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Today I learnt that tigons and ligers are what you get when lions and tigers interbreed?!\\u2019 surprised listener Jamz G tells the doctors. \\u2018What determines whether species can interbreed?\\u2019

Geneticist Aoife McLysaght studies molecular evolution. She explains the modern definition of a species, built on ideas from Aristotle, Linnaeus and Darwin: a species is a group of organisms capable of interbreeding to produce fertile offspring. Hybrids \\u2013 such as ligons and tigers \\u2013 are usually infertile, because their common ancestors long ago diverged into the lions and tigers we know today. However, this definition isn\\u2019t absolute, and there are many ways a new species can be formed.

Hybrids also offer rich study subjects for scientists. Mathematical biologist Kit Yates discusses why he\\u2019s been reading research papers about hebras and zorses (horse x zebra) as their patterns offer insights into how cells spread and develop into organisms, building on a prediction made by codebreaking mathematician Alan Turing.

And it turns out that these hybrids are even more intriguing. As speciation and evolution expert Joana Meier explains, hybrids are not always infertile. Hybridisation can lead to successful new species arising, such as in Lake Victoria\\u2019s cichlid fish, who it seems have been having a wild evolutionary party for the last 15,000 years. And the picture gets even murkier when we discover that modern genetics reveals our human ancestors successfully mated with Neanderthals.

Presenters: Hannah Fry & Adam Rutherford\\nProducer: Jen Whyntie