What is a fish species ?

The formal processes of alpha-taxonomy ensure that species have unique name s and can be identified. No similar process is mandatory for infraspecific variation, so the species is a uniquely important practical term. At presen t, there is little agreement of the definition of a species. In the last 30 years, numerous concepts have been proposed. The nature of fish species is reviewed. Clonal inheritance of nuclear genes occurs in several lineages. Hybridization is frequent, often leading to introgression, which may lead t o extinction of species. Species may have hybrid origins. There is good evi dence for parallel speciation in similar habitats. There are clearly except ions to the cladistic assumption of dichotomous branching during speciation . Sibling species may exist with no discernible niche differentiation. Basic assumptions are violated for the recognition, phylogenetic, ecologica l and some formulations of the evolutionary species concepts. The most sati sfactory definitions are two of the earliest proposed in the light of evolu tionary theory. The Darwinian view is that species are recognizable entitie s which are not qualitatively distinct from varieties. A restatement of thi s concept in genetic terms provides a means of dealing with all forms of sp ecies known in present-day fishes. This modified Darwinian concept is opera ted through the application of fuzzy logic rather than rigid definition. Th is involves a search for discontinuities between species, rather than an a priori definition of how boundaries are to be determined. A subset of Darwi nian species are Mayrian or `biological species', which are characterized b y their demonstrable reproductive isolation from other species. The status of a population as a Mayrian species is a testable hypothesis. Molecular te chniques allow this hypothesis to be tested more easily than previously, at least when dealing with sympatric populations.