Figure 2. The timing of parental divergence and hybridization in five examples of homoploid hybridization speciation. The five homoploid hybrids are displayed according to the timing of origin (from most ancient to most recent): Carpinus cordata (heartleaf hornbeam; a species from sect. Distegocarpus ), Ursus thibetanus(Asiatic black bear), Ostryopsis intermedia(hazel-hornbeam), Macaca fascicularis (long-tailed macaque; sapecies from the fascicularis group), andRhinopithecus brelichi (gray snub-nosed monkey). Branch lengths are not to scale.
While the Wang et al. (2021) pipeline appears to be broadly applicable to the documentation of homoploid hybrid speciation, some ambiguity will remain. Reproductive isolation is typically polygenic, so natural selection acting on an admixed population will likely take advantage of alleles from both parents (Thompson et al., 2023). Thus, as admixed populations diverge, even if the admixture were initially maladaptive or neutral, it seems likely that they will (at some point) become homoploid hybrid species according to the Schumer et al. (2014) criteria and diagnosable following Wang et al. (2021). Given the ubiquity of admixture, this line of reasoning implies that homoploid hybrid speciation could be more common and protracted than suggested by conceptual arguments and theoretical studies (Grant, 1981; Rieseberg, 1997; Buerkle et al., 2001, 2003; Schumer et al., 2015; Comeault, 2018; Blanckaert et al., 2018, 2023). However, before such a conclusion can be made, discussion is required about how much reproductive isolation must derive from the minor parent of homoploid hybrid species and whether that isolation must be critical to the initial establishment of the homoploid hybrid species, as implied by the Ottenburghs’ (2018) classification scheme. Also, how divergent must the parental lineages be at the time of hybrid origin? When would our classification of evolutionary process shift from the ‘Grey Zone of Speciation’ (Roux et al. 2016) to homoploid hybrid speciation? Methods for more accurately estimating the ages of alleles, the timing of admixture, and the timing of selective sweeps will contribute importantly to such a debate (Y C Brandt et al., 2022).