Much of it. We have examples in recent history where we can see evolution occuring during our lives. For instance, there were some rabbits that were brought onto an island sometime in the 1800’s. Since they were on an island, they could no longer mix with their friends on land. Within 100 years, these rabbits were no longer able to breed with the rabbits on the mainland, which is one of the hallmarks of a new species.
I also recently read a really interesting article in National Geographic about an experiment they have been doing since the 1950’s in Russia trying to make wild wolves more like house dogs. They didn’t mention whether they could no longedr breed with wild wolves, but it only took 6 generations for these wolves to look and act more like dogs.
Mostly, yes. The basic principle of evolution and “survival of the fittest” is a good theory and it is backed up by experimental evidence. It is a good explanation of the mechanism of how life continues and changes.