Taxonomic ranks and species

2.2.2 Taxonomic ranks and species

A hierarchical system of taxa is currently used in biological classification (Table 2.1). Thus, a phy- lum consists of one or more classes, each of which consists of one or more orders, and so on through less inclusive taxa such as families, tribes, genera

and species. Additional divisions are also often rec- ognized within these by adding a ‘super’, ‘sub’ or ‘infra’ prefix to these ranks. Examples are ‘super- family’ between the order and family level, and ‘subfamily’ between the family and tribe level. An alternative is to slot in less commonly used ranks, such as ‘division’ and ‘cohort’. The hierarchical structure of these ranks is convenient for reflect- ing nested phylogenetic relationships, but their use has resulted in considerable confusion about the reality of rank.

The artificial nature of ranking is immediately obvious when one examines the phylogeny of extant fish order provided in Fig. 2.2. Clearly the orders of fishes are not comparable, either in the sense of relative age as indicated by their relative position in the branching sequence, or in the sense of taxonomic diversity as perhaps indicated by the number of families or species. They are a neces- sary, though uncomfortable, compromise in com- municating concepts of phylogenetic relationship within the framework of traditional Linnaean clas- sification. However, the observation that ranking is arbitrary extends to all taxonomic levels, includ- ing – perhaps disturbingly for many biologists – species. The latter observation is at odds with various practices involved with the tallying of species, usually in the manufacture of indices of diversity; because species are not comparable, such tallies allow, at best, only gross comparison although we continue this tradition below in dis- cussing such things as ‘sizes’ of orders.

The species issue deserves further discussion, if only in respect for the substantial baggage associ- ated with that rank. More so than any other taxo-

Table 2.1 Classification of the Eurasian perch (Perca fluviatilis).

Phylum Chordata Class

Actinopterygii Order

Perciformes Family

Percidae Tribe

Percini Genus

Perca

Species fluviatilis

Phylogeny and Systematics

peiformes Myxiniformes Petromyzontiformes Chimaeriformes Heterodontiformes Orectolobiformes Carcharhiniformes Lamniformes Hexanchiformes Echinorhiniformes Squaliformes Squatiniformes Pristiophoriformes RajiformesCoelacanthiformes Ceratodontiformes Lepidosireniformes TetrapodaPolypteriformes Acipenseriformes Lepisosteiformes Amiiformes Hiodontiformes Osteoglossiformes Elopiformes Albuliformes Notacanthiformes Anguilliformes Clu Gonorhynchiformes Cypriniformes Characiformes Siluriformes Gymnotiformes Remaining clupeocephalans

Fig. 2.2 Interrelationships

among extant fish orders.

Numbers refer to supraordinal

taxa: 1, Craniata; 2, Vertebrata;

3, Gnathostomata; 4, Chondrichthyes; 5, Elasmobranchii; 6, Galeomorphii; 7, Squalea; 8, Osteichthyes;

ontiformes 9, Sarcopterygii; 10, Dipnoi;

11, Actinopterygii; 12, Teleostei; Argentiniformes Salmoniformes Esociformes Stomiiformes Ateleopodiformes Aulopiformes Myctophiformes Lampridiformes Polymixiiformes Percopsiformes Ophidiiformes Gadiformes Batrachoidiformes Lo Stephanoberyciformes ZeiiformesBeryciiformes Mugil Atheriniformes Beloniformes Cyprinod Synbranchiformes Gasterosteiformes Dactylopteriformes Gobiesociformes Perciformes Pleuronectiformes Tetraod 13, Osteoglossomorpha; 14, Elopomorpha; 15, Clupeocephala;

16, Clupeomorpha; 26 17, Ostariophysi; 18, Otophysi;

19, Protacanthopterygii; 20, Neoteleostei; 21, Eurypterygii;

22, Cyclosquamata; 23, Ctenosquamata;

nomic rank, the species rank has been debated and about species. However, there is little consensus usually seen as having a special reality or proper- among biologists concerning species concepts, not ties that uniquely distinguish it from all other even among systematic biologists, the people who taxonomic ranks. For example, species are often routinely describe and revise species. For example, viewed as the ‘building blocks’ of biological diver- Mayden (1997) listed some 22 different species sity, the ‘fundamental units’ of evolution, unlike concepts (Table 2.2), which variously address dif- higher taxa, which are seen as the arbitrary units ferent interests and processes and thus attributes resulting from such evolution. Because of this of biological diversity. For example, the biological view that species have some sort of special reality, species concept places emphasis on the reproduc- many biologists feel qualified to make judgements tive interactions between individuals.

20 Chapter 2

De Queiroz (1998) has argued that there is really only one species concept, that of species as an evo- lutionary lineage, and that the arguments con- cerning ‘concepts’ as listed by Mayden are really criteria, i.e. ways to identify whether or not a group of organisms is indeed a separate lineage. As sys- tematists, we find G.J. Nelson’s (1989a; see also Nelson and Hart 1999) argument compelling that species are taxa and are nothing more nor nothing less. Moreover, we believe that this concept, which places emphasis on pattern rather than process, will ultimately lead to discovery about process. Although some systematists argue for the recognition of subspecies and other intraspecific ranks (e.g. Randall 1998), we for various reasons (some of which are summarized by Gill 1999; Gill and Kemp, in press) argue against the use of in- traspecific ranks, and that species should be the least inclusive monophyletic taxa identifiable. As such, species are hypotheses of relationships that

can be tested and modified by the collection of more data and their congruence as interpreted by cladistic analysis.

We do not expect the reader to agree with the view that species are merely taxa – the smallest taxa identifiable – and that taxa of the same rank are not comparable. Regardless, we do hope that the reader will appreciate that species are not di- rectly comparable given that divergent species concepts are in use. A recent summary of views on the species concept in fish biology can be found in Nelson and Hart (1999).