Thus in such situations the characters used to recognise the species becomes merely diagnostic, not distinctive. Biologically speaking, these are a connected series of the species in neighbouring areas which interbreed with two end populations which are too phenotypically and geographically separated that they cannot interbreed. These two genetically and phenotypically diverse populations which represent the end populations may exist in the same geographic region, yet due to genetic and phenotypic diversity would not interbreed. As an example, the case of Larus gulls can be taken, the different species of which form a ring around the North Pole. The Lesser Black-beaked Gulls in Siberia form a part of this ring, and although they descend from the same species, adjacent Herring gulls are so different from them that they do not interbreed.
Earnst Mayr's Biological species concept tends to recognise species based on defined phenotypic characters. Mayr defined species as groups of interbreeding populations, which do not reproduce across other species. This builds in a concept of reproductive isolation from other such groups. Particular species specific phenotypic characters or attributes prevent interbreeding with other species. ...Show more