Conclusion

Abstract

Speculation about the co-evolution of viruses with their hosts was justified because of relevance to mechanisms of virus maintenance. It is inconceivable that the diversity and sophistication of those viruses about which much is known have had only brief evolutionary histories. Nucleic acid fits the description ‘selfish’ and, having the capacity to replicate for maintenance at rates somewhat faster than that of host chromosomal DNA, it is not hard to anticipate that environmental circumstances will sometimes facilitate greatly increased over-production of nucleic acid species which thereby acquire some of the attributes of a virus. In some instances this over-production may result in death of host cells. Provided that suitable nearby cells are replaced at a sufficient rate one imagines that horizontal spread by contagion alone will be adequate for the survival of the nascent virus but, outside the cell, nucleic acids are vulnerable to extracellular enzymes and possession of a protective protein coat will be advantageous and likely to be a property selected for. Monophagy, deriving from a close co-evolution with one type of host, is likely to be the first option in the evolution of the nascent pathogen, but oligophagy would provide greater security and adaptation to the natural environment’s patchyness. One way in which this goal may be achieved is by chance association with a vector.