Selfish DNA

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Selfish DNA refers to those sequences of DNA which, in their purest form, have two distinct properties: (1) the DNA sequence spreads by forming additional copies of itself within the genome; and (2) it makes no specific contribution to the reproductive success of its host organism. This idea was sketched briefly by Richard Dawkins in his 1976 book The Selfish Gene and was explicitly exposed in two 1980 articles in Nature magazine. According to one of these articles:

The theory of natural selection, in its more general formulation, deals with the competition between replicating entities. It shows that, in such a competition, the more efficient replicators increase in number at the expense of their less efficient competitors. After a sufficient time, only the most efficient replicators survive.

—L.E. Orgel & F.H.C. Crick, Selfish DNA: the ultimate parasite

So, the selfish DNA can be considered an efficient replicator that follows another way of increasing in number.

[edit] Examples

Transposons copy themselves to different loci inside the genome. These elements constitute a large fraction of eukaryotic genome sizes (C-values): about 45% of the human genome is composed of transposons and their defunct remnants.

Homing endonuclease genes cleave DNA at its own site on the homologous chromosome, triggering the DNA double-stranded break repair system, which "repairs" the break by copying the HEG onto the homologous chromosome. HEGs have been characterized in yeast, and can only survive by passing between multiple isolated populations or species.

Supernumerary B chromosomes are nonessential chromosomes that are transmitted in higher-than-expected frequencies, which leads to their accumulation in progenies.