Journal of Molecular Evolution

, Volume 13, Issue 1, pp 27–34

Are evolutionary rates really variable?


  • John H. Gillespie
    • Department of BiologyUniversity of Pennsylvania
  • Charles H. Langley
    • National Institute of Environmental Health Sciences

DOI: 10.1007/BF01732751

Cite this article as:
Gillespie, J.H. & Langley, C.H. J Mol Evol (1979) 13: 27. doi:10.1007/BF01732751


Langley and Fitch (1974, 1976) have shown that the pattern of nucleotide substitutions in proteins is inconsistent with a Poisson process with constant rate. From this they conclude that the rate is temporally heterogeneous. It is pointed out in this note that a process which is temporally homogeneous but not a Poisson process is compatible with the data if the coefficient of variation of the time between substitutions is around 1.63. Furthermore, theoretical analysis of samples from neutral phylogenies shows that these samples should not appear to be samples from a Poisson process, but should deviate from a Poisson process in the same direction, though perhaps not to the same extent, as do the data.

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© Springer-Verlag 1979