What is a fish species?
Reviews in Fish Biology and Fisheries 9 (4): 281-297. The formal processes of alpha-taxonomy ensure that species have unique names and can be identified. No similar process is mandatory for infraspecific variation, so the species is a uniquely important practical term. At present, 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 to extinction of species. Species may have hybrid origins. There is good evidence for parallel speciation in similar habitats. There are clearly exceptions 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, ecological and some formulations of the evolutionary specie
