The term 'species' is a word used with wild abandon by science and the mass-media alike. We try to preserve them, when at the same time we're eradicating lots of them. And everyone has their favourite. But the strange thing is no one really knows what a species actually is.
There is no smooth transition between different life forms, but species are certainly distinct entities. However, they're not that distinct; there doesn't seem to be well-defined boundaries between them. Surely scientists must have some idea of what a species is? Well, sort of.
A Brief History of Species
Absolutely ages ago (Ancient Greece ages ago) what is now known as the 'typological species concept' managed to describe what species are, that all individuals of a species are expressions of an underlying 'type'. Each type represented one species and any variation within the species represented imperfect expression of the type. It was thought that the types were fixed and unchanging. Now we like to think we know better...
Zoologists like to use biological species (where isolation leads to divergence and independent adaptation). Geneticists prefer phylogenetic species (genetic sequence data is used to construct evolutionary trees of relatedness). Palaeontologists just love evolutionary species (groups sharing common evolutionary fate through time) while people of a certain logical persuasion think cladistic species is the way to go. There is no real consensus, the idea that comes close to being the accepted definition of a species is Ernst Mayr's biological species concept.
The Biological Species Concept
Put simply, this definition states that a species is a group of interbreeding individuals. If the idea that what makes a group one species is its ability to interbreed, then it follows that the thing separating species is a lack of interbreeding; what is known to biologists as reproductive isolation. Reproductive isolation, in the biological species concept sense, occurs when gene flow between two populations of previously interbreeding individuals ceases, or is severely restricted. Speciation can manifest itself as either allopatric speciation, where physical separation is responsible for the isolation or sympatric speciation, where speciation occurs with the two ancestral species cohabit the same area. Gene flow can stop for countless reasons, such as a change in mating call in a bird species, so that mating recognition systems no longer work, or accidental physical separation and subsequent divergence.
It is probably the most used definition because of the sheer number of zoologists working in the field of evolutionary biology. However, this is fast changing in the post-genomics era1, where vast amounts of DNA sequence information is being poured into the public domain.
Which species concept you choose to use is fairly subjective and often depends upon what type of study you're undertaking and your personal preference.
Parlez Vous Mon Genome?
When thinking about what a species is and how they have come to exist, this Researcher personally finds it useful to compare species with the myriad of languages of human culture.
Language and Species
Just like species, languages are incredibly varied. Languages, like species, have common ancestors; a shared root from which they have evolved and diverged. Isolation seems in both cases to be the key to this divergence: with species a restriction of gene flow is responsible due to some reproductive isolating mechanism. The resultant, isolated populations then go on to independently adapt to their respectively different surroundings, up until a point where their genomes become incompatible and cannot 'understand' one another if they were to come into contact once more. Cultural, political and geographical boundaries have in the past isolated groups of peoples from each other and different languages have sprung up with an obvious common root, as in the Latin-based languages of Europe and the Chinese character-based languages of East Asia. In a huge and complex form of 'Chinese whispers' languages change and mutate from generation to generation, adapting to the current cultural climate the language just happens to find itself in.
One only has to look at English and US English to see how a period of isolation starts to change a language. If technology that allowed the two cultures to come back into contact with each other never existed, would US English have diverged into a new language? Perhaps, but we will never know for certain.
Dialects and Race
Take your average Geordie2 and stick him in deepest, darkest Cornwall3. Then, attempt to get the poor Geordie to talk to the Cornishmen. The chances are that they will resort to sign language and writing things down on pieces of paper in no time, such is the dramatic difference in dialect. Also, many Scottish people on the telephone to many English people will find they can encounter nightmarish communication difficulties. Genetic polymorphism (literally meaning 'many shapes') within a species' population due to random errors and/or local adaptations give rise to a wide array of differences, upon which natural selection can work its magic. Both dialects and races have adapted to where they happen to find themselves in much a similar way, partly through local adaptation and partly via semi-isolation. It is interesting to note that since the arrival of the communications and transport technologies of the 20th Century local dialects are said to be dying out. This increase in the flow of information is akin to an increase in gene flow within a species, and would have an identical effect.
The North Germanic Languages
The North Germanic languages are a diverse set of tongues spoken in the Scandinavian countries of Norway, Sweden, Iceland, Denmark and the Faroe Islands. They all have a common ancestor in the form of an old Scandinavian language called 'North Germanic'. Around the year 800 AD North Germanic started to diverge into 'East' and 'West' variants. East North Germanic resulted in the contemporary languages of Danish, Swedish and Gutnish (a language spoken on the island of Gotland, considered by some to be a dialect of Swedish). The West North Germanic language (also known as 'Old Norse') is split into Faroese, Icelandic, Norwegian and Norn4.
In a similar mechanism that creates new species, the divergence of the Scandinavian languages is a result of political and geographic isolation. The members of the Scandinavian countries can still understand each other fairly well, such is the similarity between them. However, this varies according to just how isolated the countries are. Danes, Swedes and Norwegians can talk to one another with not too much trouble, but the remote island of Iceland has diverged more dramatically and so communication with the other cultures is more difficult. The effect of isolation is so dramatic that Icelandics can still read Old Norse! Compare this with the marsupial mammals of the long-isolated continent of Australia with the mammals roaming the savannahs of Africa. The difference is dramatic.
This supports the idea that a restriction in the flow of information (cultural with languages, gene flow with species) between two populations is responsible for the current diversity among both life and language of humanity.
Continuation of the Analogy
Just how well does the analogy of languages and species hold up under detailed scrutiny? There are some elements which are clearly comparable in both cases, and others which are not:
Both languages and species are becoming extinct at a rapid rate due to mankind's activities.
In the same way as species, a definition of language is also virtually impossible to get a grasp of.
Hybridisation of species occurs naturally, so does languages. One only has to look at the influence of French on the largely Germanic language of English.
The biggest stumbling block with this analogy is natural selection. Often parts of a language are retained for illogical reasons (any person trying to learn English will no doubt testify). Survival and adaptation of a language or elements of a language does not just require fitness. The unnecessary spelling and grammar usage are maintained as a part of the languages written tradition, and are not subject to the harsh survival conditions that life faces daily.
The definition of a language also includes what nation it happens to come from. This, of course, is not the case in species. For example, the 'dialects' of Italian are far more diverse than the 'languages' of Scandinavia, but the former is a single language and the latter is many.
Are species a man-made artefact born of a desire to neatly categorise stuff? To some extent, probably yes. But one cannot really blame mankind for trying, for on the surface of the natural world, life seems to be distinct and neatly organised. It's just when you get down to the hard definitions that species tend to blur.
In this Researcher's opinion a better understanding of species is needed to achieve a real scientific consensus, using human language as an analogy, and even comparative studies would enable a clearer and more intuitive appreciation of the subject. This, in combination with the more objective genetic, genomic and cladistic information gathered about life so far might enable biology to finally put its finger on what a species actually is.