#####Yes. But does that mean that, over time, as the process of natural selection plays its role, we should end up with a species that is much stronger (or faster, or something) than its distant ancestors?#####
Absolutely! Natural selection would reasonably be expected to strengthen some existing species, while leading to the extinction of others.
Fine. Now ponder this. Suppose a group of animals, who naturally undergo this process, became separated from the main herd (or pack, or whatever). Permanently separated, perhaps by a big earthquake, or something like that. Now you would have two groups, and they no longer mingle. Each undergoes this "strengthening process" as a result of natural selection.
Now, time passes. A thousand generations. Ten thousand generations. Suppose the two groups were to be compared, perhaps by a traveling naturalists. What would he find?