ACCIDENTAL ROLE. “Later, these regulatory mechanisms could have been co-opted to exert controlling effects over thousands of host genes as is the case for miRNAs. The evolution of such complex gene regulatory systems can be considered nonadaptive in the sense that they did not evolve by virtue of selection for the role that they play now.” (TE = transposable elements; miRNA = microRNA).
http://www.genetics.org/content/176/2/1323