Here's one link.
http://www.actionbioscience.org/evolution/irwin.html
googling on "ring species" nets many more articles.
Since wolves can breed with some dogs speciation is not complete. That's why I asked you about fertility rates.
Lions and tigers, horses and zebras also have some interbreeding potential.
The definition of a species is a population that, in nature, does not breed with another.
As they can interbreed, I wonder if everyone here accepts a lion evolving into a tiger, or vice-versa.
That is not actually true. The ability to interbreed is often used as in indication but is not a determining factor. Ring species and asexual organisms demonstrate that the ability to interbreed is not an acceptable criteria.
In reality, there really is no such thing a concrete line that defines a "species". Evolution predicts that the the concept of a species will be murky and, in fact, it is.