FACTORS INFLUENCING FREQUENCY OF THE MID‐STYLED MORPH IN TRISTYLOUS POPULATIONS OF OXALIS ALPINA

Genetic models and computer simulations suggest that modifications of incompatibility in tristylous populations of Oxalis alpina from southeastern Arizona should result in evolution of distyly, due to the inability of the mid floral morph to compete effectively with the short and long floral morphs. To evaluate this hypothesis, progeny testing of naturally pollinated individuals was carried out over a three‐year period for populations with differing mid frequencies. In contrast to expectations, mids appeared in greater than expected proportions in many cases. Detailed analysis of results from two trimorphic populations where the mid form is relatively uncommon revealed that mid morphs produced far more mid progeny among their offspring than expected. Assuming that all compatible pollen has the same probability of deposition on appropriate stigmas, mid excesses were found among progeny of shorts and longs in these populations as well. In a third population where the mid form is abundant, mid excesses were not found among the progeny of mids, nor were mid excesses typical among the progeny of shorts and longs. Mid excesses among progeny of populations where the mid form is uncommon may explain retention of this form, despite incompatibility modifications that should favor evolution of distyly. Over‐representation of mids may result from gametophytic selection favoring mid alleles. If this is the case, tristylous populations may be in equilibrium, with the frequencies of the three morphs determined by the balance of the opposing selective forces resulting from incompatibility modifications and gametophytic selection.