OK, let me try to put this in simple terms and tell me where I'm wrong. Let's assume that there is some environmental control that switches on/off a gene that ultimately changes a P (planktotrophic--multi-whorled protoconch) type to an L type (lecithotrophic--one to two whorled protoconch) in one generation. Further postulate that it is not under paternal control at all, not found on the male chromosome. Then if the P species lands on an isolated island and continues to have P offspring, all the larvae are washed away by the currents and the population dies out before a malacologist discovers it. But if this genetic change occurs in even one successful female (assuming that there are any males around at all), then the L type will be able to reproduce itself and form a colony and will not be mixed with the P type, which will have died out.
If, on the other hand there is no environmental control, and the condition is recessive (requiring both parents to pass on the genes to reproduce as an L)unless there is a distinct reproductive advantage, and the change happened in a fairly isolated population (like on the edge of viability for the species), then the L form would be overwhelmed and not have a chance of being found by a malacologist if it occurred.
Would this not explain the brooding form of the Crepidula plana species group?
Allen Aigen
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-- "M. J. Faber" <[log in to unmask]> wrote:
----- Original Message -----
From: "Marco Oliverio" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, October 15, 2003 11:49 AM
Subject: Re: Poecilogony: pairs of species
> Marien,
>
> first of all I repeat here that my practice in taxonomy is "to use
> different name for different protoconchs". But the reason is not that I
> consider this the only explanation of the facts (or even the best one for
> all cases). I consider it the more probable in general, possible for most
> cases, not necessarily for all. And I agree with most of your
consideration
> .... not with all.
>
> At 11.05 15/10/2003 +0200, you wrote:
> >....but nevertheless this is the onset of the
> >drifting apart of the L daughter gene pool...
>
> No: at least, not necessarily. Isolation means that there is no
possibility
> of gene flow. Do you have any evidence that a lecithotrophic egg cannot be
> fertilized by a sperm from a planktotrophic male (or vice versa)?
No, in fact it seems very likely (besides, it is scientifically impossibe to
prove that something does not occur) but fact is that the P male usually
(i.e. almost always) simply is not around. The proof is in de record. If
such events are likely to happen in the real world (not in the lab), we
would see, over time, many species with two types of protoconch but not
differing in any other way. However, we know this is not true. We see L
species becoming more and more different from their P parent over time, all
the time. So we must admit that P/L change is a veritable starting point for
speciation and, not surprising, succesful and genetically favoured trait in
these gastropods.
Then it comes to definitions: what is a marine gastropod species? When does
it start to be a new species? In the case of P/L shift we don't know, like
we do not know what water is, if we examine a single H20 molecule. It
depends on how we define a species. But if we consider a shell with a highly
sculptured multispiral protoconch specifically different from an otherwise
identical one with a simple 1-1.5 whorls smoothish protoconch, then we have
no choise but to accept that the single shift in larval development IS the
starting point for a new species.
> Think for a moment that egg production be under environmental control: a
> female produces eggs with more yolk when the environmental conditions
would
> conterselect planktotrophy (and vice versa). There is evidence that the
> amount of yolk in the egg DOES REGULATE the length of the embryonic phase,
> maybe in terms of speed of the embryogenetic events. In this case
> poecilogony is not only the most parsimonious hypothesis but also the more
> workable.
> There is NO definitive EVIDENCE that having tow different developments
> (lecith vs. plankt.) would mean always genetic isolation.
>
>
> > my point is that if you accept the P/L switch as crucial,
> >then there is no place for the concept op poecilogony.
>
> Your reasoning is circular:
> Different development is crucial in speciation => different development =
> different species
It is not a reasoning here, it is a hypothesis, that you either accept or
reject. I only want to indicate that both definitions are mutually
exclusive: If P and L are different (i..e a species is P OR L), then P does
not include L. If you argue that P sometimes does include L, then it will
fall under Ockham's razor.
> you miss an important link:
> different development "=" speciation (crucial is not enough)
> There are lots of example where a feature is crucial and actually
> accompanies speciation in a group (lets say changes in the radula, or in
> the feeding way, or in the courtship, or in the colour pattern) and the
> same feature is simply a polymorphism in another group.
> In the first case you can use the feature as a diagnostic character. In
the
> other group ... simply you can't.
Very true, that is why cladistics are an oversimplification - but that's
another story.
And of course you can argue that the P/L speciation is an oversimplification
too but it is practical and it does not (yet) contradict facts.
>
> >We may not be right, but we're practical.
> Taxonomy in fact is a tool in the hands of thos that use it. Monographs,
> checklists, inventory of the biodiversity, ecological stusies, all require
> a taxonomy usable. But we also know that the biological classification
must
> reflect the reality in the Nature.
> All in all I keep considering our practice of giving different names to
> different protoconchs as a good practice. It probably does reflect most of
> the reality of Nature. BUT ... we must always keep in mind that Nature is
> not rigid, likes to make exceptions, sometimes uses alternative, less
> parsimonious ways.
> That's what makes Evolution and Biology in general so different from
> Physics and Chemistry.
> And I love it.
>
>
>
>
>
>
> =========================================================
> Marco Oliverio - Evolutionary Biology PhD
> Research Scientist
>
> Dipartimento di Biologia Animale e dell'Uomo
> Viale dell'Universita' 32
> I-00185 Roma ITALY
>
> phone +39.06.49914307
> FAX +39.06.4958259
> e-mail: [log in to unmask]
>
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