Autopolyploidy in angiosperms: have we grossly underestimated the number of species?

Many species comprise multiple cytotypes that represent autopolyploids, or presumed autopolyploids, of the basic diploid cytotype. However, rarely has an autopolyploid been formally named and considered to represent a species distinct from its diploid progenitor (Zea diploperennis and Z. perennis represent a rare example). The major reasons why autopolyploids have not been named as distinct species are: (1) tradition of including multiple cytotypes in a single named species; and (2) tradition and convenience of adhering to a broad morphology-based taxonomic (or phenetic) species concept. As a result, plant biologists have underrepresented the distinct biological entities that actually exist in nature. Although it may seem "practical" to include morphologically highly similar cytotypes in one species, this practice obscures insights into evolution and speciation and hinders conservation. However, we do not suggest that all cytotypes should be named; each case must be carefully considered. A number of species comprising multiple cytotypes have been thoroughly investigated. Drawing on the literature, as well as our own experience with several autopolyploids (Tolmiea menziesii, Galax urceolota, Chamerion angustifolium, Heuchera grossulariifolia, Vaccinium corymbosum), we reassess the traditional view of plant autopolyploids as mere cytotypes. When considered carefully, many "unnamed" autopolyploids fulfill the requirements of multiple species concepts, including the biological, taxonomic, diagnosability, apomorphic, and evolutionary species concepts. Compared to the diploid parent, the autopolyploids noted above possess distinct geographic ranges, can be distinguished morphologically, and are largely reproductively isolated (via a diversity of mechanisms including reproductive and ecological isolation). These five autopolyploids (and probably many others) represent distinct evolutionary lineages; we therefore suggest that they be considered distinct species and also provide a system for naming them.