I'd like to point out that physical characteristics are not the only things that define a species and prevent it from mating with other species. Take tarantulas, for example. There are thousands of identified species of tarantulas. While some of these can cross breed with one another and produce sterile offsprings, most of these can't cross breed at all. Yet, they look virtually identical to each other.
This is one reason we cannot say that a species overall have not changed over the thousands or millions of years simply by looking at historical or fossil record. While the physical changes may be minute enough to not be obviously noticed, we cannot say about the molecular and chemical changes that occurred.
I know this is a little side tracked, but I think it is important to keep in mind, especially for creationists, that differences in appearance are not the only ways species are distinct from one another.
Owing to the deficiency of the English language, I have occasionally used the academic jargon generator to produce phrases that even I don't fully understand. The jargons are not meant to offend anyone or to insult anyone's intelligence!