Chapter 6: Species Concepts and Speciation Analysis Joel Cracraft Systematic biologists have focused on species concepts because they recognize that the origin of taxonomic diversity is the fundamental problem of evolutionary biology. Questions such as, What are the units of evolution? and, How do these units originate? continue to attract attention. It is likely that most believe the systematic aspects of the problem have been solved to some extent, while the current task is to understand the genetic and ecological components of differentiation, which are often seen as the real mechanisms of speciation. A study of speciation is largely a study of the genetics and evolution of reproductive isolating mechanisms. A new mechanistic taxonomy of speciation is needed before population genetics can be properly integrated with speciation theory. The various modes of speciation should be characterized according to the various forces and genetic mechanisms underlying the evolution of isolating barriers. In terms of the systematic component of speciation, it is now widely accepted, especially in vertebrate zoology, that the units of evolution are "biological" species, characterized by reproductive discontinuity from other such units. Ornithologists have contributed significantly to this view. The interests of classification theory and systematic analysis have long been seen as conflicting. The problem is that our typologically based system of nomenclature cannot adequately deal with speciation phenomena. However, the problem lies elsewhere. Our classificatory system is reasonably capable of dealing with the results of speciation, but speciation analysis is incapable of accommodating the prevailing concept of species. The idiosyncratic nature of the polytypic species concept and the definition of species as discontinuous reproductive units rather than phylogenetic units prevents an adequate assessment of speciation patterns and processes. Speciation can be viewed as the phylogenetic deployment of differentiated taxonomic units through space and time. If reproductive discontinuity does not precisely correlate with this deployment, then any definition of these taxonomic units solely in terms of that discontinuity will logically result in phylogenetic history being reconstructed incompletely or incorrectly.Chapter 6: Species Concepts and Speciation Analysis Joel Cracraft Systematic biologists have focused on species concepts because they recognize that the origin of taxonomic diversity is the fundamental problem of evolutionary biology. Questions such as, What are the units of evolution? and, How do these units originate? continue to attract attention. It is likely that most believe the systematic aspects of the problem have been solved to some extent, while the current task is to understand the genetic and ecological components of differentiation, which are often seen as the real mechanisms of speciation. A study of speciation is largely a study of the genetics and evolution of reproductive isolating mechanisms. A new mechanistic taxonomy of speciation is needed before population genetics can be properly integrated with speciation theory. The various modes of speciation should be characterized according to the various forces and genetic mechanisms underlying the evolution of isolating barriers. In terms of the systematic component of speciation, it is now widely accepted, especially in vertebrate zoology, that the units of evolution are "biological" species, characterized by reproductive discontinuity from other such units. Ornithologists have contributed significantly to this view. The interests of classification theory and systematic analysis have long been seen as conflicting. The problem is that our typologically based system of nomenclature cannot adequately deal with speciation phenomena. However, the problem lies elsewhere. Our classificatory system is reasonably capable of dealing with the results of speciation, but speciation analysis is incapable of accommodating the prevailing concept of species. The idiosyncratic nature of the polytypic species concept and the definition of species as discontinuous reproductive units rather than phylogenetic units prevents an adequate assessment of speciation patterns and processes. Speciation can be viewed as the phylogenetic deployment of differentiated taxonomic units through space and time. If reproductive discontinuity does not precisely correlate with this deployment, then any definition of these taxonomic units solely in terms of that discontinuity will logically result in phylogenetic history being reconstructed incompletely or incorrectly.