Let me throw in my two cents by now...
If the switch from lecitotrophic to planktotrophic is controlled by some
environmental variable, like e.g. population density, then it could go forth
and back from one generation to the next, depending on the environmental
conditions. An animal issued from a planktotrophic egg will probably come to
another location, where it could lay lecitotrophic eggs (provided it finds a
mate; land pulmonates, which I mostly deal with, are not so demanding. OK,
dispersal of land snails would be another thread).
The mate could also be the descendant of a local, currently lecitotrophic
population. As long as there are no mechanisms restricting the gene flow
between different subpopulations, there will be no species split.
Greetings from Germany
Michael

> -----Ursprüngliche Nachricht-----
> Von: Conchologists of America List [mailto:[log in to unmask]]Im
> Auftrag von M. J. Faber
> Gesendet: Freitag, 17. Oktober 2003 10:40
> An: [log in to unmask]
> Betreff: Re: Poecilogony: pairs of species
>
>
> ----- Original Message -----
> From: "Allen Aigen" <[log in to unmask]>
> To: <[log in to unmask]>
> Sent: Wednesday, October 15, 2003 3:46 PM
> Subject: Re: Poecilogony: pairs of species
>
>
> > 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.
>
> This process, although with two "old" species involved, is known from the
> isolated north Atlantic islet of Rockall. The case is described in the
> following paper:
>
> Johannesson K 1988 The paradox of Rockall: why is a brooding gastropod
> (Littorina saxatilis)  more widespread than one having a planktonic larval
> dispersal stage (L. littorea)? Mar. Biol. 99:507-513
>
> > 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
> > [log in to unmask]
> >
> >
>