Have two previous species ever evolved convergently enough to successfully mate?
A well-known example are the donkey and the horse, which will produce a mule.
I watched a documentary yesterday about the Sukhumi Institute of Experimental Pathology and Therapy, which was legendary in Soviet times but it declined during the 1992-1994 Georgian-Abkhaz war (I'm not sure how they came through through the more recent wars).
Their lead scientist Ilya Ivanovich Ivanov was interested in interspecific hybridization and was quite successful. He apparently interbred zebra x donkey, wisent x cow, antelope x cow, mouse x rat, mouse x guinea pig, guinea pig x rabbit, rabbit x hare, etc.
Ivanov also tried make http://www.ncbi.nlm.nih.gov/pubmed/...el.Pubmed_DefaultReportPanel.Pubmed_RVDocSum", by artificial insemination (both ways around!), but never succeeded. There are research groups working hard today to make human hybrids with other species, but of course ethically these hybrids will never be allowed to develop into a fetus (the idea is to isolate stem cells from these hybrids).
Here is a text about the EC-funded Chimbrids project, from the University of Frankfurt: http://www.euv-frankfurt-o.de/de/forschung/institut/institut_ize/3_schwerpunkte/3_bioethik_medizinethik/Chimbrids_Zusammenfassung_f__r_IZE-Internetseite.pdf" [Broken]
A recent study showed that coyotes that have interbred with wolves have been able to migrate to the Northeast and fill the ecological niches once held by wolves (see: http://www.scientificamerican.com/blog/post.cfm?id=wylie-coywolf-the-coyote-wolf-hybri-2009-09-23). Of course, it is not a surprise that coyotes and wolves can interbreed because they are very closely related species. I'm not sure if there would be any examples of organisms from very different evolutionary lineages being able to interbreed.
A related phenomenon which might be more plausible could be transfer of genetic material between species that are not very closely related (e.g. between pigs, humans, and birds). I mention these three because they are all able to be infected by influenza viruses and viruses have the ability to transfer genetic material between species (although influenza is unlikely to do so, other families of viruses are better able to do so).
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