The second point was confirmed by watching 2, 348 matings between flycatchers in the overlap zone.

ECONOMIST: Evolution

From these matings, they generated 100 hybrid lines, each with a different combination of genes from the parents.

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As the eggs produced by these matings were laid, they were removed and analysed for their testosterone content.

ECONOMIST: Animal behaviour

The fourth point was confirmed by looking at the results of natural matings.

ECONOMIST: Evolution

Hybrid offspring from matings between engineered males and wild females are fine because they have one A and one B.

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Dr Saetre reckons that if the birds had been mating at random, almost 14% of matings should have been across species.

ECONOMIST: Evolution

But evolution sometimes works at one remove, and when the researchers looked at matings which themselves involved a hybrid bird they found that only 25% of the eggs were fertile.

ECONOMIST: Evolution