The Role of Hybridization in Evolution: Old Wine in New Skins1'2

Perhaps no one has had a greater influence on the development ofhybridization theory than Dr. Ledyard Stebbins. However, his contributions have not been restricted to his writings or lectures alone. Many of you have marveled at the great authority with which Dr. Stebbins identified the hybrid status, parentage, and ploidal level of plants in the field or on herbarium sheets. He didn't seem to need all the modem techniques that the current generation of botanists is both blessed and burdened with. For example, I remember a short field trip with Dr. Stebbins in the fall of 1984 to look at some sunflower hybrids near Davis. The plants were dead, only a few dried leaves, stalks, and floral heads remained, yet he confidently identified parental, hybrid, and backcrossed generations. Ironically, after 2 yr of careful molecular studies, I essentially confirmed his initial observations (Rieseberg, Soltis, and Palmer, 1988). Thus, I came to the conclusion that he was able to see things in plants that escaped less careful or less knowledgeable observers uch as myself. The interest of the botanical community in plant hybridization appears to be somewhat cyclical. From the late 1940s through the middle 1960s, much of the morphological and cytological variation in plant populations was attributed to introgressive hybridization (e.g., Anderson, 1949; Stebbins, 1959). Over the next two decades, these interpretations were viewed with greater skepticism (Barber and Jackson, 1957; Gottlieb, 1972; Heiser, 1973; Doebley, 1984), and I think appropriately so, leading to perhaps less interest in hybridization a d its evolutionary role. In the early 1990s, however, there appears to have been a renewed interest in hybrids, stimulating several recent reviews (e.g., Abbot, 1992; Arnold, 1992, 1994; Rieseberg and Brunsfeld, 1992; Rieseberg and Wendel, 1993; Strauss, 1994). This resurgence in interest has led to a reevaluation of some of the central tenets or dogmas ofhybridization theory (e.g., Rieseberg and Ellstrand, 1993; Arnold and Hodges, in press), as well as to the origin of some new ideas and approaches to the study of hybrids (e.g., Whitham, 1989; Rieseberg et al., 1993). The purpose of this paper is to discuss some general misconceptions that many botanists and zoologists appear to hold about hybrids, as well as to present several of the