Genetic Variability, Divergence and Speciation in Trees of Periodically Flooded Forests of the Amazon: A Case Study of Himatanthus sucuuba (Spruce) Woodson

Contrary to the theory that geographic isolation is the main trigger for speciation, recent studies emphasize the continuous adaptation to different habitats as the driving force initiating diversification. In this way, adaptive divergence in response to contrasting selective pressures of populations of the same species in geographically or ecologically continuous environments may occur if long-lasting barriers are induced by biotic or abiotic events. Plants of the Amazon floodplains withstand annual periods of flooding which can last seven months. To verify if the regularity of the “flood pulse” of the Amazon River can induce speciation, we investigated populations of Himatanthus sucuuba (Apocynaceae) colonizing whitewater floodplains (varzea) and non-flooded uplands (terra-firme) in the region. In independent experiments, we simulated flooding conditions, to evaluate the germination and growth of seedlings from both environments. The two populations showed significant differences for most parameters evaluated. Thus, flooding is apparently a feature strong enough to promote phenotypic differentiation among varzea and terra firme populations. Indeed, molecular analysis showed genetic difference between populations, revealing that different ecological pressures may promote adaptive changes in Amazonian plants to insure establishment in different environments.