circular overlapa naturally occurring arrangement of contiguous and intergrading populations which interbreed, curving round geographically until the beginning and end overlap and behave as good species. A species that behaves in this way is called a ring species. An example is the herring gull/black-backed gull species.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005