Variation in floral morph ratios in tristylous Oxalis squamata (Oxalidaceae): an Andean alpine endemic

Negative frequency-dependent selection resulting from disassortative mating should result in equal morph ratios in tristylous populations at equilibrium. However, surveys of morph-frequency variation in tristylous species commonly report deviations from equality. Here, we report variation in morph ratios in Oxalis squamata Zucc., an endemic of the Andean region of Chile and Argentina. Absence of clonal propagation in this species allows unambiguous estimates of the morph ratio of genets. We sampled floral morph ratios in 20 populations occurring in Central Chile and investigated the relation between morph evenness in populations and their size. All populations of O. squamata were tristylous but with significant heterogeneity among populations in morph ratios. Although small populations exhibited a greater variance in morph evenness, biased ratios were also evident in several large populations. We found no evidence of morph loss or a consistent bias in morph frequencies as reported in some tristylous species. Biased morph frequencies in large populations probably arise from episodic sexual recruitment following disturbance and a slow approach to equilibrium.