The study of rare species has long fascinated biologists. Rare and common species may differ in body size, dispersal, reproduction and many other respects. The Biology of Rarity reviews documented patterns of such differences, and considers the methodological difficulties plaguing their interpretation. This book provides an unusually wide ranging picture of how patterns of species differences can be maintained. The authors consider the potential roles of community assembly rules and speciation in biasing the set of species that become rare, of extinction in `editing' that set, and the potential for changes in ecology, genetics and evolution to transform the characteristics of rare species over time. The potential role of phylogenetic comparisons in disentangling these processes is considered, along with potential applications to conservation biology.