"Guy Hoelzer"  wrote in message
news:c7dtgv$2a0c$1{at}darwin.ediacara.org...
> in article c79l1b$11a7$1{at}darwin.ediacara.org, Perplexed in Peoria at
> jimmenegay{at}sbcglobal.net wrote on 5/4/04 7:54 PM:
>
> > "Guy Hoelzer"  wrote in message
> > news:c78iba$m62$1{at}darwin.ediacara.org...
> >>>> All traits are species selected traits.
> >>> Even those that are polytypic *within* the species?
> >> Perhaps ESPECIALLY those traits that are polytypic *within* the
> >> species.  I think that it is fair to say that selection at one level
tends to
> >> coordinate the interactions among the parts that constitute that level,
> >> including the specialization of diverse roles.  For example, selection
> >> at the level of the individual organism seems likely to have guided
> >> the proliferation of tissue,
> >> organ, and organ system diversity within individuals, and engineered
> >> functionally effective paths of interaction among these parts.  I would
> >> similarly expect selection at the species level to engineer within
species
> >> polymorphism.
> >
> > If I understand you correctly, what you are saying may be very
heretical,
> > and therefore very interesting.  Three questions:
[snip vague answer to vaguer question #1.]
> > 2.  Besides sexual dimorphism, and eusocial insect castes, what examples
> > can you suggest of polymorphisms that arise through species level
> > selection?
> I'll nominate the three discrete male morphs
> of the sponge-dwelling isopods studied by Steve Shuster.  Like any good
> neo-Darwinian, Steve has explained the existence and persistence of these
> morphs as a consequence of selection at the individual level, but I think
> that the population-level balance in the relative numbers of these morphs
is
> better understood as a top-down effect of selection at higher levels.
>
> > 3.  What are the mechanisms by which the species maintains a stable
> > pattern of polymorphism?
>
> Frequency-dependent fitnesses of individuals seems to be an important
> mechanism.  As I hinted at above regarding Shuster's explanation of isopod
> male morphs, frequency-dependent selection has traditionally been modeled
as
> a form of individual level selection by parameterizing fitness only at the
> individual level.  However, I would argue that the dependence of
individual
> fitness on the state of the rest of the population implicitly asserts the
> role of top down effect.  Species-level selection is one potential source
of
> such a top-down effect, and I admit that there are other possible
mechanisms
> (e.g., means of self-organization).

I think that it is important to distinguish two different kinds of
"top down effects".   The fact of frequency dependence at the individual
level means that the structure of polymorphism is potentially an emergent
(my static sense) trait at the species level.  However,
this doesn't necessarily mean that it is a stable or heritable species-level
trait, nor that this trait is subject to and/or maintained by species-level
selection.

Let me ask about two other examples of polymorphism.  Do you believe
that the following polymorphisms involve species-level selection?
1.  Human blood-type polymorphism (O, A, B, AB).
2.  Human polymorphism for sickle-cell anemia.
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