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1. Identification and taxonomy, classification and phylogeny
The main service which the present day world expects of its systematists remains . . . the speedy and reliable identification of organisms.
R.A. Crowson, 1970
Discrimination and identification have value beyond the obvious separation of edible from poisonous, valuable from worthless, or safe from dangerous. This is a means to gain an appreciation of the richness of the environment and our human place within it . . . We start to understand our history by seeking to collect and classify.
Richard Fortey, 1997
1.1 What is the use of taxonomy?
The subclass Acari – the mites – contains about 45 000 species that have been formally named and described. This is a small percentage of the total global diversity of mites, estimated to be between 540 000 and 1 132 000 species (Walter and Proctor, 1999), making it the most diverse group of arthropods after the insects. The science dealing with the study of mites is called Acarology. To make sense of the enormous diversity of living organisms a system of description and ordering is required.
Taxonomy (literally, the naming of taxa, or groups of phylogenetically related organisms – subspecies, species, genera, families and so forth: see Table 1.1) is the science that deals with the recognition, description and defining of organisms. It involves providing taxa with an ‘identity’ that allows them to be recognised, hopefully in a reliable and repeatable manner. For practical purposes, the identity of a species is defined by comparing it with related species and by characters that are unique to that taxon. In the majority of animal taxa, and especially arthropods, such characters have been mostly morphological ones because traditionally the vast bulk of taxonomic work was done using dead specimens from museum collections. However, characters based on behaviour, ecology, biochemistry, gene sequences, protein characteristics and biogeography are also used by taxonomists to great effect. Nevertheless, most newly described species are defined by morphological differences between themselves and previously described species, and are referred to as morphospecies. The morphospecies represent the taxonomist’s ‘first cut’ in terms of accuracy of definition. A single morphospecies may, on closer investigation through the comparison of different populations of that morphospecies, turn out to contain several biological species, not separable by morphological differences but with unique characters of behaviour and biology and, if sexually reproducing rather than parthenogenetic, only capable of producing fertile offspring by mating with other members of the same biological species. So, the definition of species at a higher resolution than morphospecies requires the taxonomist to make detailed observations on the life history and biology of live populations. An example of such a study on dust mites is that showing a lack of interbreeding of populations of Dermatophagoides farinae and D. microceras by Griffiths and Cunnington (1971).
Definitions of taxonomy are numerous and some include taxonomy and systematics as separate but overlapping activities, others do not. Systematics involves the study of the diversity of organisms and their phylogenetic relationships: how they are related through evolutionary history. Taxonomy supplies the data for studies in systematics and phylogeny. I will try to explain how taxonomy works in practice, as well as to attempt a definition. It is important to state at the outset that taxonomy provides the basis for the identity of species. Its practitioners seek to separate and characterise species, even if they are morphologically very similar. Thus, when operating effectively, taxonomic procedure provides scientists in other disciplines with as much assurance as possible that they are studying a single entity and not a complex of species. Why is this important? Imagine studying the allergens of what had been thought of as a single species of dust mite, but which turned out to be two following a taxonomic investigation. Suppose they have specific allergens and their distribution and biology are different? The result would be that one would draw inaccurate conclusions about the clinical importance of each species; how many people are exposed to it and in which centres of human population, with all the ensuing consequences for the management of allergic reactions caused by those species. This situation has happened, to a limited extent, with at least one pair of dust mite species (Dermatophagoides farinae and D. microceras), as we will see later, and has caused some confusion.
Table 1.1 Classification of the grain mite Acarus siro Linnaeus, showing major categories of the taxonomic hierarchy. (Note that not all categories, or taxa, have common names. The ordinal-subordinal classification of the mites is currently unstable: the Astigmata has been proposed to have been derived from within the oribatid sub-order Desmonomata and some of its characters are shared with this group of oribatids (Norton, 1998).)
Category of classification (taxon) | Scientific and common name (in brackets) of taxon, and important defining characters |
Kingdom | Animalia (animals, i.e. those multicellular, heterotrophic organisms that develop from a ball of cells – the blastula). |
Phylum | Arthropoda (arthropods, i.e. those animals with external skeletons and jointed limbs). |
Sub-phylum | Chelicerata (i.e. those arthropods with chelicerate mouthparts and no antennae). |
Class | Arachnida (arachnids, i.e. those chelicerates with eight legs and a body divided into two distinct regions). |
Sub-class | Acari (mites, i.e. those arachnids with chelicerate mouthparts plus a subcapitulum, with reduced segmentation of the posterior body region, and with a six-legged larva). |
Order | Acariformes (i.e. those mites with leg coxae fused to the body, anisotropic setae, a dorso-sejugal furrow and anamorphic postembryonic development). |
Infra-order | Sarcoptiformes (i.e. those Acariformes with a toothed rutellum, prodorsal differentiation and no solenidia on tarsus IV). |
Sub-order | Astigmata (i.e. those Sarcoptiforms with lateral glands and reduced setation of the opisthosoma). |
Superfamily | Acaroidea (i.e. those Astigmata with a clear propodosomal and hysterosomal division). |
Family | Acaridae (i.e. those Acaroidea with solenidion ω1 at the base of the tarsus and usually with a rectangular prodorsal shield). |
Genus | Acarus (i.e. those Acaridae with 12 pairs of notogastral setae and solenidion σ1 on Genu I more than 3 × longer than σ1). |
Species | Acarus siro (i.e. that species of Acarus with dorsal setae d1 not more than 2 × length of h1 and with setae d1 or e1 no longer than the distance between its base and the base of the seta immediately posterior to it). |
E.O. Wilson in his autobiography, Naturalist (1994), makes clear the importance of identification and taxonomic skills in the armoury of the evolutionary biologist:
If they are also naturalists – and a great many of the best evolutionary biologists are naturalists – they go into the field with open eyes and minds, complete opportunists looking in all directions for the big questions, for the main chance. To go this far the naturalist must know one or two groups of plants or anim...
