Gradual vs Punctuated

Adam Shohet (
Mon, 01 Jun 1998 15:31:35 +0100

New Thread:

There exists a Planarian worm called Microstomum which has a rather
peculiar behaviour. (One which certainly baffles me at any rate). It has
evolved the ability to ingest Hydra (a small marine creature that clings to
aquatic plants and uses specially evolved stinging cells called nematocysts
for defense. Each cell has a hair trigger which when stimulated fires a
poisoned barb). Microstomum actually alters it's normal diet to feed
exclusively on Hydra. It then directs the nematocysts through it's tissues
to the surface where it utilizes them itself. Once fully armed with
nematocysts Microstomum returns to it's normal diet. When stocks of
stinging cells are low it feeds on Hydra again to replenish it's arsenal.

In order for this to occur the animal must be able to `not digest' what it
eats and instead direct these cells only, to it's skin surface. This
demands the co-operation of three different types of tissue (endoderm,
parenchyma and epidermis) coupled with specialized diet regulation.
Microstomum has no brain or nervous system. How does the theory of
gradualism account for this process? How could having a nematocyst lodged
within the endoderm possibly bestow a survival advantage? If the answer is
none, then natural selection would not have selected for this adaptation.
What mechanism can we propose to explain this?