Phenotypic differences are not explained by pre-zygotic reproductive barriers in sympatric varieties of the Humiria balsamifera complex (Humiriaceae)

Biosystematic studies addressing mechanisms of speciation in widespread, phenotypically variable Neotropical plant taxa are scarce. In Viruá National Park, Roraima State, Brazil, we demonstrate that nine phenotypic variables measured from sympatric populations of two varieties within the Humiria balsamifera complex (Humiriaceae) are sharply discontinuous for both vegetative and floral characters. However, these differences were not explained by either phenological separation in anthesis, suppressed reproductive success in experimental inter-varietal crosses, or pollinator preferences, suggesting that pre-zygotic reproductive mechanisms do not contribute to the demonstrable phenotypic differences. Post-zygotic barriers such as reduced germination success in crosses, balancing selection at fine scales, or historical factors such as recent contact among formally isolated populations may account for the maintenance of phenotypic variation among these sympatric varieties. Future studies relating genetic structure with phenotypic variation may shed light on the mechanisms which promote and maintain large Neotropical species complexes such as in this case.