Résumé
The long generation time and large effective size of widespread forest
tree species can result in slow evolutionary rate and incomplete lineage
sorting, complicating species delimitation. We addressed this issue with
the African timber tree genus Milicia that comprises two morphologically
similar and often confounded species: M. excelsa, widespread from West to
East Africa, and M. regia, endemic to West Africa. We combined information
from nuclear microsatellites (nSSRs), nuclear and plastid DNA sequences,
and morphological systematics to identify significant evolutionary units
and infer their evolutionary and biogeographical history. We detected five
geographically coherent genetic clusters using nSSRs and three levels of
genetic differentiation. First, one West African cluster matched perfectly
with the morphospecies M. regia, which formed a monophyletic clade at both
DNA sequences. Second, a West African M. excelsa cluster formed a
monophyletic group at plastid DNA and was more related to M. regia than to
Central African M. excelsa, but shared many haplotypes with the latter at
nuclear DNA. Third, three Central African clusters appeared little
differentiated and shared most of their haplotypes. Although gene tree
paraphyly could suggest a single species in Milicia following the
Phylogenetic Species Concept, the existence of mutual haplotypic
exclusivity and non-admixed genetic clusters in the contact area of the
two taxa indicate strong reproductive isolation, and thus, two species
following the Biological Species Concept. Molecular dating of the first
divergence events showed that speciation in Milicia is ancient (Tertiary),
indicating that long-living tree taxa exhibiting genetic speciation may
remain similar morphologically.