Natural selection for spotted leaves: parallel morph ratio variation in three species of annual plants

Three sympatric species of annuals, Collinsia parviflora, Mimulus guttatus, and Trifolium tridentatum, are polymorphic for anthocyanin spots on the upper epidermis of the leaves. In each case the presence of spots is determined by a single dominant gene. Spotted morphs are most obvious in young plants, and in Collinsia and Mimulus the spotted phenotype is expressed only at low temperatures. Phenotype frequencies were determined in population samples in the Flat Top Islands, British Columbia, and at Anacortes, Washington. In all three species spotted morphs are more frequent in cold, exposed habitats, and the three species show parallel patterns of morph ratios in the sites studied. This provides strong evidence that the polymorphism is under the control of natural selection. Direct evidence of the adaptive significance of spotted leaves is lacking, but indirect evidence suggests the hypothesis that leaf spots may be advantageous in cold microhabitats because they increase leaf temperature during the winter growing season of these annuals.