Usually they are infertile, as a consequence of the chromosomes not matching up, meaning meiosis cannot take place. However, in plants, polyploidism can occur: the doubling of chromosomes. This is usually fatal in animals, but in plants it means hybrids can be made to be fertile, and is frequently a force behind plant evolution, as new species are frequent.
Though last year a new species of blue butterfly was observed to emerge by hybridisation.
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