Alan M Feuerbacher

I have some questions for evolutionary biologists about convergent
evolution in carnivorous plants.  The Thursday, Sept. 24,1992
_Oregonian_ newspaper, Portland, Oregon, printed an article
"Meat-eating plants not quite as unusual as scientists thought" in
section B, p. 1.  The article said:

	Plant evolution has been a little shop of horrors. Scientists have
	discovered that in the history of the normally placid plant kingdom,
	the habit of killing and eating animals has arisen at least seven
	times.

	The research refutes the common assumption that carnivorous plants
	are all closely related.  It shows that even plants with the same
	method of trapping prey may be only distantly related and arrived at
	their similar ways of life through convergent evolution.

	For example, the various `pitcher plants,' whose scents lure insects
	into a vat of digestive juices, arose independently at least three
	times.  The `flypaper' way of life, with its sticky secretions on the
	surfaces of leaves, was `invented' at least five times.

	On the other hand, the Venus fly trap, whose jaws snap shut on
	hapless small creatures - or even bits of hamburger in the homes of
	hobbyists - is a close relative of a family of flypaper-type
	meat-eaters.

	The research, published in Science, was done by Victor A. Albert and
	Mark W. Chase of the University of North Carolina in Chapel Hill and
	Stephen E. Williams at Lebanon Valley College in Annville, Pa.

How can several plant varieties have evolved the complex mechanisms
required to catch insects, then digest and utilize the nutrients,
completely independently?  The evolutionary mechanisms alluded to by
Dawkins in _The Blind Watchmaker_ seem inadequate to the task of
having *all* the required mechanisms evolve simultaneously.  If
they did not evolve simultaneously, what advantages can be proposed
to allow the individual mechanisms to be selected for?

For example, if a plant mutation resulted in sticky leaves, what selective
advantage might be proposed to carry that along until the plant evolved
the appropriate digestive enzymes and the mechanism to secrete them?
What selection pressure might be further proposed to allow the combination
of sticky leaves and digestive enzymes to exist until the plant evolved
the ability to absorb and utilize the digested material?  What selection
pressures would be involved in carrying along any partially formed
mechanisms?

Furthermore, since plants, at least to my knowledge, do not normally
make enzymes capable of digesting animal tissue, what are the probabilites
involved in a plant mutating so that the right enzymes resulted, given
the complexity of enzymes and the fact that virtually unlimited protein
molecular structures are possible?  This is especially significant in
light of the above article's conclusion that this has happened
independently several times.

Alan Feuerbacher
alanf@atlas.pen.tek.com
------------------------------------------------------------------------

Andy Peters

In article <7063@tekig7.PEN.TEK.COM> alanf@tekig6.PEN.TEK.COM (Alan M
Feuerbacher) writes:
>[Quoting from _The Oregonian_:]
>
>   The research refutes the common assumption that carnivorous plants
>   are all closely related.

This has never been the common assumption among biologists who
study carnivorous plants; witness pitcher plants.  _Sarracenia
sp._, the most widespread in the U.S., and _Darlingtonia sp._,
found mainly in the southeast (in the US) are in different
families (Sarraceniaceae and - oh, damn, I forgot which.  Well,
I remember it's different.  I'm more familiar with Sarracenia).
There are so many different kinds of traps (pitfall, active flypaper,
passive flypaper) that it's never been assumed that they were
all closely related.

It shows that even plants with the same
>   method of trapping prey may be only distantly related and arrived at
>   their similar ways of life through convergent evolution.
>
>   For example, the various `pitcher plants,' whose scents lure insects
>   into a vat of digestive juices, arose independently at least three
>   times.

See above.

>   The research, published in Science, was done by Victor A. Albert and
>   Mark W. Chase of the University of North Carolina in Chapel Hill and
>   Stephen E. Williams at Lebanon Valley College in Annville, Pa.

>How can several plant varieties have evolved the complex mechanisms
>required to catch insects, then digest and utilize the nutrients,
>completely independently?

I don't have the complete reference here (I'll get it by tomorrow),
but Thomas Givnish has a book chapter entitled something like
"The evolution of plant carnivory".  It is an excellent overview of
this stuff. All of the following are from my memory of this paper
and others I read when I was studying this stuff.

Basically, there's a straightforward series of steps to the evolution
of plant carnivory.  First of all, you've got to keep in mind that the
major benefit which carnivorous plants receive from their prey is raw
nutrients (nitrogen, phosphorus, potassium), not proteins, carbon
chains, etc.:

(1) A plant population is in a nutrient-poor habitat

(2) A mutation arises which allows a plant to absorb nutrients through
	 its leaves (no digestion necessary; decomposing plant & animal
	 matter provide the necessary nutrients); it spreads because it
	 allows an extra source of the limiting nutrient.  Increased
	 efficiency of this absorption would be strongly selected for.

(3) Another mutation arises which allows a plant actually to capture
	 prey, and hold it against its leaves.  This mutation spreads
	 because it increases the nutrients received; increased efficiency
	 of this mechanism would also be strongly selected.

(4) Some method of active digestion arises.  You're right; digestive
	 enzymes do seem like a pretty big leap for a plant.  However,
	 not all carnivorous plants use enzymes to digest their prey.
	 _Sarracenia purpurea_ probably doesn't have an enzyme; it's
	 more likely that it has a mutualistic relationship with
	 mosquito larvae which live in its trap fluid (Reference to come,
	 though I can attest, through the personal experience one
	 can only gain through counting and identifying the prey
	 of a couple of thousand pitcher plants, that the mosquito
	 larvae are very prevalent in the trap, and _are not digested_).
	 In addition, any increased ability to utilize prey is
	 very strongly selected for.  I'll look into this a bit more.

(5) Congratulations!  You have a carnivorous plant.  It's all fine-
	 tuning from there on out.