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).