Dollo's law

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Dollo's Law or Dollo's Principle is a hypothesis proposed by French-born Belgian paleontologist Louis Dollo (1857-1931) in 1890 that states that evolution is not reversible. This law was first stated by Dollo in this way: "An organism is unable to return, even partially, to a previous stage already realized in the ranks of its ancestors."[1]. According to this hypothesis a structure or organ that has been lost or discarded through the process of evolution will not reappear in that line of organisms.

According to Richard Dawkins, the law is "really just a statement about the statistical improbability of following exactly the same evolutionary trajectory twice (or, indeed, any particular trajectory), in either direction." [2] Stephen Gould viewed the idea less strictly, suggesting that "irreversibility" forecloses certain evolutionary pathways once broad forms have emerged: "[For example], once you adopt the ordinary body plan of a reptile, hundreds of options are forever closed, and future possibilities must unfold within the limits of inherited design."[3]


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  1. ^ Dollo, quoted in "Evolution: Ammonites Indicate Reversal," in Nature, March 21, 1970
  2. ^ Dawkins, Richard [1986] (1996). The Blind Watchmaker. New York: W. W. Norton & Company, Inc.. ISBN 0-393-31570-3. 
  3. ^ Gould, Stephen J. [1993] (2007) "Eight little piggies," Vintage Books. ISBN 978-0-099-50744-4

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