Re: NANFA-- molecular clocks and Pteronotropis

Joshua L Wiegert (
Mon, 23 Feb 2004 11:48:55 -0500

> There's a lot of disagreement about whether molecular evolution is
> clocklike, and it's clear that the rate varies considerably among gene
> regions and among taxonomic groups, and may not always behave clocklike
> you need it to! What you're likely to get is an estimate that has a
> confidence limit larger than the estimate... but just for fun, let's
> that it does work...

Don't get me started on molecular clocks, Dave. :) I have to strongly
agree with the side that says molecular evolution is not clock-like....
Basically, the argument for molecular clocks states that the mutation rate
for organisms is more or less constant, as long as the organisms are
similar. I.e., if we monitor molecular mutations in human beings and
determine that in the past 1000 years, the genes have changed 0.001% (I'm
making these numbers up for the sake of example), then we have a rate of
0.001% per thousand years. If we are molecularly eight percent different
from chimps, then we split 8,000,000 years ago. (Hey, that's actually
pretty good for numbers I just made up!) This can be determined by
8%/0.001% * 1000 years.
Now, sometimes this seems to work -- and sometimes it just don't. For
instance, measuring molecular clocks in African Cichlids of Lake Victoria
indicates they invaded the lake ... before there was a lake. Those
scientists who believe strongly in molecular clocks stand quite firmly in
their belief, claiming that the lake did not dry up, that there were
refugia, or what not. However, geologists have determined rather pointedly
that the lake did dry up. Without getting into details, its hard to believe
in molecular clocks when one of the most common uses (The Victorian
Question) seems to overestimate the timescale by factors between two and
Everything we understand about evolution states that it may jump in leaps
and bounds. Ken Kaneshiro's work on Drosophilids demonstrates that its
possible for speciation to occur overnight (almost literally!). Molecular
clocks assume that gene mutations are like pebbles collecting on the bottom
of a hill. If you count the pebbles on the bottom of the hill, monitor the
number of pebbles falling in the course of a day, and do a little simple
arithmetic, you can determine how long pebbles have been rolling down the
hill. The only problem -- every so often a tiny pebble starts to roll
downhill and sets off a rockslide.
IMNHO, molecular clocks just should not be used right now. I think in
the future, eventually, we may be able to determine things with a bit more
accuracy, as we understand cascading mutations, linked mutations, and the
like. But, right now, they're simple not accurate enough to be used.
Certainly they should not be abandoned as a science -- nor even those who
work with them or use them derided -- but it has to be understood that (for
now) they're not sensitive enough to be used.
Haven't completely forgotten about the lampreys, btw, will ask about
refs. for that today from at least two people I've heard stories on lamprey
attacks from.


Joshua L. Wiegert
AIM UID: JoshuaWiegert                     ICQ UIN: 276060292
Feel free to contact me by any of the above means for any reason.
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