Several questions on evolution, and mutation (rate)

Gerhard Steiner GSTEINER at zoo.univie.ac.at
Fri Aug 23 02:33:33 EST 1996



> >1- How does someone determine how long it would take for a series of 
> >mutations to occur (all being presumably favorable mutations)?
> 
> In my opinion we are nowhere near being able to pose this question for a
> structure such as the vertebrate eye.  How many mutations *does* it take
> to make an eye?  It depends, critically, on how much of the code was
> created more or less *de novo* and how much was cribbed from other
> applications.  If a lot of the stuff needed to make an eye is already 
> implicit in the genome, serving other functions, you might be able to
> make an eye very easily indeed.  Need a muscle to focus the lens?  Just
> tweak the muscle that closes the eyelid.
> 
> To know how probable the eye is, we would have to know how many
> different ways you could use a reasonable number of mutations to 
> adapt the pre-existing code into an eye.  But since we don't know
> how many ways there are to make an eye (at least three, as another
> poster points out) or what the details of the pre-existing code were,
> there is no way to make a meaningful probability estimate.
 
> You could get a start by identifying all the eye genes, and then asking
> where they seemed to have come from--do they have close relatives in
> other systems, or are they new genes?  How different are they from their
> neighbors?  We have not done this yet, except for a very few genes.
> The genes that make human red-detecting and green-detecting pigment
> are siblings--about half a dozen mutations were needed to change from
> bichromatic to trichromatic vision.  But eyes are old, and some of the
> evidence may be lost in history.
> ...
> In my opinion we cannot make meaningful estimates of the probability
> of evolution for any complex structure, much less all of life; we
> just don't know enough.  We certainly can't say with any confidence that
> it was too improbable to happen.

For recent ideas on meaningful estimates on probabilities and 
necessarities take a look at:

Nilsson, D.-E. 1994. Eyes as optical alarm systems in fan worms and
ark clams. Philosophical Transactions of the Royal Society London B
346:195-212.
.... and refs therein.

Osorio, D. 1994. Eye evolution: Darwin's shudder stilled. Trends in
Ecology and Evolution 9:241-242. 

Salvini-Plawen, L. 1982. On the polyphyletic origin of
photoreceptors. In: Visual cells in Evolution. 

Cheers 

----------------------------------------------------------
Gerhard Steiner 
Institute of Zoology
University of Vienna           Phone: +43 1 31336 1299
Althanstr. 14                      FAX:   +43 1 31336 778
A1090 Vienna, Austria
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