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John Donald Collier jcollier at ariel.ucs.unimelb.edu.au
Fri Sep 20 08:06:18 EST 1991


In <9109191549.AA22718 at bambi.ccs.fau.edu> tomh at BAMBI.CCS.FAU.EDU (Tom Holroyd) writes:

]This is not a modification of the genotype, right?  The snails
]develop a different shape depending on the environment they mature in?
]In other words, the genotype has the potential for both kinds of
]shell, and the environment forces one or the other to be expressed.
]Or do their offspring remain rounded even in placid waters?  How
]could this be passed on?  Some enzymes in the egg?  I can imagine
]that, but wouldn't it be better to let the environment determine it?

As usual, I should have been less terse. My students have learned
to fill in the gaps, assuming that what I said makes sense, but
I forget other people don't have any reason to assume that. 

I should have said that the modified shape is retained in the
descendents in placid water. The effect takes several (I am not
sure how many, but small enough for Paiget to have observed)
generations. The only peculiarity in the case is that the
behaviour change is what permits the migration into the
different niche, which then allows the behaviour to become
genetically fixed.

Presumably the ones in the turbulent environment that have a
genetically fixed round shape are more successful than the ones that
have to try to stay round. I would guess that there is a propensity
for the round genetic form, but it usually doesn't survive very in the
placid environment. Why that propenstiy is there, I wouldn't know. It
could be either past selection on the lineage, or else it might be
chance. Perhaps there is some other explanation, e.g., the turbulent
environment induces mutations that favour the round form. (I 
understand that heavy metals induce a mutation of the melanic form
in pepper moths -- a possible contributor to the effect of
industrial melanism?)

-- 
John Collier 				Email: Collier at HPS.unimelb.edu.au
HPS -- University of Melbourne		  jcollier at ariel.ucs.unimelb.edu.au
Parkville, Victoria, AUSTRALIA 3052	Fax:   61+3 344 7959



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