The notion that species are kinds of organisms delineated not by the decisions of the classifiers but by the reproductive behaviors and results of the organisms themselves is an old one. Modern views of species are widely known and discussed except for the ways that the most well-known reproductive isolation concept, the class of which we shall call for brevity isolation concepts which is of course Ernst Mayr's biological species concept. To a greater or lesser degree, it has been a component of nearly all species concepts since Linnaeus and even now, it is a key component of phylogenetic species concepts and most other operational definitions. Evolutionary species concepts derive from attempts to deal with the time dimension, something that the biological species concepts of Mayr and Theodosius Dobzhansky tend to avoid—for them species exist at a given time horizon, and over evolutionary time, of course, they can change and split.