Reticulate evolution is thought to accelerate the process of evolution beyond simple genetic drift and selection, helping to rapidly generate novel hybrids with combinations of adaptive traits. However, the long-standing dogma that reticulate evolutionary processes are likewise advantageous for switching ecological niches, as in microbial pathogen host switch events, has not been explicitly tested. We use data from the influenza genome sequencing project and a phylogenetic heuristic approach to show that reassortment, a reticulate evolutionary mechanism, predominates over mutational drift in transmission between different host species. Moreover, as host evolutionary distance increases, reassortment is increasingly favored. We conclude that the greater the quantitative difference between ecological niches, the greater the importance of reticulate evolutionary processes in overcoming niche barriers.ecology | reticulate evolution | influenza | host switch | reassortment

12Reticulate evolution is thought to accelerate the process of evolution beyond simple genetic drift 13 and selection, helping to rapidly generate novel hybrids with combinations of adaptive traits. 14 However, the long-standing dogma that reticulate evolutionary processes are likewise 15 advantageous for switching ecological niches, as in microbial pathogen host switch events, has 16 not been explicitly tested. We use data from the influenza genome sequencing project and a 17 phylogenetic heuristic approach to show that reassortment, a reticulate evolutionary mechanism, 18 predominates over mutational drift in transmission between different host species. Moreover, as 19 host evolutionary distance increases, reassortment is increasingly favored. We conclude that the 20 greater the quantitative difference between ecological niches, the greater the importance of 21 reticulate evolutionary processes in overcoming niche barriers.