Are there "Identical Twin" plants?

Andrew Dalke dalke at acm.org
Sun Dec 17 13:49:59 EST 2000


I was recently rereading Phil Morrison's "The Ring of Truth".
There was an interesting plot of the fastest bicycle speeds.
There was a slow increase in speeds until about 1972.
Extropolating from that plot you would judge that the best
humans could do is 44 miles an hour.

After 1972 that limit was broken by the discovery of
streamlining.

That's why I don't find your argument interesting.  Yes,
of course there is a limit on sizes and the genetics has
a strong impact on the result.  Realistically there's no
way we can discover that limit because the environment also
has a big impact, and there's no way we can explore all of
the possible environmental effects.

> GH:  Not a problem.  For my purposes the only environment
>     of interest is the "real prevailing natural environment".

There are no more natural environments, excepting perhaps
deep oceanic vents.  Human production of CO2 and other waste
products over the last 150 years has changed the overall
composition and temperature of the atmosphere from its
previous "natural" state.  It is unknown what the effect
of those changes might be in the long term.  It may cause
some species of plants to experience a completely new environment
(eg, higher UV from reduction of the ozone layer or higher
CO2 levels) which instills larger plant sizes even without
changing the genome.

Additionally, if your concerns is only on the limits placed
by natual environments then switch definitions for a moment
and talk about food production.  One of the reasons California
is such a good place to grow food is because there is almost
no rain, so irrigation can be done at precise times.  Some
plants react to drought conditions by putting more effort into
reproduction - eg, bigger strawberries.  By timing the watering
just right you can produce harvest yields which are not
natural.

While this is harvest size and not species size, the point
is that the "real prevailing natural environment" has an
impact on plants.  To determine the "theoretical genetic maximum
adult size" you will have to explore all variations of the
environment - even those which you might otherwise assume are
not natural.

>GH:  interestingly, VIRUSES are probably the only living
>     organism that have actually REACHED their theoretical
>     growth curve.  I say this because you can actually
>     COUNT the number of atoms in a T-4 virus for instance,

That is incorrect for various reasons.  First, I specifically
mentioned enveloped viruses while bacteriophage T4 is
non-enveloped, so we're talking about different viruses.
I deliberately chose a structure where it was easy to
judge that there were differences in size.

Even T4 can have size variations.  Here are a few cases:
  - different mutants may produce different capsid proteins.
      Suppose a glycine was replaced with a tryptophan.  That
      causes the protein to be larger.

  - translation to protein is not perfect.  As I recall,
      the error rate is something like 1 in 100,000 bases
      (but don't quote me on that).  That means you will
      naturally have some variation in the capsid proteins.
      Not enough to affect the xtal structure, which I assume
      is why you think you can count every atom.

  - there can be assembly errors which cause different sizes.
      I don't know about T4, but for tobacco mosaic virus
      I've seen a nice plot showing the effects of temperature
      and pH on structure sizes.

  - AZT has an effect on HIV because reverse transcriptase
      incorporates the nucleotide-like AZT into its copying.
      The AZT doesn't completely act like a nucleotide and
      so inhibits HIV reproduction.  A similar effect should
      be present in bacteriophage T4 - that is, modified
      nucleotides and amino acids present in the environment
      may be included during viral synthesis.  The modified
      residues may be larger than the original, so the overall
      final structure be larger than the average.


>     and every one of them is perfect right down to the
>     last atom

In addition to the above, if I wanted to be picky, there will
be waters associated with the virus structure.  These appear
in xtal structures and are pretty well bound.  I don't know
about T4, but in other systems some even appear to have a
structural consequence - meaning they should be considered
part of the structure.  The number of waters in a virus is
going to vary.  (Again, from xtal structures the occupancy
of some of the water sites can be high or low.)  So the virus
structures are not "perfect right down to the last atom."

If I wanted to be *really* picky, there will still be isotopic
differences in the atoms used to make the structure.  This
causes a very small difference in mass, but still a
difference.

>     A virus is uneffected by NURTURE (i.e.
>     every brick is in place regardless of nutrition).

Wrong.

>GH:  Hmmm.. wouldn't you know I'd run into one of my own,
>     even on a "plant" newsgroup.

Ummm, you've cross posted to bionet.general and bionet.info-theory.

>     it the "Equivalence Principle".  If you didn't
>     know that the Equivalence Principle was the foundation
>     of General Relativity, I suppose you could call
>     Einstein's renaming of gravitational acceleration
>     an "uninteresting tautology".

Einstein provided a predictive framework for gravity.   Same for
Newton before that.  The "unintesting tautology" would
correspond more with the statement "things fall to the earth
because things fall to the earth."

There is no predictive power to your statements which is why
I argue it is neither useful nor even interesting (in the
scientific viewpoint of falsifiability).

                    Andrew
                    dalke at acm.org








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