The selection and breeding of domesticated animals over thousands of years has provided us with extremely well-documented examples of evolution. These include: the aforementioned descendence of dogs from wolves, as well as the barnyard variety of livestock. Cows, sheep, and pigs all descended from wild animals that were domesticated thousands of years ago. Wild mountain sheep no more resemble domesticated sheep than do wolves resemble cocker spaniels. This same record exists for many types of plants as well. Wheat has been bred to survive in different climates and seasons. Tobacco, corn, and cabbage all have a detailed evolutionary history. This is very strong evidence that species are mutable and can give rise to others.
So we have many pieces of a puzzle which fit together very nicely, but with a few of them missing. But we can see a pretty clear picture. So why are there some people who dismiss this?
A St. Bernard with a Chihuahua. The latter is actually a dog--the same species. Not a barking rat.