Re: What goes around comes around

Preston:

You say that "Common mechanism" is not a hypothesis that is specific enough
to be evaluated, but you agree that it could be tested. In fact, the common
mechanism hypothesis *is used* by evolutionists, albeit on a much more
limited basis. They use it in those cases where it appears it cannot be
avoided, though they have only speculation about the details of why it would
occur.

This is relatively new evidence and we do not understand it very well yet.
What we need are some empirical studies (as you mention) to understand these
inserts better. Lacking that, all we can safely say is that common mechanism
is the obvious explanation for a few cases. There is no theory-neutral
argument (that I'm aware of) for why common mechanism would be limited. You
say:

> Common descent provides a very simple explanation for the overwhelming
> majority of millions of individual events.

Agreed, as does geocentrism for the overwhelming majority of planetary
movements. But this is science, not statistics. When we look at all the
cases, common descent is less simple, as it must draw on a common mechanisms
hypothesis, and we have no compelling, theory-neutral reason to limit that
hypothesis to the cases that common descent doesn't handle.

I agree with your common sense appeal. Could there really be a common
mechanism for all these site and all these inserts? OTH, we can make an
opposing common sense appeal: given all the problems with evolution we ought
to at least understand the nature of these data a bit better. I would like
to see some empirical studies that perhaps could shed more light on why the
common mechanism explanation should be concluded to be quite limited.

--Cornelius

> Cornelius and all,
>
> I have been away from the computer for several days, so hi again.
>
> "Common mechanism" is not a hypothesis that is specific enough to be
> evaluated. I pointed out the paper to show that there is more than a
> handful of evidence that has to be accounted for. (There are somewhere
> over a million sites in the chimp/human comparison. The chimp genome
> hasn't been fully evaluated so the total is uncertain.) Common descent
> provides a very simple explanation for the overwhelming majority of
> millions of individual events. For the "common mechanism" hypothesis
> (independent occurrence in separate organisms) to be taken seriously
> outside of anti-evolution circles, you would have to make a detailed
> proposal for how each subsubtype of L1 and Alu (and other classes of
> transposable elements) could have a complex but well defined spectrum of
> target sites which is different from all the others. You also have to show
> that most of these elements achieved something near saturation of all the
> sites in the genomes that each was compatible with. This is because the
> possible sites would be randomly sampled separately in each species, and
> the observed fraction of total events that is common to the two species is
> high. This could be made somewhat more plausible if there was positive
> selection for all the insertions. There are a few insertions of HERVs that
> seem to have a function, and there is some evidence that L1 elements
> participate in X chromosome inactivation for dosage compensation in
> females. Seems pretty hard to think of a function that would select
> individually for all those L1s and Alus, though. L1s don't code for
> anything but the enzymes necessary for transposition and Alus don't code
> for anything at all. And of course to be taken seriously, all this would
> have to show that your hypothesis made some predictions that were
> confirmed.
>
> There are conceivable ways to test the hypothesis. You might get some
> evidence by studying the genome sequences carefully. (The details are left
> to the student.) :) You can make the master elements transpose in
> cultured cells by forcing expression of the element. There has been some
> work to do this and look at the target sites to see if there is any
> sequence specificity, but I haven't looked at that work in any detail. You
> could pursue this. Maybe the people funding the Discovery Institute would
> fund you. :) Maybe you could get a post-doc in a transposon lab that
> studies this and not tell them what your motivation is. :) Are you
> confident enough of your hypothesis to spend your time and other peoples'
> money?
>
> Preston
Received on Wed Oct 12 03:05:57 2005