Creationists argue species can and do arise via evolution but are limited to a certain natural limit based on their potential existing genome. The idea is that new information cannot really be added, just modified. So you could have one common cat ancestor as an explanation for all cats today, although I think the argument is there were 2 cat kinds, since the Sabre-tooth tiger was considered to have stemmed from a different ancestor.
This is conceivably true within a short timeframe. A land mammal is not going to suddenly evolve into a fish over a few gernerations, for instance. Ther genetic code is just too different.
What creationists don't seem to understand is that we are talking about
millions of generations, not just a few. Ever played Telephone? Your message is usually pretty distorted at the end of the line from tiny differences along the way. Imagine a game of telephone
millions of people long, that branches at every link (symbolizing multiple offspring). At the end of these chains, how different do you think the messages would be from the original? How different would they be from
each other? This would be an imperfect analogy to evolution (there is no selection, so all species survive), but it does demonstrate the sheer amount of difference that can result from millions of imperfect copies.