How A Virus Evolves

The mutations of SARS-CoV-2, according to a new study, have led to the emergence of a dominant virus type, Type A2a, distinctly different from the original virus, Type O, that emerged from Wuhan, and spreading with much higher frequencies than the original version.

- R. Ramachandran

Viruses readily mutate. There is nothing surprising about this because it is their nature to do so. This happens due to the imperfect copying mechanism at work as viruses replicate in the cells of infected hosts.

The complete set of genetic information needed to sustain an organism, such as the virus, is its genome, which, in the case of viruses, can be made up either of the DNA or the RNA molecule. The DNA and the RNA can be thought of as a string of (genetic) letters, and a genome can be imagined to be long stretches of these letters with different parts of it encoding for different proteins required for the organism’s existence. Mutations are just random errors that occur during the process of copying these letters during viral multiplication and such errors accumulate during every replicating cycle, which can occur within hours or even less. RNA viruses mutate faster than DNA viruses because their replication mechanism is intrinsically more error-prone. Likewise single-stranded viruses mutate more often than double-stranded ones.

Viruses cannot exist in isolation; they need a host to replicate and survive. Mutations generate a diversity of virus population in a single infected host. This amazing ability of viruses to mutate is what drives their evolutionary change. Most mutations may be inconsequential. But mutations that adversely affect some virus function or the other, which impede its sustenance, will get removed by natural selection. If during an outbreak, a mutated virus with a greater (or lesser) degree of infectivity or virulence were to appear in a population, it does not immediately follow that the mutation will sustain and continue to spread with high frequency, unless it gives the virus a selective advantage as instances during the current COVID-19 pandemic that we consider below illustrate.