I doubt very much that the quoted examples of Physella gyrina and Haitia
heterostropha (=acuta) are in fact examples of phenoplastic switch.
Predation should readily explain a shift in allele frequencies within a few
generations, yielding the observed results.
Criterion 1), the chemical agent required, gives the base for a test
however: if the mere presence of, say, tissue extract from the different
predators would lead to the same results, then the hypothesis of
phenoplastic switch would receive support, at the same time falsifying my
idea of allele frequency shift through selective predation.
I my opinion, every species has a range of possible reactions to
environmental factors, and this capacity has a genetic base, so we must
carefully examine which factors (external or internal) could have influenced
the appearance of the individuum we are just looking at. In fact, freshwater
snails change their appearance very much depending on temperature, chemical
factors, water body size etc. etc.
Speciation occurs when populations have become unable to cross-breed, and it
becomes permanent when a switch back to the former conditions has become
improbable. The safest way to get there is a number of genetic differences
(=mutations) fixed in each population. Of course, these are not in those
genes molecular systematics are looking at, but they could be anywhere e.g.
in some regulatory genes.
Greetings from Germany
Michael Hoelling
> -----Ursprungliche Nachricht-----
> Von: Conchologists List [mailto:[log in to unmask]]Im Auftrag von
> Burton Vaughan
> Gesendet: Samstag, 8. Januar 2005 23:29
> An: [log in to unmask]
> Betreff: Homeobox Switches
>
>
> Re: Doug Stemke's question in regard to switches in Conus
>
> Developmental plasticity under the influence of a differing specific
> environmental factor is in fact fairly well established for mollusks
> and quite common across the whole phylogenetic spectrum. Such switches
> have been almost totally ignored by most geneticists or dismissed as
> rare developmental aberrations. Some definitive criteria for a true
> phenoplastic switch are that: 1) an identifiable chemical, hormonal, or
> tissue factor induces morphological change during development; 2)
> spreads rapidly within the P generation; and 3) may or may not show in
> the F2 and later generations, depending on environment. It may also be
> reversible in P, F2 or later generations. Moreover, such switch
> mechanisms may be activated by tissue or external factors at any
> cellular level, ranging from the timing of homeobox gene expression, to
> the modification of RNA transcripts after translation, to the assembly
> of protein domains in the microsome, or even later in the inactivation
> of protein domains during growth.
>
> I searched through M. J. West-Eberhardt's, recent compendium of
> research, "Developmental Plasticity & Evolution" (Oxford Press, 2003),
> where I find numerous references to such snail/mollusk research (pp.
> 307-362). A fairly typical example involved rearing Physa gyrina, or P.
> heterostropha, in controlled pair groups in either water in which
> crayfish co-existed or water in which fish but no crayfish co-existed.
> Within the month, facultatively induced differences in shell morphology
> appeared; i.e., snails exposed to shell-crushing fish predators showed
> wide apertures and very much strengthened, rotund shells. Fish exposed
> to crayfish predation, only, showed narrow-apertured, thin elongate
> shells, with barricading teeth (crayfish feed from the inside of Physa
> species).
>
> Such phenomena are strongly suggestive of a phenoplastic switch. This
> is one reason why I think the sooner we toss neo-Darwinian explanations
> into the trash can, and focus instead on the environment in which a
> species develops, the better we will understand the basis of
> speciation. Study of DNA cannot help much here, because the real issue
> is one of regulatory silencing of genes, RNA transcripts, or protein
> domains by external factors. The genome, itself, may remain unchanged
> throughout.
>
> Burt Vaughan
> Evolutionary Biology
> WSU-TC
>
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