NEOTENIC EVOLUTION OF DELPHINIUM NUDICAULE (RANUNCULACEAE): A HUMMINGBIRD‐POLLINATED LARKSPUR

Takhtajan (1972) contended that many morphological discontinuities among related plant taxa are illusory because we commonly compare only adult forms. He suggested that we will discover a greater continuity of form within lineages if we consider ontogenies as well. The application of comparative ontogenetic data to questions of animal phylogeny has a long and sometimes tumultuous history (for review see Gould, 1977). Botanists, however, have only rarely dealt with broad theoretical aspects of the relationship of ontogeny and phylogeny in plants (Takhtajan, 1943, 1969 for review, 1972, 1976; Doyle, 1978). In this study, ontogenetic data together with phylogenetic and ecological information are interpreted in the context of models of ontogeny and phylogeny recently developed by zoologists, to address the evolution of the unique floral form of Delphinium nudicaule. Gould (1977) rekindled interest in the study of ontogeny and phylogeny by his thorough historical treatment of the subject. One of Gould's contributions was to distinguish between evolutionary processes (e.g., neoteny and progenesis), and their morphological or phylogenetic products (paedomorphosis). He also modified and simplified de Beer's (1930) categorization of heterochrony (i. e., changes in rates or timing of development) by reducing the basic processes to relative acceleration and retardation. Building on this foundation, Alberch et al. (1979) have developed a framework using size, shape and age as independent variables to illustrate how heterochrony can account for morphological differences among related taxa (Fig. 1). Paedomorphosis (evolutionary juvenilization), and its opposite, peramorphosis (literally 'shapes beyond'), can each be produced by three processes: paedomorphosis by neoteny, progenesis, and post-displacement; and peramorphosis by acceleration, hypermorphosis, and pre-displacement. One of the strengths of the Alberch et al. methodology is that it can be applied simultaneously to many arbitrarily chosen developmental processes within an organism. This allows different components of overall form to be conceptually dissected from one another. These can then be interpreted in the context of the whole organism. For example, Alberch and Alberch (1981) discuss the effect of several dissociations, or decouplings, of developmental processes within and among various portions of the body, on the form of the generally progenetic salamander, Bolitoglossa occidentalis. To understand the effect of heterochrony on the evolution of adult forms, it is necessary to compare the size, shape, and age to maturity of related organisms. This is problematic since maturity is not a unitary concept, but rather consists of two distinct phenomena. The ability to reproduce sexually, and a culmination of growth, are separately and together termed maturity. Because of the embryonic separation of germinal cell lines from somatic tissues in the majority of metazoans, sexual maturity in most animals is a property of whole individual organisms. An animal typically reaches sexual maturity just once, and a culmination of somatic growth is often loosely associated with the onset of sexual maturity. This is not usually the case with plants because of their indeter-