The objection raised by bray turns on time scale and the lack of ability to test interbreeding between ancient ancestors and living descendants. However, there is a phenomenon that shows that small incremental changes in genome can result in different species (though it also in the process calls into question the utility of the notion of species): Rassenkreis (also called “ring species”, though they don’t meet the usual definition of species), as for example Larus gulls and greenish warblers in Asia.
In each case there is a region where the organisms cannot survive (the Arctic Ocean or the Himalayas in the two examples), and traveling around it one finds birds which can interbreed with their neighbors, until one comes to a place where there are two similar sorts of birds which can’t interbreed (two species), though one can interbreed with its neighbors going clockwise and the other with its neighbors going counterclockwise.
bray: Try this on for size: The identification of the Hebrew (and Greek) words in Genesis usually Englished as “kind” with the modern scientific notion of species is a mistake.
But you must understand that the whole field of biological classifications has undergone profound changes with the advent of DNA analysis.
It's not just that whole new categories have been created, others deleted, new relationships confirmed which were previously unsuspected, but the very ideas of what constitutes a breed, or species or genus has changed based on new DNA comparisons.
And I think it's critical to our understandings of what makes different species, genera, family, etc., is the degree, or indeed eagerness, with which different populations can interbreed.
And that in turn is clearly a function of similarities in their DNAs.
To pick out just one example: so far as we know, humans and chimpanzees can not interbreed, period, but evidence shows that humans and Neanderthals did, in fact, occasionally interbreed.
This clearly suggests a range for speciation -- six million years of evolution puts chimps into a different biological family, while a few hundred thousand years may keep Neanderthals within the same species as modern humans.
In short, the "testing" which you say cannot be done, was done naturally, and the evidence for it is found in our DNA.