SummaryRecent reproductive biology studies reveal high levels of self‐incompatibility in species diverse tropical communities. Many tropical trees are animal pollinated, pollen transfer patterns closely following the energetics of foraging. The energy output (in the form of nectar, pollen and other food bodies) of a single massively flowering tropical tree is far in excess of the demands of a single pollinating individual. This being so, a large proportion of the pollen transfers in tropical trees are geitonogamous and genetically equivalent to self‐pollination. It is here argued that the tendency for high levels of geitonogamy, which presumably increase with decreasing population density or increasing species diversity has been the major factor in the spread and perhaps initial evolution of pre‐zygotic self‐incompatibility mechanisms in the angiosperms.