Are there "Identical Twin" plants?

Andrew Dalke dalke at acm.org
Sun Dec 17 02:07:40 EST 2000


>Hammond:  the question then: Is there direct evidence from
>          crop yield and other agricultural data on parthogenetic
>          (asexual-clonal) plants, which indicates a significant
>          VARIANCE in adult size, from which we may conclude
>          that there is such a thing as an "ADULT GROWTH DEFICIT"
>          in these plants (in the actual environment), whereby
>          they are generally always somewhat smaller than some
>          "theoretical genetic maximum adult size"?
>            And further, from the Normal Distribution Curve
>          for the adult size of these (clonal) plants, can
>          we ESTIMATE the "theoretical genetic size" of the
>          plant, statistically.... by for instance assuming
>          that it must be near "2-Standard Deviations" above
>          the mean clonal size?

That question isn't very interesting.  Because of variations in
the environment (soil, water, light, pests, etc.) there will
be variations in size even with with identical genes.  The
largest of these sets a minimum value on the size for that
genotype as a whole.  It is well neigh impossible to estimate
what the maximum size would be for a species, only set a lower
bound.

In other words, it is tautological that all plants must be
smaller than their "theoretical genetic maximum adult size."
You will never find a plant larger than that limit.  At best
all you can do is, as you say, make observations to the
distribution of sizes and apply some arbitray cutoff in
the extrapolation to determine what that size might be.
So the answer to your question '[may we] conclude that there
is such a thing as an "ADULT GROWTH DEFICIT" is "yes" but
it isn't interesting or useful.

The environment does have a big impact on the size.  For
example, some plants would do much better in a CO2 rich
environment compared to what there is now.  Others are
affected by the timings of the rains.  I'll bet even more
are limited by being in a 1g gravity field and would grow
a lot bigger at 1/2 g.  So the environment needed to produce
the "maximum adult size" might not even be available on
this planet and any extrapolation you make must necessarily -
if only implicitly - include that environmental limitation
in its qualifiers.

Why is your concern on the adult plant size?  Wouldn't other
attributes be more important, like ability to produce food
or to reproduce?  Or drought or pest resistance?  It isn't
good to be a big plant if you can't survive seasonal droughts.

Hmm.  Thought of a related issue to point out why I don't
think the question is all that intesting.  Consider viruses.
Nice, simple reproductive factories.  You can apply your
same arguments to them to an even higher degree - after
all, there is a much smaller set of genes needed to make
the virus and hence less possible variation called by
environmental effects on gene expression.

Take HIV as an example virus.  It's an enveloped virus and
the size of the envelope can vary somewhat.  Again, by
definition, any HIV virus can be no bigger than the biggest
possible HIV virus.  Given the distribution of observed sizes
it's a pretty safe bet that we've never seen the largest HIV
virus which has ever been produced, much less the biggest
which could be produced.  Hence, viruses also suffer from
an "ADULT GROWTH DEFICIT"!  (Because of differences in
human cells, perhaps a better choice would be a bacteriophage
virus. I don't know enough about that field to pick a good
example.)

Since your statement can be applied to almost every species
and for every possible variable characteristic, I can only
conclude that it is not a very useful concept.

> I'm not advising you to take a graduate degree in theoretical
> physics either

Too late.  Already did that.  :)

                    Andrew
                    dalke at acm.org








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