How does a variant of the SARS-CoV-2 coronavirus spread?

There are two possible reasons for the spread of a variant: chance and natural selection.

A variant that arrives first in a host population starts the race ahead and benefits from what is called a "founder effect". In the case of SARS-CoV-2, risky host behaviors or social habits unrelated to the viral mutation may cause superpropagation episodes (See question What is a super-propagating situation for the COVID-19 ?). A mutation may occur by chance in these individuals involved in superpropagation episodes, or it may be transmitted to a large number of people by a "windfall effect". These exceptional transmission events can lead to a dramatic increase in the frequency of certain variants without necessarily being due to an adaptation of the virus. Indeed, with the exponential growth of the virus, a slightly more frequent mutation may become the majority within a few weeks or months.

Natural selection is another mechanism that can lead to the propagation of a variant. Mutations appear randomly and three effects are possible: some will be neutral (no effect on the coronavirus), some harmful to the coronavirus (mutation that decreases its transmissibility) and some beneficial (mutation that increases its transmissibility). In the case of a mutation that is beneficial for the coronavirus, the variant will have an advantage over the initial form, it will spread more rapidly to a larger number of people and, thus, become more common in the population.