The debate on rethinking evolutionary theory (see Nature 514, 161–164; 2014) should include viruses. By integrating into host DNA, viruses have markedly influenced the evolution and development of cellular organisms (see, for example, F. Baluška Ann. NY Acad. Sci. 1178, 106–119; 2009).
Viruses are the most abundant genetic entities on the planet. Almost all genomes of cellular organisms contain viral sequences, elements of which are now essential in gene regulation.
Persistent endogenous retroviruses, for example, have contributed crucially to the evolution of the mammalian placenta. And the genetic variations that led to the evolution of adaptive immunity in vertebrates, or the equivalent system in prokaryotes, were not a result of random errors in DNA replication but of viral infection events (see L. P. Villarreal Viruses 3, 1933–1958; 2011).