Searching 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, recent genomic reanalyses of classic examples of sympatric speciation reveal complex histories of secondary gene flow from outgroups into the radiation. In contrast, the rich theoretical literature on this process distinguishes among a diverse range of models based on simple genetic histories and different types of reproductive isolating barriers. Thus, there is a need to revisit how to connect theoretical models of sympatric speciation and their predictions to empirical case studies in the face of widespread gene flow. Here, theoretical differences among different types of sympatric speciation and speciation-with-gene-flow models are reviewed and genomic analyses for distinguishing which models apply to case studies based on the timing and function of adaptive introgression are proposed. Investigating whether secondary gene flow contributed to reproductive isolation is necessary to test whether predictions of theory are ultimately borne out in nature.

Evidence for sympatric speciation in a Wallacean ancient lake

“…This temporal proximity between admixture and divergence suggests that these admixtures triggered their initial sympatric divergences (Richards et al. ). Further analyses on the functional roles of introgressed genes are necessary to test if the initial divergence between O. nebulosus and O. orthognathus , and the split of O. nigrimas as well, were triggered by the secondary gene flow from Lake Tiu.…”

“…If this is the case, the divergence of O. nigrimas can still be categorized as hard sympatric speciation (Richards et al. ). The other possibility is that the introgressed genes were not neutral and aided stalled divergence in the ancestral O. nigrimas population, a situation categorized as gene flow induced sympatric divergence (Richards et al.…”

“…For example, genetic variants supplied by secondary gene flow may be absorbed into the gene pool, leading to formation of a hybrid swarm and increased genetic variation within the population, which may be sufficient to trigger later sympatric divergence (Richards et al. ). In contrast, population divergence might occur without reaching panmixia, which will be the case when reproductive isolation between colonists and occupants is strengthened by reinforcement and/or ecological character displacement (Seehausen ; Pfennig and Pfennig ).…”

“…On the other hand, secondary gene flow may trigger initial sympatric divergence (Richards et al. ), where differential admixture will not occur. In all above cases, an initial level of divergence between species is triggered by allopatric divergence; this is therefore called “gene flow induced sympatric divergence” as opposed to “hard sympatric speciation” (Richards et al.…”

“…In all above cases, an initial level of divergence between species is triggered by allopatric divergence; this is therefore called “gene flow induced sympatric divergence” as opposed to “hard sympatric speciation” (Richards et al. ).…”