Searching for sympatric speciation in the genomic era

Sympatric speciation illustrates how natural and sexual selection may create new species in isolation without geographic barriers. However, recent genomic reanalyses of classic examples of sympatric speciation have revealed complex histories of secondary gene flow. Thus, there is a need to revisit how to connect the diverse theoretical models of sympatric speciation and their predictions to empirical case studies in the face of widespread gene flow. We summarize theoretical differences between sympatric speciation and speciation-with-gene-flow models and propose genomic analyses for distinguishing which models apply to case studies based on the timing and function of adaptive introgression. Investigating whether secondary gene flow contributed to reproductive isolation is necessary to test whether predictions of theory are ultimately borne out in nature.