circular overlap

circular overlap

a 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.
References in periodicals archive ?
amplus may represent one divergent end of a ring species demonstrating circular overlap.
254-255) proposed that these two forms may be extremes of a single ring-species demonstrating circular overlap.
It is curious that Mayr should include circular overlaps as examples of geographic speciation given that a complete chain of intergrading populations is nowhere extrinsically separated.