Re: Species Selection Redux
From: Guy Hoelzer (hoelzer_at_unr.edu)
Date: 03/11/05
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Date: Thu, 10 Mar 2005 20:47:07 -0500 (EST)
in article d0on9t$gus$1@darwin.ediacara.org, Perplexed in Peoria at
jimmenegay@sbcglobal.net wrote on 3/9/05 9:51 PM:
> "Guy Hoelzer" <hoelzer@unr.edu> wrote in message
> news:d0nbfi$1ci$1@darwin.ediacara.org...
>> in article d0m6mq$2ndm$1@darwin.ediacara.org, Perplexed in Peoria at
>> jimmenegay@sbcglobal.net wrote on 3/8/05 10:56 PM:
>>
>>> "Tim Tyler" <tim@tt1lock.org> wrote in message
>>> news:d0kipq$2654$1@darwin.ediacara.org...
>>>> bryophyta@hotmail.com <bryophyta@hotmail.com> wrote or quoted:
>>>> Also, it is widely expected that species selection will be a rather weak
>>>> force - since its effects will often be swamped by individual selection -
>>>> which will act to destroy some of the variation on which species
>>>> selection would otherwise act.
>>
>> The strength of selection at any level is always determined by the same
>> factor: the extent of heritable variation in fitness.
>
> I agree, but there is a need to be cautious here. In comparing fitnesses
> at different levels, you need to be sure you are not comparing apples
> and oranges. The most important thing is to be sure that you are using
> the same time scale in both cases. You can't naively take fitness to be a
> growth rate per generation but use organism generations in one case and
> species generations in another.
While I appreciate the importance of temporal (and spatial) scale
compatibility in the interaction among processes, I don't think that your
concern is relevant here. I don't think that the interaction between
selection manifested at different levels of organization generally depends
on the relative fitnesses of entities existing at the different scales. For
example, it is not like a species can be more fit than an individual
organism, or vice versa. Instead, the strength of selection at any level
depends on "the extent of heritable variation in fitness" at that level
alone, and the evolutionary response to selection can (but does not
necessarily) include a cascade of effects that spill over into systems at
other scales of organization.
You seem to be focused on situations where selection within systems existing
at different scales conflict, so let's say that individual selection favors
bigger individuals and the effects of species selection favors smaller
individuals. One outcome may be that the individuals within species evolve
to be larger and larger, while species of larger individuals suffer a
greater rate of extinction. This particular example actually has widespread
support in the literature. The question is then, what is the size
distribution of individuals (independent of species assignment) on the
landscape? If species selection had greater effect than individual
selection, then the biosphere will become composed of smaller individuals.
If individual selection had greater effect, then the opposite would be true.
The fact that the selection processes are happening on different time scales
doesn't come into play. Of course, it would be a necessary part of such a
study design that you include data that spans a sufficient time scale to
detect the effects of the slower process. I don't think that this would
bias the outcome in favor of the slower process, unless the faster process
was self-cancelling because it fluctuated over long time scales. However,
if this was the case it would only further substantiate my point that
differences in process rate itself does not obviate consideration of
selection effects derived from both levels.
> The same time scale caution applies to the comparison of heritability.
> What does heritability mean in the case of long-lived entities such
> as species in which the "genome" of the reproducing or non-surviving
> "adult" may have changed since the species was "born".
I don't see the problem here.
> For both fitness and heritability, I think that it is necessary to use
> definitions that involve rate of change per year, rather than per generation.
I don't see your point regarding heritability, and I would not compare
fitness values across scales between which process (function) is independent
(a higher order process has emerged).
>> The view that forms
>> of group selection, including species selection, is usually weak implies
>> that heritable variation in fitness among groups is usually small. That may
>> be the case, but I think it would be useful to have someone explain why this
>> situation might be common in today's biosphere, or in general (temporally
>> speaking) if that is part of the argument.
>
> Well, in some sense (regarding group selection at least) this is simply
> a theorem of analysis-of-variance. The (per capita) variance within
> the population as a whole is partitionable into within-group and among-groups
> components. The part is always smaller than the whole.
How does this lead to the conclusion that group selection is usually weak
compared with selection at the individual level?
>> I am personally persuaded by
>> arguments suggesting that the primary level of selection shifts upward in
>> scale as the biosphere undergoes evolutionary complexification. For
>> example, I would argue that selection at the level of the individual
>> organism asserted a great deal of control over selection at the level of the
>> gene when organisms with multigene genomes emerged. I'm not convinced that
>> species selection has usurped control from individual selection, but I am
>> open to arguments that forms of group selection have succeeded where groups
>> have attained a sustainable and functional integration (i.e., they have
>> emerged as physical agents). Possible examples might include colonies of
>> eusocial organisms, Portuguese Man of War, Dictyostelium slugs, and cultural
>> groups of humans.
>
> Agreed.
>
>> I would be quite disinterested in responses claiming that
>> such phenomena can be explained as a consequence of individual selection,
>> because I am more interested in understanding the level at which selection
>> is mainly having effect, rather than an academic exercise exploring how
>> skilled we might be in defending the argument that one shoe can fit all feet
>> (to draw on a metaphor suggested by JE).
>
> Mostly agree, though my own criterion might be more along the lines of
> "the level with the most explanatory power" rather than "the level with
> the larger effect". Frequently they are the same, but not always.
Right. I am indeed personally more interested in the source of effect, but
I will take explanatory power in most cases because that is often as close
as we can get.
>>> I agree that it is weak, but not for the reason you give. In saying
>>> that individual selection "will act to destroy some of the variation
>>> on which species selection would otherwise act" you seem to be saying
>>> that convergent evolution is common. I think you are confusing
>>> species selection with group selection here.
>>
>> That is rather like saying that someone has confused a Biology department
>> with a university. They aren't exactly the same thing, but they aren't
>> entirely different either. One is embedded within the other.
>
> Here I disagree, as I have said earlier in this thread to Tim. I don't
> see species selection as a form of group selection.
Hmm. Surely you would agree that the term "species" refers to a group of
organisms, so...
> The entity that evolves is different.
I presume you are referring to the entity that evolves in DIRECT response to
the selection pressure, as opposed to entities that evolve due to indirect
correlation with the entity under selection.
> In group selection, as in individual selection,
> it is the species that evolves.
Who told you this? The network of populations evolves under group
selection. This may be a metapopulation or a species or any other kind of
network of populations that might exist (including networks of populations
involving multiple species!).
> That is, in both cases, the effect is
> to change the average character of individual organisms within the species.
This is major misconception, IMHO. The average character of individual
organisms will only evolve (indirectly) as a result of group selection when
individual level traits are correlated with factors affecting heritable
variation in fitness among groups. This need not be the case, and I know of
no reason to think that it would be the norm.
> However, species selection just cannot cause the evolution of a species.
> It causes the evolution of something else - a taxon, perhaps, or a guild.
Yes. Selection at the level of the species would cause a network of species
to evolve. The literature has mostly concentrated on phylogenetic networks
(phylogenetic trees are graphs of interconnected nodes), but I think it will
ultimately be more interesting to consider functional (ecological)
connections among species within ecosystems as the system evolving in
response to species selection.
>>> It is true that the process of individual selection does tend to reduce
>>> variation between groups.
>>
>> Funny. I would make exactly the opposite argument. I think that selection
>> at higher levels is better able to reduce the effects of selection at lower
>> levels than it is for the exertion of control to reach upward. Higher level
>> phenomena emerge from the activities of their lower level components, so
>> their existence and their potential as a unit of selection is automatically
>> promoted by the interests of lower level agents. I see no advantage to
>> individual organisms in somehow stifling group selection (including species
>> selection).
>
> You seem to have misinterpreted me. I agree with the points you are making
> about "emergence" and about control hierarchies. I was talking about
> versions of group selection (such as Price's or D.S.Wilson's) in which
> there is no "emergence" and in which groups act as pure units of selection
> rather than as levels of functional organization.
I'm not sure that D.S. Wilson still holds that view, but I think I
understand your perspective. I guess I would argue that the functionalist
viewpoint solves most of these issues.
>> Let me give you an example of the usurping of selection effects
>> by a higher level of organization as I see it. A defining event in the
>> origin of organisms from a "chemical soup" was the emergence of the rules of
>> Mendelian inheritance. I am extending Mendel's rules to haploid organisms
>> in the sense that every bit of the genome (e.g., every gene) gets copied
>> exactly once during cellular reproduction. Under these rules, every bit of
>> the genome has exactly the same fitness (reproductive rate) as every other
>> bit, meaning that the process of cellular reproduction generally undermines
>> selection at the level of gene or allele. Genes ought not fight back much,
>> because the functionality of the organism generally enhances the fitness of
>> genome bits above what they would expect to manifest on their own. I think
>> that to the extent that forms of group selection might come to dominate
>> selection at the individual organism level, it would happen (or has
>> happened) in the same way.
>
> A fine example. But you are talking about "groups" as units of functional
> organization, rather that as collections of individuals with a partially
> shared fate.
I think it is a big mistake to define a group by the identities of the
individuals that compose it, as opposed to its functional coherence.
Otherwise we can be talking about ghosts, which as we all know don't really
exist. Do you see a problem in simply limiting the purview of group
selection theory (multilevel selection theory, really) to functional
entities, without changing any of the conceptual aspect of the theory?
Cheers,
Guy
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