New criteria for sympatric speciation in the genomic era

Abstract

Sympatric speciation illustrates how natural and sexual selection may create new species in isolation without geographic barriers. However, so far, all genomic reanalyses of classic examples of sympatric speciation indicate secondary gene flow occurred. Thus, there is a need to revisit criteria for demonstrating sympatric speciation in the face of widespread gene flow. We summarize theoretical differences between sympatric speciation and speciation-with-gene-flow models and propose genomic criteria for sympatric speciation: 1) timing of fine-scale introgression; 2) timing of selective sweeps and 3) functional annotation of this introgressed variation; and 4) the absence of similar sweeps in outgroups. Monophyly is an insufficient criterion for sympatric speciation; we must take a locus-specific approach to investigate whether any introgression contributed to reproductive isolation.