From: colby@bu-bio.bu.edu (Chris Colby)
Newsgroups: talk.origins
Subject: hierarchical selection
Date: 16 Oct 92 04:14:16 GMT
Organization: animal -- coelomate -- deuterostome

In article <Bw1JJC.7z1@gpu.utcs.utoronto.ca> lamoran@gpu.utcs.utoronto.ca (L.A. Moran) writes:

>It is not possible IMHO to claim that
>any of the early nodes rank higher (or lower) than later nodes or living
>species.

Um Larry, if you can spare some time out of your busy schedule, could
you try reading what I wrote and responding that? I am claiming that
_traits_ are distributed in a hierarchical (or nested) pattern. I am
not saying organisms (or species) are arranged in a hierarchy. At
(or before) the node on any dendrogram or cladogram would be the
_traits_, not the species. To use plants as an example, the trait
"flower" would be at the top of the angiosperm hierarchy. Branching
below this would be two lineages that both share this trait. But, one has
a collection of monocot traits (characteristic number of cotyledons (1) 
and venation pattern) and other a collection of dicot traits (i.e. two
cotyledons and a different venation pattern.) This pattern of branching
leads to some _traits_ (have I mentioned that I'm not talking about
organisms?) being at higher levels than others.

A cladogram is a graphic representation of how traits are distributed
amongst species. The process of evolutionary descent with modification
is _inferred_ from this. So, (IMHO) you can't say that species are at
any one node _from cladistic information alone_. But, you can say a trait
is. You need to discriminate between the representation of the data and 
the inferences drawn from it.

>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? 

No Larry, you are supposed to believe that it is in theory possible
for selection to work at different levels. For example, genes can be
a unit of selection (an example would be transposable elements or
segregation distortion genes -- these have a differential reproductive
success compared to other genes _within_ a genome). Individuals can
be a unit of selection. They can have a differential reproductive
success compared to others in a population. The individual is (IMHO)
probably the most common unit of selection. And, populations or
groups can be selected. Group selection is theoretically possible
and demonstrated in _Tribolium_ once (and that's it to the best of
my knowledge). In order for the group to be the unit of selection,
it must be the group as a unit that exhibits differential success
-- this difference must not reduce to a sum of differential repro
successes of individuals comprising the group. Put another way,
the group must exhibit an emergent phenomena that is heritable (at
the group level) and this heritable emergent property must confer
a differential reproductive success to the group as compared to
other groups. If selection is working at different levels of
organization (gene, organism, population) then the process of
evolution is hierarchical (or nested if you object to the term
hierarchy).

Endler, in his book "Natural Selection in the Wild", has a discussion
of this. Sober, in his anthology "Conceptual Issues in Evolutionary
Biology", has a section devoted to this. Richard Lewontin, Stephen
Jay Gould, Richard Dawkins, William Wimsatt and Eliot Sober all
have a chapter devoted to this subject. 

I think that selection operates primarily at the level of the
individual, although lower level selection is more common than
is generally recognised (IMHO). In addition, evolution (not
due to selection) also has a hierarchical (or nested) component
when you consider concerted evolution (a pattern of nucleotide
diversity seen within repetive genetic sequences). I do not 
think "higher" levels of selection are important in the evolution
of life on this planet. In particular, I think the idea that
entire species are units of selection is ludicrous (although I'll
buy that populations may, on occasion, have the prerequisites for
selection to act among them.)

>Laurence A. Moran (Larry)

Chris Colby 	---	 email: colby@bu-bio.bu.edu	---
"'My boy,' he said, 'you are descended from a long line of determined,
resourceful, microscopic tadpoles--champions every one.'"
 	--Kurt Vonnegut from "Galapagos"

Newsgroups: talk.origins