It's not a case of looking for wiggle room. The evidence is consistent with what we expect to see. Rapid evolution in a few hundred years would agree more with the ideas put forward by some creationists. (The idea is that Noah took relatively few animals on the ark - to get them to fit - and most modern species evolved form there. e.g. there would be one lot of "elephants" and all three existing species, as well as most extinct species, including mammoths and mastodons evolved from that one small group in a few hundred years at most).
Mutation is known to happen and does expand the gene pool. Horizontal transfer (i.e. the transfer of genetic material by mean other than descent) can have a similar effect but it's mainly important to bacteria.