Fig. 2. The impacts of founder effects and population bottlenecks on the evolution of human-amplified arthropod-borne viruses.

Schematic depicting diverse viral populations (mutant swarms) that circulate. These swarms comprise a range of mutants that are dominated by a master sequence, which is typically characterized by high fitness in the current replicative environment. An infected human traveller initiates a founder effect by introducing a virus into a new geographic region, which results in a new population with randomly derived genetic variants, some of which may have reduced fitness depending on whether the master sequence is eliminated stochastically. Following population expansion during viraemia in the human host, further bottlenecks occur during infection of the vector and dissemination in the vector, as well as during transmission to another human host. Founder effects and other bottlenecks lead to the loss of genetic variation and fitness declines that can result from Muller’s ratchet, as random mutations become sequentially fixed in the population in the absence of efficient recombination to restore master sequences without infrequent direct reversions¹⁸.