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Dollo's law

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.

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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]

References

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