ABSTRACT
This chapter begins with a quick review of the history and current variety of species concepts. It is clear that chaos among differing ways of viewing species has to do at least partly with the fact that the evolutionary processes operating in different branches of the tree of life really are different, and thus specialists in different groups of plants, animals, and fungi have rightly emphasized different criteria when lineages are diverged enough to be called species. Assuming we are going to keep the traditional species rank, a pluralistic approach would be needed in ranking criteria, as argued by Mishler & Donoghue (1982). Many types of causal processes influence the cohesion and divergence of lineages; Mishler (1985) brought in developmental constraints as an underappreciated process acting in the divergence of what have been called species. Mishler & Budd (1990) emphasized the study of asexual organisms, a natural experiment illustrating that bonds of interbreeding may not be an important process in the cohesion of what have been called species.
