Reproductive isolation of sympatric forms of the understorey palm Geonoma macrostachys in western Amazonia

The evolution of a mechanism for attaining reproductive isolation between two diverging populations is a key step in the speciation process. We studied phenotypic variation, genetic differentiation, spatial distribution and reproductive ecology in two sympatric forms of the understorey palm Geonoma macrostachys in lowland tropical rainforest in Amazonian Ecuador. The two forms were morphologically and genetically distinct and differed in habitat preference, with one form being confined to moist flood plain habitats and the other occurring in terra firme and flood plain. Pollen transfer between the two forms was limited, but not totally prevented, by differences in flowering time, with flowers of the small form opening 2–3 h earlier than those of the large form. Differences in floral scent probably reduce the number of shared pollinators. A crossing experiment showed that pistillate flowers of either form produced fruit with pollen from the other form. We conclude that flowering biology may be a key factor in causing reproductive isolation between these closely related sympatric taxa.