It is the process of natural selection. The mussels that are the easiest
for the crabs to crack and eat are the ones with thinner shells. The
process of survival favors those with thicker shells as they have a
greater chance to survive. Over time, the population shifts to a thicker
shelled population. This requires a bit of genotypic variation in the
population upon which natural selection can work.


> Alan; can you or someone explain how a mussel shell can be induced to grow
> thicker because a crab is chewing on it? obviously the mussel can't make a
> conscious decision to do that. Is it the irritation that causes thicker
> shell growth? If so then captive mussels could be induced to grow thicker
> shells by scraping them with a file?
>
> I understand how a colony of mussels could, over generations, grow thicker
> shells because of this type of predation but i can't grasp how a single
> organism can perform that feat. pete krull
>
> -----Original Message-----
> From: Conchologists List [mailto:[log in to unmask]] On Behalf Of
> Alan
> Kohn
> Sent: Wednesday, March 25, 2009 7:48 PM
> To: [log in to unmask]
> Subject: Re: [CONCH-L] Crab Induced Snail Evolution in Our Generation?
>
> Alex Menez's comments are quite correct, accurate and helpful. The term
> adaptation can apply both to change in an individual organism or in a
> group
> (population or species). The jargon term "phenotypic plasticity" is often
> applied to the former (the case described for Nucella), to distinguish it
> from genetic change. The latter typically does involve genetic change over
> time, i.e. evolution.
>
> Alan
>
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Regards,
Charlie
.................................................
Charlie Sturm
Research Associate - Section of Mollusks
Carnegie Museum of Natural History
Pittsburgh, PA, USA

Assistant Professor - Family Medicine
Fellow-American Academy of Family Practice
Fellow-Academy of Wilderness Medicine

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