4.1 Homostyles replace short-styled individuals within populations
The variation in the frequency of floral morphs among populations provides an opportunity to elucidate how homostylous phenotypes originate and spread. The homostyles of Primula vulgaris , typically characterized by long styles and high anthers (Huu et al., 2016; Li et al., 2016), display the stigma type of LS-morph but retain the anther position and pollen type of the SS-morph (Crosby, 1940). Therefore, the origin of HO-individuals from SS-individuals should cause an initial reduction of the latter, triggering downstream consequences that further increase the number of homostyles. Firstly, pollen of homostyles can be used for both self- and cross-fertilization of LS-individuals, but not for fertilization of SS-individuals (Figure 1). Homostyles can also be fertilized by compatible pollen of SS-morphs, but this cross is thought to occur rarely in nature due to stigma clogging with self-pollen in homostyles (Crosby, 1959; but see Bodmer, 1959). Hence, homostyles have a reproductive advantage over both LS- and SS-morphs as a result of their ability to both self-fertilize and cross with LS-morphs (Richards, 2003). Finally, self-fertilization of homostyles (Figure S4A and B) and occasional crosses from homostyles to LS-individuals produce homostylous and LS-progeny, but no SS-progeny (Figure S4C and D). In accordance with these expectations and findings of previous studies (Crosby, 1949, 1959; Curtis & Curtis, 1985)⁠, our results demonstrate that, as homostyles increase within populations, more SS- than LS-individuals are lost within populations (Table 1 and Figures 2A and S1). The fact that LS-individuals are maintained at low frequencies within populations, even in the absence of SS-individuals, suggests that they are occasionally fertilized by homostyles. Taken together, these results corroborate the hypothesis that long-homostyles originate from SS-individuals and explain why the latter occur at lower frequencies than LS-individuals across populations with all three morphs.