These pages have been left in this location as a service to the numerous websites around the world which link to this content. The original authors are no longer at the University of Leeds, and the former Centre for Human Biology became the School of Biomedical Sciences which is now part of the Faculty of Biological Sciences.
Mayr E (1940) Speciation Phenomena in birds. American Naturalist 74,249-278.
Simpson G G (1961) Principles of Animal Taxonomy New York, Columbia University Press
Wiley E.O. (1978) The evolutionary species concept reconsidered. Systematic Zoology 27,17-26.
Why do we classify animals?
How do we classify animals?
We start off from a series of assumptions:
1. That all animals are not the same
This assumption is built into our view of the world. In modern terms we would probably argue something like this:
First of all, we are told that no two individuals are alike except identical twins. Identical twins, we are told, are derived from a single egg which splits at some point in the developmental process to make two individuals with the same set of genes, the same DNA. Any differences between identical twins must therefore be due to differences in their environments. Twins are usually brought up together, and so usually share an environment. Differences are maximised if the pair is split up at some point and brought up separately: this may maximise differences in things like IQ.
We can see that individuals who are not identical twins will have less similarity to each other, less genes in common: brothers and sisters will be similar, so will parents and offspring. Uncles and nephews, aunts and nieces, first and second cousins will all share less and less similarities. Beyond that we might suppose that English and Americans, Australians and Chinese, African bushmen and Esquimaux might be less similar.
But we all form part of a recognisable group. The best test of group membership is this: any two of us in this room of opposite sexes could breed with each other. But there are different probabilities of any two of us mating. There is a low probability of any female here finding me sexually attractive. Amongst the rest of you, well, look around and write your own list. There is a lowish probability of any of us mating with an Australian Aborigine or a Bushman or an Esquimo, because we don't meet many. But this probability is increasing with cheap travel, and were we to do so we could produce fertile offspring. Man is a single breeding group.
At another level of difference we recognise that all of us are more like each other than any of us is like a chimpanzee. Although we share 90 odd percent of our genes with chimpanzees there are major differences. Man is one kind of animal, the chimpanzee is another. There are great similarities, but also differences. Man and chimpanzee do not interbreed, perhaps because of inappropriate mating behaviour (which is a posh way of saying they don't fancy each other). 'I couldn't fuck a gorilla' ( Dr Hfaar, The Man with 2 Brains).
Often it is difficult to see what this difference is. Animals living in the same bit of countryside and apparently similar to us often do not interbreed, for reasons best known to themselves.
So we see a continuum, man, all of whose members are different from each other, but are all really the same (on the breeding criterion) separated from another continuum, chimpanzee all of whose members are also similar to each other but all really the same. Classification is splitting these continua apart: sometimes this is easy, sometimes not.
We can classify animals that are more and more different at other levels, and the more different they are the easier it becomes; sheep and pigs are more different from cows than, say, African cows are from British ones. Birds are more different from pigs than sheep are. And so on making up a series of groups of animals which are less and less alike. We thus make up a pyramid of different animals with different amounts of similarity. Eventually we get to a stage where our animals are very unlike each other, lets say a fish, an earthworm, a sea urchin and a sponge. These are all animals, but as different as we can get.
A decent zoologist will tell us that man, chimp, sheep and fish are really pretty similar: all have a single dorsal nerve cord, notochord, gill slits in their pharynx, are bilaterally symmetrical, triploblastic (that is develop from ectoderm, mesoderm and endoderm) have a well developed digestive tract, sense organs, sexual reproduction.
2. That the differences are real and important.
Well they probably are real: but this is really a philosophical question. Man, for some reason is a classifying animal. Imagine a beach in India; on the beach sits a man. What is he doing? He is sorting grains of sand into piles. He is classifying. So is a stamp collector, so is a Zoologist. This is what we all do when we look at objects. We take a small subsample which we imagine to represent the whole (he can't look at all the grains on the beach), we establish criteria (shape, size, colour) and we sort the objects into piles. Is this important? It may be very important to the man: he might be looking for oil bearing strata or industrial diamonds, he might be looking for God or Enlightenment, he might be looking for a new meaning to his life. It probably isn't important to the sand, because he probably isn't using the criteria that the sand would use. So when we classify animals we should also ask if the classification is meaningful to them as well.
Classifying things is something we do. We write lists, collect stamps, and trainspot. We endow and build museums and art galleries. Different rooms at the Tate have French impressionists, the English school, postmodernists. I don't know why, but we do. One level, of course, this is just administrative convenience. A telephone directory (OVERHEAD) would be useless if the names were inserted at random, or classified by phone number. On another level, however, we split things into categories because we think they are really, fundamentally, different. (OVERHEAD)
Classifying animals and plants
Animals have been classified by type since the time of Aristotle. Aristotle recognised the unity of plan (fundamental similarity) within groups of animals and built up a classification based on shared features and correlations - animals with tusks lacked horns and vice versa. His groups or phyla were (OVERHEAD)
Testacea (molluscs, echinoderms, ascidians.
All were grouped again as sanguineous or exsanguinous on the presence or absence of red blood.
This classification lasted until the Renaissance: pre Renaissance scholars followed Aristotle rather than looking for themselves
'when an argument arose as to how many teeth the horse has, one looked it up in Aristotle rather than in the mouth of the horse' (Mayr).
'The professor of medicine would recite Galen while an assistant (surgeon) dissected the corresponding parts of the body. This was poorly done, and the oratory and the disputations of the professors, all of them merely interpreting Galen, were considered to be far more important than the dissection' (Mayr).
Now Aristotle's ideas, and other pre-Darwinian schemes were based upon explanation of the different phyla as modifications of a single morphological archetype - which is subtly different from a common ancestor. Without the fossils there was no expectation that things had ever been different from what was seen in the current world. No one ever asked themselves, apparently, if the archetype had ever existed, and why it wasn't still around.
So Biological classification makes no claims about evolutionary relationships. It simply involves ordering organisms into groups on the basis of their anatomical similarities or differences. The system still in common use is the Linnean hierarchy and dates from 1758 and the 10th edition of Linneasus's Systema Naturae.
The Linnaean hierarchy is a pyramidal structure where each higher category includes a nest or set of one or more subordinate categories. At the minimum there are seven levels, at the maximum 20 or more (OVERHEAD) (OVERHEAD).
A taxon (Gk taxis = arrangement) is a group of organisms at any level. and taxonomy (Gk nomous = law) is the study of the arrangement of these taxa. There are extremely boring rules set out in the International Code of Zoological Nomenclature which govern the way in which names are formed (families end in -idae, subfamilies in -inea). Name bearing types (e.g. a type specimen, which we can keep in a box in a museum and refer to) make sure that we are always talking about the same animal or group of animals.
The code also specifies that all animals have a surname and a given name: the name of their Genus followed by the name of their species. The Genus name always begins with an upper case letter and the specific with a lower case one e.g. Homo sapiens.
That is all perfectly splendid for stamp collectors, but now comes the difficulty. First of all what is a species: what is a genus: what is a family, what is an order etc.? Species is generally OK. We use the definition of Mayr (1940), which corresponds to the way we were thinking earlier. He talked of biospecies;
'groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such species' (OVERHEAD).
This is only applicable to contemporaneous living organisms, animals capable of breeding now, and has been criticised for not specifically addressing the evolutionary duration or time depth of a species. Simpson proposes evolutionary species;(OVERHEAD)
'an evolutionary species is a lineage (an ancestral-descendant sequence of populations) evolving separately from others and with its own unitary evolutionary role and tendencies'
A little later Wiley suggested (OVERHEAD)
'...a single lineage of ancestral descendent populations of organisms which maintains its identity from other such lineages and which has its own evolutionary tendencies and historical fate'
These definitions of species all have a problem when applied to extinct organisms. any inference about breeding habits in extinct species must be based on morphology. Simpson suggested that if the ranges of variations of two extinct groups overlapped then they were probably a single species and if they did not then there were two species. Things are not so simple however: non interbreeding species with identical morphologies are known from living animals. If you wonder about the importance of this ask yourself ' Was Neanderthal man a different species from us?' If the answer is yes then Neanderthals could not have contributed to the modern gene pool.
So at the species level we sort of know what is what. Once we get above species level things get worse. They get worse for two reasons
1. Similarities are not all similar for the same reason (OVERHEAD)
Types of similarities
Some anatomical and biochemical similarities arise because they were present in a common ancestor. These are known as homologies or homologues (homos Gk = the same). The wing of a bat and the human forelimb (OVERHEAD) are homologous, thus similar because of their common ancestry. They do not look alike because they have different functions.
The wings of bats, birds and insects are homoplastic (GK plasis = moulding) because they do the same thing: structurally they are very different. This is referred to as parallelism, convergence or analogy according to how closely related the animals are, parallelism in near neighbours = convergence in more remotely related animals, analogy in animals so different that it probably happened twice (like bird and insect wing). Placental and marsupial mammals often resemble each other (OVERHEAD) because they do the same sort of thing. Mimicry occurs (particularly for some reason in insects, perhaps because there are so many of them) where one species gains advantage by looking like another (wasp stripes on harmless flies etc.). The last category is chance.
2. The size of similarity is not measurable (OVERHEAD)
Size of similarity.
There are no rules about how dissimilar things at a particular taxon level are. Quite obviously we cannot measure this, and it would be ridiculous to assume that animals (or plants, or fungi or protistans) which are in different orders are equally different. We can make a reasonable case for living species, because we have a measuring rod, but the rest of the structure is very rocky until we get up as far as Kingdom and Phylum. Kingdom splits off things that seem very different, animals and plants, fungi and bacteria and seems reasonable in view of the huge differences in biochemistry, physiology, genetics etc. that we see. Within the animal kingdom phyla split off grossly different sorts of things which are similar to each other in being animals (i.e. in not being bacteria, plants etc.) and in being multicellular but are very different in the way they are put together. This seems a good intuitive split, and I shall assume that it is real when we come to look at animals a little later.
Why do we classify animals?
But beneath all this is something else. We classify animals because we believe that there is some reality in what we are doing: we believe that animals are related to each other, and many of us believe that classifications represent evolutionary relationships.