Genome size

Tree of life with genome sizes as outer bars (in number of genes, not the total quantity of DNA)

From the economic way of thinking, since phosphorus and energy are scarce, a reduction in the DNA should be always the focus of the evolution, unless a benefit is acquired. The random deletion will be then mainly deleterious and not selected due to the reduction of the gained fitness but occasionally the elimination will be advantageous as well. This trade-off between economy and accumulation of non-coding DNA is the key to the maintenance of the karyoplasmatic ratio.

One example of the miniaturization of the genome occurred in the microsporidia, an anaerobic intracellular parasite of arthropods evolved from aerobic fungi.