Species are the fundamental unit of classification schemes. What is a species, and how should species be arranged ina phylogenetic classification? The early 20th century British ichthyologist C. Tate Regan (1926) defined a species as “A community, or a number of related communities whose distinctive morphological characters are, in the opinion of a competent systematist, sufficiently definite to entitle it, or them to a specific name”. This practical, but somewhat circular, definition of a species, now termed a morphospecies,does not depend on evolutionary concepts.
In the late 1930s and early 1940s, the first major attempts were made to integrate classification with evolution. Julian Huxley integrated genetics into evolution in his book The new systematics in 1940. In Systematics and the origin of species, Ernst Mayr (1942, p. 120) introduced the biological species concept. To Mayr, species were “groups of actually or potentially interbreeding populations which are reproductively isolated from other such groups”. This was an important effort to move away from defining species strictly on the basis of morphological characters. This definition has been modified to better fit current concepts of evolution: an evolutionary species “is a single lineage of ancestor–descendant populations which maintains its identity from other such lineages and which has its own evolutionary tendencies and historical fate” (Wiley 1981, p. 25).An entire issue of Reviews in Fish Biology and Fisheries was devoted to “The species concept in fish biology” (Nelson1999).