Evolutionary consequences of variation of floral traits in Phlox drummondii
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To investigate the evolutionary consequences of flower size variation in Phlox drummondii, I characterized the organization of this variation within and among populations, and performed a bi-directional artificial selection program to see whether flower size can respond to selection. By monitoring flower size, corolla tube length and stigma-anther proximity in the greenhouse, I measured direct and indirect responses to selection, and calculated genetic correlations, realized and broad-sense heritabilities. I also carried out a series of experiments to determine the form, direction and strength of selection on flower size, flower number, floral display, and corolla tube length, petal shape and petal color in artificial populations. In the field, I directly estimated response to selection by comparing offspring from natural and random hand-pollinations, and estimated correlation between floral characters and maternal fitness using simple and multiple linear and polynomial regressions. Flower size was significantly different between color races and among populations within races in the field, and was weakly influenced by the amount of the vegetative growth period. The per-generation average of the response to selection on flower size was about 5%, and realized heritability was high (0.89). In the field, directional selection on flower size in the pink race was not detectable. Floral display was under positive directional or stabilizing selection depending on flower color. When additional characters were included, positive directional selection was apparent only on floral display and flower number, whereas corolla tube length was subject to negative directional and stabilizing selection. Stabilizing selection on petal color was also significant, as was indirect selection between flower size and tube length. Based on these results, I concluded that floral traits may not be strongly integrated genetically in this species, and that there is great capacity to rapidly respond to selection, which may be in part responsible for the observed high degree of differentiation within the species. Different forms of selection are simultaneously acting on floral traits of P. drummondii in the field, and floral differentiation of races most likely arose from the way the primary pollinator perceives and behaviorally responds to the colors of flowers.