Phyletic diversity in higher plants

Abstract

The term “phyletic diversity” is used here to denote the continued co-existence and evolution of separate major phyletic groups which have a similar mode of life (e.g. within the angiosperms). This work attempts to show that an understanding of the factors which determine the degree of phyletic diversity could contribute to understanding the nature and evolution of higher taxonomic categories, such as families. The opposite of phyletic diversity, phyletic uniformity (Fig. 1A—B), should result from unequal evolutionary rates of different groups and repeated adaptive radiations of the most successful groups; the following competition for essential, limiting resources should lead to the extinction of all less rapidly evolving, inferior groups. At least some plant families are shown to have specific adaptive specializations which give them competitive advantages for part of the environmental resources only. This ensures their co-existence and the maintainance of phyletic diversity. The nature of these family specializations is considered briefly. It is shown that physiological adaptations to particular conditions, symbioses which aid in obtaining nutrients and, especially, chemical defence mechanisms could be major components of these specializations and thereby the raison d'être of plant families.