From: lamoran@gpu.utcs.utoronto.ca (L.A. Moran)
Subject: Chris Colby's Evolution FAQ: What is hierarchy?
Organization: UTCS Public Access
Date: Tue, 13 Oct 1992 03:40:24 GMT

I (Larry Moran) said,

     "I think that Chris should be using "branching" to describe the 
      pattern of dendrograms. It is much clearer to say that the 
      "branching" pattern is what one expects to see in evolution since 
      living species have evolved from common ancestors."

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". The
point is that we can construct a dendrogram relating living species and
extinct species and this is powerful evidence for evolution. The fact that
the pattern IS a dendrogram conveys both notions of branching and nesting
although the word "dendrogram" is more closely related to "branching". The 
dendrogram may superficially resemble the organizational chart of a major 
company or a military group but it is not hierarchical because it refers to
changes over time not static ranks. It is not possible IMHO to claim that
any of the early nodes rank higher (or lower) than later nodes or living
species.

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? 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
so that the process that gave rise to the ancestor of all chordates (for
example) was different from that which gave rise to the ancestors of reptiles
and amphibians? Are the mechanisms that gave rise to, say, Dipterans and 
mammals different from those that gave rise to the genus Drosophila or Homo?
And what about the evolutionary mechanism that produced individual species
such as Homo sapiens and Homo habilis? Is this a different mechanism than
those that operated earlier in time?

Even if such a bizarre idea were true it would be more proper to talk
about "temporal" mechanisms of evolution than "hierarchical" evolution. That
is, different mechanisms of evolution operated in the past rather than 
different mechanisms operated during the formation of phyla, orders, species.

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

Laurence A. Moran (Larry)