----- Original Message -----
From: "Allen Aigen" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, October 14, 2003 3:34 PM
Subject: Re: Poecilogony: pairs of species


> Marien,
> If the change from one mode of larval development to another is widespread
in
>  marine molluscs, and it causes a change from one species to a pair of
sister
>  species, then this may be a fairly random change that occasionally
succeeds.

Allan,

it does not, it seems to be rare (is there anyone who has studied thousands
or ten thousands of specimens of a single planktotrophic species and
discovered one or more non-planktotrophic "hopeful monsters"?), but it
succeeds exceedingly often in isolated or semi-isolated ilands and island
groups. Not once, but again and again: Manzonia in the Macaronesian Islands,
Schwartziella in the Cape Verdes, to name a few. Apparently, it is NOT
random chance. Besides: why should the possibility (thus the responsible
genes) of becoming non-planktotrophic be preserved in many gastropod
families over millions and millions of years? There is nothing random about
that.

>  Given enough time and species, it is common where there is an advantage
in
>  keeping larvae local.  Where there is no advantage, the change probably
does
>  not succeed, as it competes too much with the parent form.  Those random
cases,
>  which may be seen as unsuccesful "hopeful monsters", could perhaps be
>  best described as poecilogony.

This description does not give is any deeper insight in how species,
especially non-planktotrophic species, come into being, therefore it is
quite useless

If they were successful, they would eventually
>  be different species.

What is wrong, in light of the evidence, to regard and thus name them as
different species the moment they occur? after all, that is what we
taxonomists are doing all the time!

Species, however, must have a viable population to
>  continue reproducing.  One individual does not a species make, although
it may
>  be the start.

"A single specimen does not make a species" (famous last words of the last
dodo). Indeed, a species is generally defined as a viable gene-pool isolated
from all others. But the point here is that the switch in mode of  larval
development not only pushes the specimen(s) involved beyond the realm of its
pre-existing sister or, rather, mother species, but also comes in viable
numbers, unless of course this particular species hatches only one egg at
the time.


Marien