Genetic tests on bacteria, plants and animals increasingly reveal that different species crossbreed more than originally thought, meaning that instead of genes simply being passed down individual branches of the tree of life, they are also transferred between species on different evolutionary paths. The result is a messier and more tangled \”web of life\”.
There\’s a good reason why at one time the definition of \”species\” was a group that was capable of breeding into fertile offspring.
We\’ve rather changed that definition in recent years/decades: which leads to this particular conclusion.
Donkey/horse crosses are mules or hinnys. Which are almost always infertile. But not quite always. So whether donkeys and horses are actually different species under that old definition is, umm, debateable.
Lions and tigers produce ligers and tigons. Always (I think?) infertile: thus they are different species.
Wolves and dogs produce fertile puppies: they\’re the same species.
This new finding is really a result of the way we\’ve changed our definition of species.