What can we learn about homoploid hybrid speciation from these
five studies?
First, all five examples involved hybridization that took place deep in
time (from ∼1.9 to ∼22.5 mya; Figure 2). This not only demonstrates the
power of modern genomic approaches for identifying ancient reticulations
(Stull et al., 2022), but it also helps assuage earlier worries that
hybridization and hybrid speciation were mainly a consequence of
anthropogenic disturbance and therefore unlikely to have a major
evolutionary role (Schemske, 2000). More broadly, these studies add to
growing evidence that hybridization is the rule rather than the
exception during the divergence of plant and animal lineages (Taylor &
Larson, 2019; Daglis et al., 2022), and that it can contribute to
post-hybridization diversification (Meier et al., 2017; Stull et al.,
2023).
Another interesting observation is the vast difference in the extent of
divergence of the parental lineages at the time of hybrid speciation,
with the animal hybrid lineages arising fairly recently after the
divergence of the parental species (Figure 2): snub-nosed monkey (∼0.11
mya), bear (0.25 mya), and macaque (∼0.35 mya). In contrast, the two
plant hybrid species arose ∼1.8 mya and ∼6.5 mya after parental lineage
divergence, respectively, which is 5-fold to 59-fold greater than the
seen for the animal hybrids. Does this mean that successful hybrid
speciation in animals is likely to involve more recently diverged
lineages than for plants? Probably not. Keep in mind that evolutionary
rates are slower in woody species compared to annual or perennial herbs
(Smith & Donoghue, 2008), and homoploid hybrid species involving
annuals appear to have arisen closer in time to parental divergence than
for Ostryopsis and section Distegocarpus (Osborne et al.,
2013; Owens et al., 2023). Likewise, there is evidence that some animal
hybrid species have originated from more divergent parental lines than
the primates and bears discussed here (e.g., Ottenburghs et al., 2016;
Ravinet et al., 2018). Furthermore, on average, reproductive barriers
appear to be more rapidly established in plants than animals (Monnet et
al., 2023). A meta-analysis would be useful to assess the timing of
hybrid speciation more broadly, with the goal of asking whether
homoploid hybrid speciation is most common at intermediate levels of
divergence as predicted by models and experimental studies (Buerkle et
al., 2001; Chapman & Burke, 2007; Comeault & Matute, 2018; although
see Blanckaert et al., 2023).