Corolla Color as an Inhibitor of Interspecific Hybridization in Phlox

Populations of Phlox pilosa typically are composed of organisms with pink corollas. In areas of contact with the pink-flowered P. glaberrima, the corolla phase of P. pilosa is replaced by a white one. The purpose of this investigation was to determine whether the selective advantage of the white phase lies in its ability to inhibit interspecific hybridization by reinforcing previously existing ethological barriers. The relationship between hybridization and corolla color was determined in two synthetic populations each containing ca. 200 P. glaberrima, with 20 pink P. pilosa transplants in one and 20 whites in the other. The percentage hybrid seed produced by the two P. pilosa phases was determined by analyzing seed-storage-protein profiles. The profiles of P. pilosa and P. glaberrima are quite distinctive, and we would expect hybrid profiles to be a summation of the parental profiles. Additive profiles, interpreted as those of hybrids, were found in the seed pools of both P. pilosa phases. However, the percentage hybrid seed in the pink pool was 8.2% as compared with 2.1% in the white. Thus it would appear that white corollas have a profound effect on interspecific hybridization. The inhibiting effect of the white corolla upon hybridization is not as pronounced as it would seem. The populations from which the pink and white transplants were obtained have different crossabilities with P. glaberrima. Interspecific crosses with pink P. pilosa yielded 16% seed-set as contrasted to 8% for the white phase. Since the probability of hybridization with the pink phase was twice that with the white, the 8.2% hybridity of pink seed should be interpreted as 4.1% when compared with the 2.1% hybridity of white seed. We suggest that the different crossabilities are the product of natural selection. The population from which the white transplants were obtained had undergone character displacement for a floral signal which ostensibly was accompanied by selection for reinforcement of the interspecific incompatibility barrier. We conclude that character displacement in P. pilosa served to reduce hybridization and the potential for gene flow by accentuating species' floral differences, thereby aiding pollinators to discriminate between them and maintain their flower constancy.