Animals are classified according to a system established by Carolus Linnaeus (or Linné) in 1758. Each distinguishable form is given a genus (plural, genera) and species name, such as Homo sapiens, Tyrannosaurus rex, and Canis familiaris. The generic name is first, and it has a capital letter. The specific name is second, and it has a lower case letter. Generic and specific names are always shown in italics, or underlined.
Living species are defined according to the biological species concept, as all the members of different populations that naturally interbreed, and produce viable (i.e. fertile) offspring. In practice, of course, taxonomists do not carry out intricate breeding experiments, and they apply the morphological species concept, which defines a species in terms of unique characters. This is close to the phylogenetic species concept, that a species is a small clade of diagnosable geographical forms of the same basic kind. Palaeontologists use the morphological and phylogenetic species concepts.
Species are grouped together in genera, and each genus may contain one or more species. Genera are then grouped in families, families in orders, and so on. This pattern of inclusive hierarchical grouping reflects the splitting pattern of evolution, and the way that evolution is represented in a cladogram. The basic traditional classification of humans is:
Species sapiens Genus Homo Family Hominidae Order Primates
Subphylum Vertebrata Phylum Chordata
'Superphylum' Deuterostomia Kingdom Animalia
Traditional classifications of vertebrates and other groups often include non-monophyletic groups, although these should be avoided wherever possible. The commonest examples are paraphyletic groups, which include only the most primitive descendants of a common ancestor, but exclude some advanced descendants (Figure 2.9(b)). A well-known paraphyletic group is the Class 'Reptilia', which almost certainly arose from a single ancestor, but which excludes some descendants, the birds and the mammals. All members of the para-phyletic group share one or more derived characters, but other organisms, excluded from the paraphyletic group, do too, although they may have acquired other features. So, for example, all reptiles lay a shelled egg (as do birds and basal mammals), but the upper bounds of the group 'Reptilia' are defined only by the absence of characters such as feathers and hair.
The other kind of non-monophyletic groups are polyphyletic, those that arose from several ancestors, and that are characterized by a convergent feature (Figure 2.9(c)). Two examples of polyphyletic groups of vertebrates are the 'Natantia', the classic grouping of fishes and whales together, because they look similar in shape and they swim in the sea, and the 'pachyderms', a group of thick-skinned, greyish mammals such as elephants, hippos and rhinos.
The sorting of characters in cladistics into primitive and derived is an exercise in determining character polarity, in other words, the direction of evolution. The polarity should be made clear by outgroup comparison, and polarity can reverse, depending on the context. For example, in the analysis of deuterostome relationships, absence of a tail is the primitive character state, and possession of a tail is the derived state. In the context of human relationships, however, loss of the tail is one of the synapomorphies of the Family Hominidae (apes and humans).
There are often problems in distinguishing just what are shared derived characters, and what are not: the classic evolutionary dilemma of separating homologies from analogies. A homology is a feature seen in different organisms that is the same in each—it is anatomically and generally functionally equivalent, and shows evidence of derivation from a single source —while an analogy is a feature that may look or act in broadly similar ways in different organisms, but which gives evidence of separate origins. An example of a homology is the wing of a robin and the wing of an ostrich. Although the ostrich wing is not used in flight, its location in the body and its detailed structure show that it is a direct equivalent to the robin wing, and the latest common ancestor of robins and ostriches would have had such a wing. The wing of a robin and the wing of a fly are analogies because their detailed structure shows that they arose independently, even though they perform similar functions. Homologies, then, are synapomorphies, the clues that indicate common ancestry.
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