Biologists and philosophers have long recognized the importance of species, yet species concepts serve two masters, evolutionary theory on the one hand and taxonomy on the other. Much of present-day evolutionary and systematic biology has confounded these two roles primarily through use of the biological species concept. Theories require entities that are real, discrete, irreducible, and comparable. Within the neo-Darwinian synthesis, however, biological species have been treated as real or subjectively delimited entities, discrete or nondiscrete, and they are often capable (...) of being decomposed into other, smaller units. Because of this, biological species are generally not comparable across different groups of organisms, which implies that the ontological structure of evolutionary theory requires modification. Some biologists, including proponents of the biological species concept, have argued that no species concept is universally applicable across all organisms. Such a view means, however, that the history of life cannot be embraced by a common theory of ancestry and descent if that theory uses species as its entities.These ontological and biological difficulties can be alleviated if species are defined in terms of evolutionary units. The latter are irreducible clusters of reproductively cohesive organisms that are diagnosably distinct from other such clusters. Unlike biological species, which can include two or more evolutionary units, these phylogenetic species are discrete entities in space and time and capable of being compared from one group to the next. (shrink)

The phylogenetic species concept is applied for the first time to a major radiation of birds, the birds-of-paradise (Paradisaeidae) of Australasia. Using the biological species concept, previous workers have postulated approximately 40–42 species in the family. Of these, approximately 13 are monotypic and 27 are polytypic with about 100 subspecies. Phylogenetic species are irreducible (basal) clusters of organisms (terminal taxa) that are diagnosably distinct from other such clusters. Within the context of this concept, approximately 90 species of paradisaeids are postulated (...) to have diversified within Australasia. The phylogenetic species concept more accurately describes evolutionary diversity within the family and provides a better theoretical and empirical framework for analysing speciation, historical biogeography and patterns of morphological, behavioral and ecological diversification within this group than does the biological species concept. (shrink)