Good article, which again explains origin of new specie (cataceans) as EVOLVED from previousely existing different ones (in this case - from VERY different land mammals).

I recently learned that despite lions and tigers are considered separate species, they are still "transition" specie, as they can still interbreed being brought together in captivity, and produce hybrid offspring. (This ability to interbreed can probably be lost in the future as tigers and lions continue to change due to natural selection). This ability to interbreed again points on close relationships between such species.

By the way, some hybrid offspring can no longer produce their own offspring by breeding with either parent specie or with itself (like mule = hybrid of horse and Language Removed), and thus Language Removed and horse can also be called "transition" specie - they still interbreed, but the difference in DNA is already slightly more than needed for viable offspring.