Among-species rate variation produces false signals of introgression

Abstract

AbstractThe role of interspecific hybridization in the context of diversification dynamics has recently seen increasing attention. Genomic research has now made it abundantly clear that both hybridization and introgression – the exchange of genetic material through hybridization – are far more common than previously thought. Moreover, even highly divergent species were found to hybridize and backcross. These findings raise the question whether commonly used methods for the detection of introgression are applicable to such divergent hybridizing species, given that most of these methods were originally developed for analyses at the level of populations and recently diverged species. In particular, the assumption of constant evolutionary rates, which is implicit in many commonly used approaches, is more likely to be violated as evolutionary divergence increases. To test the limitations of introgression detection methods when being applied to divergent species, we simulated thousands of genomic datasets under a wide range of settings, with varying degrees of among-species rate variation and introgression. Using these simulated datasets, we were able to show that commonly applied statistical methods, including the D-statistic and tests based on sets of phylogenetic trees, produce false-positive signals of introgression between highly divergent taxa when these have different rates of evolution. These misleading signals are caused by the presence of homoplasies that occur at different rates when rate variation is present. To distinguish between the patterns caused by rate variation and genuine introgression, we developed a new test that is based on the expected clustering of introgressed sites and implemented this test in the program Dsuite.