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The Wiki entry on the evolution of biological complexity states that "[m]utations causing loss of a complex trait occur more often than mutations causing gain of a complex trait". There is no inline citation supporting this claim, nor could I find anything of the sort in the bibliography. So I was wondering whether this is thought to be the case and what key work has been done concerning this claim i.e. major papers either for or against the thesis.

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    $\begingroup$ Many mutations are needed for complex gain of function. Only a tiny mutation is needed for loss. Only one change is needed to break a long chain of synthesis. For example, the chain needed for melanin sythesis - see albinism albinism.org/information-bulletin-what-is-albinism $\endgroup$ Jul 8, 2021 at 23:56
  • $\begingroup$ @PolypipeWrangler While the principle you mention is a good general guide, there are exceptions. For example, a single mutation in a regulatory region can change the location and/or timing of expression of a protein and produce complex changes. See the often dramatic results of mutations in insect Hox genes. $\endgroup$
    – Armand
    Jul 9, 2021 at 4:39

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While I don't have a citation at hand, I don't think that one is needed. The principle follows directly from the notion of mutation in a complex system. Put simply, once you've got a functional complex trait in play, there are many ways to break it and much fewer that can further improve it.

This fact is often mis-interpreted by creationists to claim the evolution of complex traits is impossible. With a population under selective pressure, it doesn't matter if more mutations are negative than positive, because the positive traits produce many more descendants.

To take a current example, the SARS-CoV-2 virus has an important complex trait in the form of the spike protein that enables it to invade human cells. An infected person produces a vast number of copies of the virus, many of which have mutations that destroy the functionality of the spike protein. So those mutations disappear immediately, while the non-mutated or neutrally-mutated copies infect as usual and the rare improvement in efficacy brings us developments like the Delta strain.

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  • $\begingroup$ +1 I think it is worth qualifying what is meant by more often. In a particular individual the loss of trait is certainly more likely than the gain. On the other hand, the direction of evolution as a whole demonstrates acquisition rather than the loss of traits: complex organisms evolved from simple ones, contrary to the creationist argument. $\endgroup$
    – Roger V.
    Jul 9, 2021 at 8:38
  • $\begingroup$ @RogerVadim "Direction of evolution" and "creationist argument" should have some scientific definitions attached if they're to be posted here, I think. This question seem to be simply about the mutation event (based on the context of the referenced wiki quote). $\endgroup$
    – Armand
    Jul 9, 2021 at 19:52
  • $\begingroup$ @Armand I am saying that "more often" is both correct and wrong, depending on what one is talking about. $\endgroup$
    – Roger V.
    Jul 10, 2021 at 0:51

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