The widespread existence of selfish genetic elements in biological kingdoms is the result of their ability for efficient replication and propagation during host cell division. A subset of these elements can also transmit horizontally, thereby modulating the fitness of cell populations within an ecological niche and influencing selection under specific conditions. Multiple mechanisms have evolved for the spread of distinct classes of selfish elements. Some involve the duplication of an element during its movement, others not necessarily so. Retrotransposons and retrohoming introns mediate their mobility through RNA intermediates. Certain extreme forms of selfish elements ensure their persistence in a host by killing cells that fail to acquire them. Not all of the elements grouped under the 'selfish' label may be exclusively selfish. Some of these elements may serve important functions in gene regulation, confer selective advantages to the host under stress conditions, and contribute in general to genome organization, dynamics, and evolution.
Rowley, P. A., Kachroo, A. H., & Jayaram, M. (2013). Selfish DNA. In Brenner’s Encyclopedia of Genetics: Second Edition (pp. 382–389). Elsevier Inc. https://doi.org/10.1016/B978-0-12-374984-0.01394-2