Jumping genes

''Transposons are sequences of DNA that can move around to different positions within the genome of a single cell, a process called transposition. In the process, they can cause mutations and change the amount of DNA in the genome.
...
The most common form of transposon in humans is the Alu sequence. The Alu sequence is approximately 300 bases long and can be found between 300,000 and a million times in the human genome.'' [source]

See also: Margulis - Acquiring genomes [source]