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Exterior features within these entities are described by Allen's, Bergmann's, and Gloger's rules. "Ring" ones form a genetic difference gradient, which often wraps in a ring around some geographical feature, such as with gulls in the Arctic. One theory about them involves the use of cladograms to determine evolutionary relationships, the phylogenetic concept, while another theory uses Ernst Mayr's definition, the ability and propensity for organisms to produce fertile offspring, the biological concept. FTP, name this unit of taxonomic classification which can be created allopatrically or sympatrically.          |
Exterior features within these entities are described by Allen's, Bergmann's, and Gloger's rules.
{Species}