PHENOTYPIC PLASTICITY IN PHLOX. I. WILD AND CULTIVATED POPULATIONS OF P. DRUMMONDII

We investigated the changes in amounts and patterns of phenotypic plasticity which have arisen in the Texas annual Phlox drummondii during domestication. Character means and plasticities were compared for five populations: a wild population, three cultivated varieties (a Tall cultivar and two Dwarf cultivars), and a population of an escaped Tall cultivar naturalized in Texas. To measure plasticity, we scored the responses of 10 characters to six treatments and analyzed both the amount and direction of plastic response. Wild plants are phenotypically distinct from the Tall and Escaped cultivar and from the two Dwarf cultivars. Despite its substantial phenotypic divergence from the Wild population, the Tall cultivar's plasticity has changed little during domestication. Traits most strongly correlated with fitness show the least change in their plasticities. The two Dwarf varieties have very similar plasticities, despite strong phenotypic divergence from the Tall population and despite the fact that they were derived from different Tall lines. This suggests that indirect selection on phenotypic plasticity related to selection for the Dwarf habit has resulted in the characteristic plasticity of the Dwarf lines. The Escaped cultivar has substantially different plastic responses from those of the Wild or cultivated populations. ALTHOUGH PLANT POPULATIONS may differ in their phenotypic plasticity due to selection, drift, and genomic disruptions such as polyploidy, hybridization, and inbreeding (Schlichting, 1986), the factors responsible for divergence of amounts or patterns of response are seldom clear (Scheiner and Goodnight, 1984; Schlichting, 1986; Schlichting and Levin, unpublished). Classic examples such as Cook and Johnson's (1968) study of plasticity in Ranunculus are the exception rather than the rule. A major difficulty in interpreting plasticity differences between taxa is the lack of information on the environmental conditions or other selective agents promoting divergence. One way to overcome this difficulty is to study the changes in phenotypic plasticity which have arisen during the domestication of a wild species. Domestication typically involves changes in environments as well as in the di' Received for publication 1 1 August 1986; revision accepted 17 March 1987. Many thanks to the following for help with experiments and data collection: J. Kimbel, K. Kramer, W. Thomas, B. Fulginiti, L. Watkins, P. Phillips, and M. C. Rossiter. Comments on the paper by M. C. Rossiter, B. Simpson, V. Grant, N. Fowler, and reviewers are gratefully acknowledged. Katie Bear prepared Fig. 1. This research was supported by NSF Grant BSR 8189484 to DAL, and by University Fellowships to CDS from the University of Texas at Austin. Computer resources were provided to CDS by Pennsylvania State University. 2 Current address; Department of Biology, 208 Mueller Laboratory, Pennsylvania State University, University Park, PA 16802. rection of selection, and these environments and artificial selection regimes are often known or can be inferred. Simmonds (1981) has suggested that increased phenotypic plasticity has been the prime object of unconscious selection for crop plants as human agricultural techniques have evolved. An increase in plasticity (environmental sensitivity) is an expected result of selection for increased yields in improved environments (Falconer, 1981, 293). Positive plastic responses to conditions of cultivation such as irrigation or fertilization have been strongly selected so that performance far exceeds performance under natural conditions. In this study we investigate the changes in the amounts and patterns of phenotypic plasticity of Phlox drummondii during the transition from wild to cultivated ornamental varieties, and for an escaped cultivar. Because we know the direction of evolution, the results of the artificial selection program, and can infer the conditions of cultivation, we might more confidently assess the causes of variation in phenotypic plasticity. Here we report substantial variation in phenotypic plasticity between wild and cultivated populations, and discuss possible factors responsible for similarities and differences among taxa for plastic response. THE SPECIES-Phlox drummondii Hook. is a winter annual native to central Texas. In 1 8 3 5, Thomas Drummond collected seed of this species and sent it to Europe, where it was