XVI International Botanical Congess

Reproductive isolation is the sine qua non of macroevolution, and must normally originate within species and intensify through time and space. Partial crossing barriers do exist within angiosperm species and often are distributed seemingly graphic or taxonomic separation of individuals. At the shortest scale, partial barriers can occur within local metapopulations. Because outbreeding refers to a continuum of genetic dissimlarity (as inbreeding refers to a continuum of identity by descent), these barriers are also termed outbreeding depression. Mechanisms of outbreeding depression range from purely genetic (nonadditive gene action) to purely ecological (disruption of local adaptation), but have been characterized in few systems. In future it will be useful to study barriers and their mechanisms on a hierarchy of scales within single species, to shed light on the evolution of reproductive isolation and its relation (or non-relation) to morphological diversification of angiosperms.