Morphostasis in alveolate evolution

Three closely related groups of unicellular organisms (ciliates, dinoflagellates and apicomplexans) have adopted what might be the most dissimilar modes of eukaryotic life on Earth: predation, phototrophy and intracellular parasitism. Morphological and molecular evidence indicate that these groups share a common ancestor to the exclusion of all other eukaryotes and collectively form the Alveolata, one of the largest and most important assemblages of eukaryotic microorganisms recognized today. In spite of this phylogenetic framework, the differences among the groups are profound, which has given rise to considerable speculation about the earliest stages of alveolate evolution. However, we argue that a new understanding of morphostasis in several organisms is now throwing light onto the intervening history spanning the major alveolate groups. Insights into the phylogenetic positions of these morphostatic lineages reveal how documenting the distribution of character states in extant organisms, in the absence of fossils, can provide compelling inferences about intermediate steps in early macroevolutionary transitions.