  Since the sequencing of large genomes, many statistical features of their
sequences have been found. One intriguing feature is that certain subsequences
are much more abundant than others. In fact, abundances of subsequences of a
given length are distributed with a scale-free power-law tail, resembling
properties of human texts, such as the Zipf's law. Despite recent efforts, the
understanding of this phenomenon is still lacking. Here we find that selfish
DNA elements, such as those belonging to the Alu family of repeats, dominate
the power-law tail. Interestingly, for the Alu elements the power-law exponent
increases with the length of the considered subsequences. Motivated by these
observations, we develop a model of selfish DNA expansion. The predictions of
this model qualitatively and quantitatively agree with the empirical
observations. This allows us to estimate parameters for the process of selfish
DNA spreading in a genome during its evolution. The obtained results shed light
on how evolution of selfish DNA elements shapes non-trivial statistical
properties of genomes.

Populations and Evolution;Biological Physics;Genomics