The minotaur – the mythical half-man half-bull, conceived when Pasiphae, the wife of king Minos of Crete consummated her passion for an unusually beautiful bull – was probably impotent and almost certainly sterile. We can be certain of this because animals produced by crossing very different species – zonkeys (zebra-donkey), tigons (tiger-lion) and the nameless cross between the cow and the yak are always partly or completely sterile.

In nature this hybrid inviability as it is known doesn’t matter: the definition of a species is that its members will normally only mate with each other. So the fact that the results of such matings would be infertile is not a problem. But speciation, as the process of forming different species is known, is a vital part of evolution and biologists are eager to understand how it takes place.

Genetically programmed behaviour can be the means of restricting breeding to a single species. There are many cases of neighbouring bird species – like the herring gull and the lesser black-backed gull in the UK – that could breed with each other but choose not to. They learn the characteristics of their own species in the nest and use them when they look for a mate. We know this because their mating preferences change if they are fostered by another species.

The need for speciation is perfectly clear. “Species are crucially important” says Bryan Clarke of the University of Nottingham, “the point at which a species becomes separate is the point at which it can evolve idependently”. The principle is straightforward. Genetic advantages are not like money or property – you cannot write a will to say who shall inherit them. You have no choice. You pass exactly half of your genes on to each of your children. Consequently the only way to be sure that your children inherit your genetic advantages is to make sure that your mate – who supplies the other half of their genetic inheritance – has the same advantages. “Speciation makes it possible to protect genetic adaptation” says Roger Butlin of the University of Leeds. “Without it we’d just have a mish-mash of vaguely similar forms.”

Of course neither we nor our genes choose to form a species. It is all done by natural selection which ensures that the best adapted individuals in each generation tend to contribute more to subsequent generations. Computer models of the process predict how different mating behaviours and different types of variation in the environment should lead either to the formation of species or to an increase in variation within a single species. The general rule is that if you and all your potential mates come from a similar environment you will form a species adapted to that environment. On the other hand, if your potential mates come from an area that includes a wide range of habitats, your species may not adapt to any of them.

Since Darwin’s work on the origin of species, islands have been very useful for testing theories of speciation. They provide self-contained capsules of habitat in which evolution proceeds relatively unperturbed. Since the late sixties Clarke has studied the snails on a chain of islands in Polynesia. “Snails are almost ideal for studying interactions between neighbouring species” he says ” they are easy to catch, they are easy to census when they are alive – you simply mark the shells of a number of snails by drilling small holes in them and then record the proportion of marked snails that you find subsequently – and when they die they leave behind a shell.”

Data from censuses of the different species of snail provide important tests for models of species formation. Unfortunately the snails Clarke studies have now almost been wiped out in the wild by a voracious carnivorous snail introduced in order to control a giant African snail that “some nitwit had introduced for food”. However Clarke still has plenty of work to do both on snails kept in the laboratory and on field data that it has been impossible to analyse completely because until recently the available computers have not been powerful enough.

However, although models predict when species should form, they don’t tell us how the process takes place. Butlin is hoping that meadow grasshoppers from the Pyrenees will shed some light on this question. Grasshoppers from the Balkans and from Spain that have been evolving separately since the last ice age come together in a hybrid zone in the Pyrenees. He is looking for evidence that females in the hybrid zone are able to choose mates that will give them fitter offspring. The measurements he has made so far show that the offspring of females who choose their own mates are no fitter than those whose father was selected by the experimenter at random.

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