Definition of Species
One of the more reasonable (or less unreasonable) creationist arguments I've heard is that evolution creates changes within species but cannot split species into others. While I do not believe this to be the case, it seems to me that this argument deserves more attention than those which deny micro-evolution as well, since denying that evolution can produce changes within a species can be dismantled by simply pointing towards the nearest dog.
In any case, it seems to me that the first step towards proving macro-evolution would be to provide a non-controversial working definition of "species". The only such definition I have heard (and I may be seriously out of the loop) is that two animals are of different species if they are unable to produce viable offspring. While this definition works well enough for species whose reproduction works by meiosis, it simply does not apply to single-celled organisms who reproduce through mitosis or binary fission.
Since the easiest form of speciation to directly demonstrate is most likely that of single-celled organisms, my instinct is to next provide a definition of "species" which clearly applies there. Can someone more knowledgeable in these subjects help me in this endeavour?