PREADAPTATION AND MULTIPLE EVOLUTIONARY PATHWAYS

The origin of radically new structures has always been one of the most fascinating-and one of the most puzzlingaspects of the study of evolution. Even today, when the broad outlines of speciation and of modification in existing structures has been well established, understanding of the factors involved in the appearance of radically new structures (and with them, new taxonomic groups) is, at best, confused. Many of the unsolved problems of macroevolution 2 stem, I believe, from a failure to appreciate the full implications of two evolutionary principles-preadaptation and multiple pathways of evolution. I wish in this paper to discuss these concepts in the hopes of at least partly clarifying their operational mechanism and their significance in macroevolution. Although I place great importance on these concepts, I must emphasize that I do not believe that they provide the solution to all the problems of macroevolution. Furthermore, I am aware that I omit many important phases of macroevolution, but I have purposely