Figure 5.

Gene flow, traditionally considered to hinder divergence between incipient species, can serve to infuse variability that may foster adaptive radiation. This can take place through (a) hybrid origins of entire radiating clades (“hybrid swarm origins”) wherein admixture between one or more divergent lineages happens prior to the onset of radiation, (b) via hybridization between nonsister species within adaptive radiations that facilitates further speciation (“syngameon hypothesis”) (Seehausen 2004), and (c) speciation with gene flow between sister species. It is important to distinguish between how admixture is achieved in order to assess its effects on the process of adaptive radiation (Brock and Wagner 2018). Both syngameon and hybrid swarm origins hypotheses have now been well documented in cichlid fish (Meier et al. 2017), and the importance of gene flow and the syngameon have been well demonstrated in Heliconius butterflies (Mallet 2005; Merrill et al. 2015), as well as many plants (Barrier et al. 1999; Friar et al. 2008), and are also found in many other lineages (Feder et al. 2003; Lamichhaney et al. 2018). Indeed, the processes may be common to many adaptive radiations. Moreover, there may be a “sweet spot” in which divergent lineages can admix or hybridize and give rise to variability that is key to adaptive radiation.