A schematic representation of taxa discussed in the text. Organisms in the four eukaryote super-groups (Unikonts, Excavates, Chromalveolates and Plantae) are classified according to the “deep” eukaryote tree proposed by Keeling et al. (2005). For each taxon the proposed non-adaptive and/or adaptive explanations for POD are provided in parentheses (see key). The cases of convergent evolution identified in unicellular eukaryotes (see text) are indicated by bold arrows. LUCA = Last Universal Common Ancestor.