Bacteriophages are the most abundant biological entities in the biosphere, and this dynamic and old population is, not surprisingly, highly diverse genetically. Relative to bacterial genomics, phage genomics has advanced slowly, and a higher-resolution picture of the phagosphere is only just emerging. This view reveals substantial diversity even among phages known to infect a common host strain, but the relationships are complex, with mosaic genomic architectures generated by illegitimate recombination over a long period of evolutionary history.
Bacteriophages are the dark matter of the biological world ; a vastness of ill-defined genetic variation whose impacts we observe on the microbial population but of which we have little understanding. The phage population is estimated to contain approximately 1031 particles and is highly dynamic, with the population turning over every few days. Moreover, this rolling boil of evolution has been churning away for perhaps two billion years or more, giving rise to fantastic genetic diversity.