Abstract

Recent work in viral genomics has shown that bacteriophages exhibit a high degree of mosaicism, which is most likely due to a long history of prolific horizontal gene transfer (HGT). Given these findings, we argue that each of the most plausible attempts to properly classify bacteriophages into distinct species fail. Mayr's biological species concept fails because there is no useful viral analog to sexual reproduction. Phenetic species concepts fail because they obscure the mosaicism and the rich reticulated viral histories. Phylogenetic species concepts, even when extended to take into account reticulation, fail because there is no non-arbitrary distinction between recombination events that create a new viral species and those that do not. There is good reason to think that bacteriophages, arguably the Earth's most abundant biological agent, evolve without forming species.1. Introduction2. The Biology of Viruses2.1. Bacteriophage life cycles2.2. Mechanisms of HGT3. The Species Problem and Species Concepts3.1. Phenetic species concepts3.2. The biological species concept3.3. Phylogenetic species concepts3.4. The ecological species concept3.5. Homeostatic property cluster species4. Viruses and Species Taxonomy5. Reticular Phylogenies6. Conclusion.