A balance between stochastic processes and negative frequency-dependent selection largely determines style morph frequencies in heterostylous populations.
Costa et al. investigate variation in morph frequencies at the southern European range limit of the tristylous, wetland species Lythrum salicaria. They find populations are predominantly trimorphic, but progressively smaller in size and with larger deviations from the predicted 1:1:1 equilibrium towards the drier southern edge of the distribution. This supports the hypothesis that deteriorating environmental conditions at range margins influence demographic features of populations, and demonstrates the resilience of floral trimorphism in the face of stressful conditions.
Joana Costa, Sílvia Castro, João Loureiro, Spencer C. H. Barrett, 2015, ' Variation in style morph frequencies in tristylous Lythrum salicaria in the Iberian Peninsula: the role of geographical and demographic factors ', Annals of Botany, p. mcv173 http://dx.doi.org/10.1093/aob/mcv173