Sorry Grif but you're wrong here actually. There has been a hybridization between a bufo and a rana frog before which shows that species is not always a block. And frogs can hybridize with frogs from outside their own families. Check out the book below. It is one of the few books and in my opinion the best book on hybridization in amphibians. So you can't just flat out say it can not happen. And also its genetic barriers in the egg of the female that occur which causes it to destroy sperm from a non compatible mate and not exactlty differences in genetics that cause species to be unable to interbreed. Some animals from the same species such as hyla are not capable of interbreeding. These barriers pop up through speciation processes as animals evolve and separate from each other. Sometimes the only thing that stops interbreeding is a geographic factor and not an actual incompatibility. Reproduction of amphibians

by Ogielska, Maria