centered in the Afrotropics. We
generate a dated molecular phylogeny for ca. 80% of the known stemborer
species and assess the role of habitat opening on the evolutionary
trajectory of the group through a combination of parametric historical
biogeography, ancestral character state estimation, life history traits
and habitat-dependent diversification analyses. Our results support an
origin of stemborers in Southern and East Africa ca. 20 million years
ago (Ma), with range expansions linked to the increased availability of
open habitats to act as dispersal corridors, and closed habitats acting
as potent barriers to dispersal. Early specialization on open habitats
was maintained over time, with shifts towards closed habitats being rare
and invariably unidirectional. Analyses of life history traits showed
that habitat changes involved specific features likely associated with
grassland adaptations, such as variations in larval behavior and color.
We compare these findings to those previously inferred for an
Afrotropical butterfly group that diversified roughly in parallel with
the stemborers but distributed predominantly in closed habitats.
Remarkably, these two groups show nearly opposite responses in relation
to habitat specialization, whether in terms of biogeographical patterns,
or in terms of rates of transition between open and closed habitats. We
conclude that habitat opening played a major role in the diversification
of Afrotropical lineages through dispersal and adaptation linked to
habitat shifts.