Talking about a re-evolution: blind alleys in ostracod phylogeny

Abstract. The description of a new genus of bairdiid ostracod, on the basis of possession of eye tubercles (previously unknown in the group), has been criticized on the grounds that such ocular structures are not a valid criterion for establishing a genus, and for the authors’ failure to discuss adequately the phylogenetic implications of a sighted taxon apparently arising from blind ancestors. The latter point particularly refers to Dollo’s Law, according to which complex evolutionary traits, once lost, cannot be regained. In response to these criticisms, criteria for sightedness in ostracods are reconsidered, leading to the conclusion that ostracods without eye tubercles are not necessarily blind. A brief review of recent literature (covering vertebrates, invertebrates and plants) demonstrates that Dollo’s Law is not inviolable; on the contrary, there appear to be several well-documented examples of the reactivation of dormant genes, allowing the reappearance of ‘lost’ characters, in some cases after several million years. The implications for the occurrence of rare males in ancient asexual lineages are considered, and it is concluded that the loss of traits such as sightedness and sexual reproduction might not be irreversible.