The Cichlid Fishes of Lake Malawi, Africa

Abstract of Publication

Turner, G.F. 1999a. 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 species concepts. The most satisfactory definitions are two of the earliest proposed in the light of evolutionary theory. The Darwinian view is that species are recognizable entities which are not qualitatively distinct from varieties. A restatement of this concept in genetic terms provides a means of dealing with all forms of species known in present-day fishes. This modified Darwinian concept is operated through the application of fuzzy logic rather than rigid definition. This involves a search for discontinuities between species, rather than an a priori definition of how boundaries are to be determined. A subset of Darwinian species are Mayrian or `biological species', which are characterized by their demonstrable reproductive isolation from other species. The status of a population as a Mayrian species is a testable hypothesis. Molecular techniques allow this hypothesis to be tested more easily than previously, at least when dealing with sympatric populations.




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