Strength of the prezygotic reproductive barriers
Differential outcomes of hybridization may partly be explained by the strength of the prezygotic reproductive barriers (Lepais et al., 2009; Mandeville et al., 2017; Vines et al., 2003) which varies across species (Good et al., 2008; Sánchez-Guillén et al., 2014) and in hybridizing species, with the time since sympatry (Kronforst, Young, & Gilbert, 2007; Lemmon & Juenger, 2017; Liao et al., 2019). The frequency distribution of hybrid classes in a hybrid zone may reflect the strength and direction of reproductive barriers. A bimodal distribution of the genotypical-like classes (parental or introgressed individuals) are thought to be the consequence of strong but incomplete reproductive isolation (e.g., Harrison & Bogdanowicz, 1997), while lower levels of prezygotic isolation resulting in a unimodal distribution on the genotypic classes (the distribution spans a range of admixture and backcrosses toward both parents) (e.g., Szymura & Barton, 1991) (Gompert, Mandeville, & Buerkle, 2017; Jiggins & Mallet, 2000).
The high prevalence of admixed individuals in north-central Spain can be explained by its level of reproductive isolation as was, for instance, detected in Catostomus fish species (Mandeville et al., 2017) and intertidal snails (Littorinids) (Stankowski et al., 2020). In north-central Spain, 70% of the heterospecific matings between phenotypically I. elegans and I. graellsii were prevented, while the backcrosses to I. elegans and the backcrosses toI. graellsii were less and similarly prevented (54% and 50%, respectively). In fact, from the north-central hybrid region, five populations (Arreo, Cañas, Perdiguero, Valbornedo and Villar) show a bimodal distribution (introgressed individuals with I. elegansand I. graellsii ) and Mateo and Valpierre presented an introgressed distribution (introgressed individuals with I. graellsii ). However in the north-west hybrid region, heterospecific matings and backcrosses with I. elegans were similar and strongly prevented (80% and 79% respectively), while only 46% of the backcrosses with I. graellsii were prevented. Arce-Valdéset al. (under review) found evidence of an incipient strengthening of the prezygotic isolation in this hybrid region. From the north-west hybrid region, three populations (Doniños, Laxe, Xuño) presented an introgressed distribution towards I. elegans(Doniños, Laxe) and towards I. graellsii (Xuño), and one population (Louro) presented a unimodal pattern. Differences in the admixture-class distributions between the Spanish hybrid regions (I. elegansclass was significantly higher in the north-west hybrid region) can be explained by the reinforcement of the strength of the reproductive isolation between I. elegans and I. graellsii in the north-west hybrid region.