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Professor Anne Magurran, Dr Anna Ludlow, Dr Alfredo Ojanguren and Mr Stephen Russell.
University of St Andrews.

Cute, wide-eyed and unobtrusive, the guppy is the nation's favourite pet fish. But behind this innocent exterior lies an animal with a torrid and tangled sex life, according to a team of researchers from University of St Andrews. They are studying the guppy's fascinating mating behaviour because it can help answer one of the biggest mysteries in biology: how do new species form?

'It really goes back to Darwin and his ideas about how species arise,' says Professor Anne Magurran, who leads the group. Although scientists have developed many theories about species formation, they have only recently had tools, such as DNA fingerprinting, to test them.

The definition of a species is a group of creatures that can breed with each other, but cannot breed and merge with other groups, even if they look similar. This inability to mix, called 'reproductive isolation' is the key to understanding how life evolved to give us the vast diversity of creatures we see today.

The St Andrews team decided to study Trinidadian guppies because they may be in the process of splitting into two new species. The guppies are divided into two groups that have been living in separate river systems for about 2 million years. A fish native to one river drainage never normally meets a foreign fish from the other drainage, but Professor Magurran and her team have been introducing males and females from the different populations in the laboratory to test ideas about how reproductive isolation develops.

One possibility is that female guppies prefer to mate with native males. Males charm females with a courtship dance, and a female will usually pick several of them to father her offspring. But the team found that females do not have it all their own way. Wily males often creep up behind unsuspecting females and quickly mate with them by surprise. This sneaky mating tactic probably ruins the effect of female choice on reproductive isolation.

Another possibility is that in the female's body, the sperm of a native male somehow out-competes that of a foreign male. To test this idea, the team impregnated females with equal numbers of sperm taken from a native and a foreign male. They then performed DNA fingerprint tests - similar to the ones used in paternity tests in people - on the baby fish to find out who the father was and to determine whether native males sire more of a female's offspring.

But what about the offspring of the foreign males? The St Andrews team have found that hybrids are worse at reproducing than the offspring of native males. These unfit offspring are another bar to interbreeding, and it seems that this mechanism is evolving simultaneously with sperm selection to cause reproductive isolation.

Come and watch the guppies' courtship dances at this exhibit, and test your potential as a paternity lawyer by predicting which fish fathered which offspring. The answers will be revealed in DNA fingerprints, and there will be small prizes for the winners.

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