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).