changes in colouration of the oriole cline across geographic range
In biology a cline refers to a continuous spectrum of species or subspecies as they exhibit gradual phenotypic or genetic difference over time and geography (Wikipedia, 2013)
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The term cline was first used in 1938 by evolutionary biologist Julian Huxley. Some evolutionary biologists feel that paleotology should socus on clines and not the distinction of species or subspecies since morphological changes are the basis of most of our knowledge of human evolution.
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Image from http://itc.gsw.edu/faculty/bcarter/histgeol/paleo2/pbgeog3.htm