Re: Plausible evolutionary scenario for metamorphosis?

On Sat, 17 Nov 2007 12:22:46 -0800 (PST), Friar Broccoli
<EliasRK@xxxxxxxxx> wrote:

On Nov 16, 8:53 pm, r norman <r_s_norman@xxxxxxxxxxxx> wrote:
On Fri, 16 Nov 2007 16:18:45 -0800, Mark Isaak <eci...@xxxxxxxxxxxxx>
wrote:

On Fri, 16 Nov 2007 10:00:23 -0500, r norman wrote:

On Nov 15, 1:22 pm, pineapple.l...@xxxxxxxxx wrote:
Plausible evolutionary scenario for metamorphosis?
[...]
Metamorphosis is an ancient and very widespread phenomenon . . .

In fact, Ovid wrote a whole book on the subject.

Here, however, is a reference which may be more what you are looking for:

Truman, J. W. and L. M. Riddiford, 1999. The origins of insect
metamorphosis. _Nature_ 401: 447-452.

That's the problem with t.o. --- just whenb the discussion gets good
and heated, somebody goes and injects fact into it. Spoils all the
fun!

A nice review of their work is at
http://www.artsci.washington.edu/news/Winter00/metamorphosis.htm

The problem I have with this is that insect metamorphoses is just one
example. There are rather large changes in life form in a very large
number of different animal phyla. The parasitic flatworms are
notorious for having multiple forms with very different ecologies
(hosts). That hormones are responsible for the metamorphic event
doesn't really explain its evolution.

Just so I am clear here, it is your belief/conviction (whatever) that
the larval stage in insects arose from variation in the juvenile stage
rather than modifications/extensions of the egg stage?

Note that I don't have a problem with this since lose of
function/features from the juvenile stage probably requires
fewer and simpler steps than adding to the egg stage.

Initially I just wanted to insure that pineapple had a simple
reply that he could understand. I wonder if he even read any
of the replies, or has this all been for nothing.


What I am trying to emphasize is that a succession of body forms, aka
metamorphosis, is commonplace in organisms. The details of exactly
how insects do it and how the specialized process of the
holometabolous insects, with larval, pupal, and adult stages is of
much less interest to me although it is of major interest to
developmental entomologists.

In general, in plants and animals the zygote (fertilized egg)
initially grows protected in some form and is provided with nutrients
to support early development in a variety of ways. The protected,
encased early stages of development is generally called the 'embryo'
(yes, plants have embryos, too!). What emerges from the protection
and nutrient supply of the seed case (technical point: all true
'plants' have protected embryos, the embryo develop inside a seed only
in a subset, the seed plants) or from the egg must usually then fend
for itself and therefore be capable of photosynthesis or feeding on
its own. As a result, embryonic development has to produce enough
structure (complexity, if you will) to get the free-living organism
growing. There are special cases where the earliest stages are not
free and are dependent on the mother (almost always) for continued
nutrient and protection. For example, the moss sphorophyte (diploid
stage) is totally dependent on its mother. Placental and marsupial
mammals are other special cases. But in general, the earliest stage
lives independently and separately from its parents. Unfortunately,
it really can't compete very successfully for the same resources as
the older, larger, fully developed, and perhaps even more experienced
and wiser parent and so often takes a very different form living a
totally different kind of existence. This is a main ecological reason
for larval stages being completely different from the adult. Another
is the problem of dispersion. Many aquatic critters that are
completely are relatively stationary or that live in habitats limited
in geographic area have larval stages that float freely in the water
current (planktonic) and so are widely dispersed. That is another
important ecological reason. A third reason, developmental in nature,
is that the earliest larval stages may not have all the adult
structures yet formed. Yet they have to live their lives and so often
have special larval tissues and organs and structures that are then
reabsorbed when the adult form is finally built.

Whether the larval stage is a "reduced adult", an adult that has "lost
features" or is its own "undeveloped adult" that has not yet gained
features is a technical question. Perhaps some evolved one way,
others another. My suspicion, not having research the evolution of
development, is that most animals go through their larval stages as
simply part of the sequence in development: zygote, morula, blastula,
gastrula, embryo, larval form-1, larval form-2, ...., adult form.
Probably many of the diverse life stages of the specialized flatworm
and roundworm parasites are secondary in evolution. It seems that the
earliest insects had a more direct form of development where what
hatches from the egg looks like an adult. Yet it is not an adult and
does not have wings or reproduction organs. No, the earliest insects
probably didn't have wings as adults, either.

Incidentally, the amniotic embryo growing inside the egg shell or the
placental fetus growing inside the maternal uterus is most distinctly
a stage quite different from the newly hatched organism. It has a
different pattern of circulation, of respiration, of nutrition and
feeding, and excretory system with its own specialized organs that are
discarded at hatching/birth. Is the human fetus a "regressed adult"?
No, it is a life stage of its own, part of the normal developmental
sequence.

And, frankly, I don't care whether the original poster is still
around. As long as there are people who seem to be genuinely
interested in metamorphosis, that is all I care about.

.