Evolution of generalism under Muller's ratchet.

Host-range expansion in parasites is usually framed as an adaptive response to heterogeneous environments and trade-offs. In this opinion article, I propose Ratchet-Driven Generalism (RDG): in some parasites, broad host range may instead arise non-adaptively after the loss of sexual recombination. Under Muller's ratchet, deleterious mutations accumulate in asexual or rarely recombining lineages. Over time, genomic erosion can degrade host-specific adaptations, including host-interaction genes. In parallel, reduced competitive filtering among parasite genotypes can weaken selection for host-specific performance. Consequently, genotypes that are merely viable across hosts may persist. RDG predicts genomic signatures of functional decay and reduced efficacy of selection. It also suggests that such generalism may be transient, because continued genomic erosion can increase extinction risk. I outline comparative genomic tests to examine RDG.