Researchers Iranzo and Koonin ask: Typically, natural selection results in deletions of harmful genes, so the main question is, why hasn’t natural selection wiped out genetic parasites? They mean “transposons, plasmids, viruses” etc., that offer no benefit to the hosts. They offer a hypothesis:
In a new study published in EPL, researchers Jaime Iranzo and Eugene V. Koonin at the National Institutes of Health in Bethesda, Maryland, have found that horizontal gene transfer may be one of the keys to understanding the persistence and spread of genetic parasites over evolutionary timescales.
In horizontal gene transfer (HGT), genetic information is transferred to an organism by a variety of mechanisms other than the traditional parent-to-offspring process of transferring DNA. For example, an organism may receive genetic material directly from another organism—even one of a different species—or pick up genetic material from the surrounding environment. For genetic parasites, HGT offers a way to infect new hosts, providing a potential mechanism to compensate for the losses due to natural selection.p
“By quantifying the cost of genetic parasites and the minimum HGT rate that parasites require to persist, we gained a fundamental understanding of why selfish genes differ in their parasitic strategies,” Iranzo told Phys.org. “For example, genetic parasites that are very deleterious need very high HGT rates, which can only be achieved by evolving autonomous mechanisms for HGT (the case of viruses and conjugative plasmids) or by piggybacking on another parasite that is autonomous for HGT (the case of toxin-antitoxin modules). Mild genetic parasites, such as transposons, have more options, such as evolving mechanisms that allow them to proliferate within their host genomes.” Lisa Zyga, “How did genetic parasites overcome natural selection for billions of years?” at Phys.org
Paper. (open access) Jaime Iranzo and Eugene V. Koonin. “How genetic parasites persist despite the purge of natural selection.” EPL. DOI: 10.1209/0295-5075/122/58001
See also: Natural selection: Could it be the single greatest idea ever invented?
Horizontal gene transfer: Sorry, Darwin, it’s not your evolution any more