If you think that the difference between two species involves changes in certain genes, you can swap those genes between the species. (discovermagazine.com)
The animals can serve as mixing vessels where human and bird viruses swap genes, creating new flu strains. (sciencenews.org)
Around 1.6 billion years ago, sex was being invented as a way of improving on bacteria - swapping genes occasionally, the primitive gene - mixing mechanism. (williamcalvin.com)