I thought that ring species could include more than two species, but the canonical ring species is an unusual geographic distribution that demonstrates a snapshot speciation. A ring species occurs when two populations of a species separate to go around a barrier, are lucky enough to find habitat all the way around the obstacle, and move slowly enough that by the time they meet on the other side they are no longer interested in mating. You end up with what we would call two species if they were isolated, but they are joined by a string of populations with transitional features and a gradient of characteristics and genetics between the two. Except at the isolated ends, it’s impossible to draw a line between the two.
This one gets a good word from Action Bioscience:
To learn more about speciation, a fascinating, very understandable, and very enjoyable read is Frogs, Flies, and Dandelions. Speciation — The Evolution of New Species by Menno Schilthuizen. This book gives an excellent introduction to the major theories regarding speciation and summarizes many recent findings