The Genetic equidistance phenomenon was first noted in 1963. The author wrote: "It appears that the number of residue differences between cytochrome C of any two species is mostly conditioned by the time elapsed since the lines of evolution leading to these two species originally diverged. If this is correct, the cytochromes c of all mammals should be equally different from the cytochromes c of all birds. Since fish diverges from the main stem of vertebrate evolution earlier than either birds or mammals, the cytochromes c of both mammals and birds should be equally different from the cytochromes c of fish. Similarly, all vertebrate cytochrome c should be equally different from the yeast protein. For example, the difference between the cytochrome C of a carp and a frog, turtle, chicken, rabbit, and horse is a very constant 13% to 14%. Similarly, the difference between the cytochrome C of a bacterium and yeast, wheat, moth, tuna, pigeon, and horse ranges from 64% to 69%. Together with the work of Emile Zuckerkandl and Linus Pauling, the genetic equidistance result directly led to the formal postulation of the molecular clock hypothesis in the early 1960s.
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