The "Genomic Lineages" graph of Covid variants from the UKHSA gives a good view of how variants follow on from one another, at least with Covid. Flu certainly had three (?) popular variants doing the rounds (one of which was thought to have been eliminated by lockdown) but at the moment, it seems like there's usually one Covid to rule them all at any one time:

https://ukhsa-dashboard.data.gov.uk/topics/covid-19