ABSTRACT
The recent availability of extensive genetic and genomic data within and between named species has made it abundantly clear that the biological species concept does not apply in plants, animals, or microbes. There is interbreeding (and other forms of horizontal transfer) going on at multiple hierarchical levels, and contrariwise, there is lack of interbreeding and emerging phylogenetic structure at many levels as well. There is no particular level where rampant interbreeding abruptly transitions to no interbreeding, despite many diagrams in textbooks and papers illustrating just such a magical distinction in order to justify treating the species level as unique. The situation is richer and more interesting than Mayr or Hutchinson ever imagined – there are nested entities smaller and larger than traditional species that have important roles in ecology and evolution. Systematics needs to be completely rankless, and discover and name clades down to the finest scale that the data allow (the SNaRC – smallest named and registered clade in the sense of Mishler and Wilkins (2018). Studies of evolutionary and ecological processes need to take into account clades at multiple, nested levels, not just the level formerly known as species. A revolution in many areas of study, including diversification (formerly known as ‘speciation’), niche evolution, biogeography, coevolution, and conservation, will follow once a rigid focus on the species level is replaced by a multi-level view.
