From: adpeters@sunflower.bio.indiana.edu (Andy Peters)
Subject: Re: Chris Colby's Evolution FAQ: What is hierarchy?
Organization: Program in Evolution, Ecology, and Behavior, Indiana University
Date: Tue, 13 Oct 1992 16:08:01 GMT

In article <Bw1JJC.7z1@gpu.utcs.utoronto.ca> lamoran@gpu.utcs.utoronto.ca (L.A. Moran) writes:
>Andy Peters responded,
>     "This is where I disagree with Larry. I don't think "branching" is 
>      a descriptive enough word in this case. While a branching pattern
>      is expected if evolution has occurred, the most striking feature 
>      expected is that of *nesting*; more recently-split groups nest 
>      perfectly within groups formed by more ancient splits, and
>      those groups nest perfectly within groups formed by yet-more-ancient
>      splits, etc. While this is a logical result of branching, I think it
>      is explicitly nesting, and not branching, which is observed, and
>      cited as evidence for evolution."
>
>OK, I don't see much of a difference between "branching" and "nesting". 

*Nesting* is the pattern of character distribution we see
which allows us to infer the *branching* patterns of dendrograms.
Since the nesting is what is actually observed, it is (IMHO) the
preferred word.

>Chris Colby talked about a "hierarchical theory of evolution" in his FAQ.
>I wondered what this meant and Andy Peters replied,
>
>     "What I think Chris is saying is that "some PE proponents" suggest 
>      that different mechanisms of evolution work at different levels of
>      nesting/branching - work at different levels in the hierarchy.  
>      Since different processes work at different levels in the hierarchy 
>      (given that one accepts the use of the word "hierarchy" in the first
>      place), the processes themselves can be seen to be hierarchical. In 
>      other words these "PE proponents" are proposing that species-level 
>      processes and individual-level processes are two different things."
>
>How can there be different machanisms of evolution at different levels in the 
>hierarchy if there is no hierarchy? 

I was merely using the word "hierarchy" to conform with the argument I
thought Chris was describing.  I think we agree that there are different
levels of nesting/branching within an evolutionary lineage; in that
terminology, I would say, instead of "the processes themselves can be
seen to be hierarchical," "different processes can be seen to work at
different levels of nesting."

>In a hierarchical CLASSIFICATION we
>group some species into orders and some orders into phyla. If the 
>classification scheme truely represents evolution (I'm a cladist!) then each
>species in an order is descended from a single ancestral species and groups
>of such "order ancestors" are themselves descended from a single ancestor
>which gave rise to a complete phylum. Are we supposed to believe that 
>different mechanisms of evolution operated at different periods in the past

First, let me clarify that neither Chris nor I claimed to fully accept
the scenario I described above; Chris rejects it entirely (I think), and
I accept it in an extremely limited sense.  The sense in which I accept it
has nothing to do with different mechanisms acting at different times; it
has to do with additional mechanisms acting on different levels of nesting,
specifically because they _are_ different levels of nesting.  This was
illustrated by my "species selection" for sex example, the point of
which was that lineages which are more "prone" to speciate (say, because
of the increased variation offered by sex) will be "selected for," in that
they will be represented by more species than those which are less prone
to speciate.  This is a mode of selection which acts on the species level,
and not at the individual level - it acts on one level of nesting, but
not another.  I do _not_ propose any mechanism of "genus selection," or
"order selection;" as a naive cladist (i.e. one who doesn't really
know all that much about it), I recognize that the only "real" phylogenetic
event is speciation (there's no such thing as "genusation" or "familiation").

>I do not believe that the process(es) of macroevolution or speciation are
>any different from those of microevolution. Chris and I agree on this
>point. 

I agree wholeheartedly that all of the important processes of macroevolution 
and speciation are the same as those of microevolution.  However, I maintain 
that the "species selection" (which I really hate to call it) I describe 
above _has to_ exist, and that it may explain some of the patterns of
diversity we see.  It _does not preclude_ the standard processes of
selection and drift; in fact, it depends on them.  In addition, it
probably is not all that important as an explanatory tool.  But I
maintain that it exists.

>But if there is a difference I fail to see what we gain by referring 
>to this as "hierarchical" evolution.

I agree; as I thought I had made clear, I was only using the word
to conform to the original argument.

>Laurence A. Moran (Larry)

-- 
* Andy Peters                     * "Slugs are small and portable    *From
* Program in Evolution,           * Just stuff 'em up your nose      *"Slugs,"
*      Ecology, and Behavior      * They'll fit beneath your armpits *by David
* Indiana University, Bloomington * Or right between your toes"      *Greenberg
Newsgroups: talk.origins