Apparently there are some 18(? must check) definitions for 'species'. The creationist's arguments imply that 'evolutionists' only use one of the definitions - or use the one that suits them at the time...
In the former case, it would be interesting to note any examples of speciation for each particular definition.
In the latter case, well, much the same - but if a speciation event by each of the different definitions can be shown, is that not sufficient to prove that speciation does occur via natural selection?
Things to look up:
1) How many definitions of 'species' or 'speciation' are there (How many does evolution claim? How many do creationists claim?)
2) Examples of speciation under each definition.