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I'm curious on how genetically different can two animals be in order to reproduce. Could they belong to different species?

One example is the mule which is the offspring of a donkey and a horse, in this case they share a common ancestor. Putting it another other way: do we know what is the minimum genetic percentage of similarity needed for two animals to reproduce?

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It's not a "percentage similarity" that's needed - it's a compatibility of the sperm's attachment mechanisms and what attachment sites the ovum provides. If all the fusion machinery is compatible, a viable embryo should be able to form, so strictly it should be possible for the rest of the genome to be entirely different. Although of course the higher the overall similarity, the more likely it is the fusion machinery will be compatible. –  Armatus Mar 28 '13 at 18:03
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In addition to sperm and egg mating compatibility, most of the resulting genome combinations would have fatal phenotypes. The odds are against such cross species combinations at least for animals. Plants are much more amenable to crossings as I understand it. –  shigeta Mar 28 '13 at 18:15
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In fact, one of the many definitions of a species is that if two animals cannot reproduce, they are different species. This is kind of simplistic of course, but it is one of the definitions. –  terdon Mar 28 '13 at 18:33
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