Progenies from nine populations of six species of Clarkia, including the new species C. calientensis, were compared using a similarity index based on the sum of sign/ifcant differences for 22 morphological and developmental traits. The two most similar populations were conspec/ifc. Other populations of different species were more similar to each other than to respective conspec/ifc populations. Characteristics associated with pollination tend to have low plasticity and high interpopulation differences, suggesting rigorous selection for precisely integrated systems. Extremely plastic traits, such as the time to first flower and the number of seeds per capsule, result from a series of morphogenetic events, each of which in itself, may be highly plastic. Despite continuous adjustment, these highly plastic traits are efficient and consistent in differentiating between populations. A group of populations in woodland habitats and another in grassland habitats both show intragroup similarity in a number of characteristics, which indicate common adaptations, and some differences, which probably reflect a mode of isolation. Sympatric populations often have similar vegetative and developmental characteristics but different breeding-system characteristics; usually some characteristics of each system differentiate sympatric or adjacent populations. The greatest similarities occur in sympatric or environmentally equivalent situations, emphasizing the importance of environmental selection. Clarkia section Phaeostoma consists of C. xantiana and the C. unguiculata complex (Vasek, 1 968a). Clarkia xantiana occurs in oak-Digger Pine woodland of California's southern Sierra Nevada foothills and is easily differentiated from other species of the section by its conspicuously bilobed petals. Most populations consist of large-flowered, protandrous outcrossers, but some populations of early-flowering, small-flowered synandrogynous selfers occur in marginal habitats (Moore & Lewis, 1965). The C. unguiculata complex consists of C. unguiculata and four derivatives (Vasek, 1968a). Clarkia unguiculata is widely distributed in California's coast ranges and Sierra Nevada foothills where it commonly inhabits oak and oak-Digger Pine woodland, but it also occurs in grassland (Lewis & Lewis, 1955; Vasek, 1968a). Its many populations are highly variable in morphology, habitat, and cytology (Lewis, 1951; Lewis & Lewis, 1955; Mooring, 1958, 1960). It is generally a large-flowered, protandrous, lateblooming outcrosser (Vasek, 1958, 1964a, 1965). The derivatives have more or less restricted distributions at low elevations near the hot dry 1 Experimental portions of this study were supported by Grant GB-5914X from the National Science Foundation. I thank H. Lewis and L. Gottlieb for comments on an earlier draft of this paper. 2 Biology, University of California, Riverside CA 92521