Highlighting convergent evolution in morphological traits in response to climatic gradient in African tropical tree species: the case of genus Guibourtia Benn

Adaptive evolution is a major driver of organism diversification but the
links between phenotypic traits and environmental niche remains little
documented in tropical trees. Moreover, trait-niche relationships are
complex because a correlation between the traits and environmental niches
displayed by a sample of species may result from (1) convergent evolution
if different environmental conditions have selected different sets of
traits, and/or (2) phylogenetic inertia if niche and morphological
differences between species are simply function of their phylogenetic
divergence, in which case the trait-niche correlation does not imply any
direct causal link. Here, we aim to assess the respective roles of
phylogenetic inertia and convergent evolution in shaping the differences
of botanical traits and environmental niches among congeneric African tree
species that evolved in different biomes. This issue was addressed with
the tree genus Guibourtia Benn. (Leguminosae, Detarioideae) which contains
13 African species occupying various forest habitat types, from rain
forest to dry woodlands, with different climate and soil conditions. To
this end, we combined morphological data with ecological niche modelling
and used a highly resolved plastid phylogeny of the 13 African Guibourtia
species. First, we demonstrated phylogenetic signals in both morphological
traits (Mantel test between phylogenetic and morphological distances
between species: r=0.24, p=0.031) and environmental niches (Mantel test
between phylogenetic and niche distances between species: r=0.23,
p=0.025). Second, we found a significant correlation between morphology
and niche, at least between some of their respective dimensions (Mantel’s
r=0.32, p=0.013), even after accounting for phylogenetic inertia
(Phylogenetic Independent Contrast: r=0.69, p=0.018). This correlation
occurred between some leaflet and flower traits and solar radiation,
relative humidity, precipitations and temperature range. Our results
demonstrate the convergent evolution of some morphological traits in
response to climatic factors in congeneric tree species and highlight the
action of selective forces, along with neutral ones, in shaping the
divergence between congeneric tropical plants.