The line between viruses and parasitic elements of the genome is thin and delicate, and probably often crossed. Parasitic elements that acquire the means to escape the host cell become viruses and, conversely, viruses can lose this ability and return to a more limited lifestyle as transposable elements (TEs). This back-and-forth occurs alongside a propensity for gene exchange, with genes acquired horizontally from various hosts, and at various periods of evolution.The result is that, although viruses and other "selfish" elements are obviously related, their relationship cannot be represented by the satisfying, graceful branches seen on phylogenetic trees depicting the evolution of more pedestrian organisms such as primates, for example. Instead, a virus or TE is like a quilt whose patches were added by different quilters, at different times.
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