The role of natural hybridization in the evolution of plant and animal species complexes has been debated at least since the early part of this century. Due to the work of Ledyard Stebbins and Edgar Anderson, natural hybridization as an evolutionarily creative force was considered to be probable in certain groups of plants. However, this viewpoint was largely supplanted during the last several decades by the viewpoint of natural hybridization as an evolutionary deadend. This framework has been the predominant model for most of the recent studies of natural hybrdization in both plants and animals. In the last several years a new interest in the evolutionary consequences of natural hybrdization has led to innovative tests of the underlying assumptions of the hypothesis that natural hybridization creates nothing but evolutionary flotsam. Rather than supporting this viewpoint, these analyses produced results that are consistent with the paradigm put forward by Stebbins and others. Thus, in plants and animals there is now evidence that natural hybridization can lead to both fit and unfit hybrid individuals, and may indeed result in the transfer, or the de novo origin, of adaptations. Future studies of the genetical basis of adaptations and the fitness consequences of natural hybridization will act as rigorous tests of these competing paradigms.

Key words: Adaptations, Introgression, Louisiana Irises, Natural Hybridization