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Re: Punk eq vs gradualism



From: Jeff Poling <jpoling@infinet.com>
 > At 03:30 PM 1/23/96 -0500, Rob Meyerson wrote:
 > >
 > >One of the biggest debates in evolutionary theory deals with the 
 > >appropriate=
 > > model for the evolutionary process.  The traditional view of gradualism=
 > > states that evolution is a constant process, while the new view of=
 > > punctuated equalibrium states that evolution occurs in short bursts.  The=
 > > big question focusing on which model is correct.
 > 
 >    Maybe those who have made evolution their life's work can elaborate
 > (briefly, natch) on the theories of evolution.

First, the above characterization of the difference between punctuated
equlibrium and the "standard" model is a little misleading.  Both
accept continuous evolutionary change.  The P.E. model merely states
that *significant* change is restricted to geological brief (centuries
to 10's of millenia) periods and to geographically restricted areas.

The standard model considers that any evolutionary change, even chamge
in large, widespread populations, can be lasting and significant.
 > 
 >    ie What was Darwin's theory of evolution (wasn't it natural selection /
 > adaptation rather than an actual theory of evolution?)

That *is* a theory of evolution.  A theory of evolution is a model
of the *mechanism* of evolutionary change.

 >    and I thought I read that the gradualist model was not proposed by Darwin
 >    and I thought I read that the gradualist model was challenged in favor of
 > a "punctuated" model in the 19th century as soon as the gradualist model was
 > proposed.

After Darwin published _Origins of the Species_ there were *several*
widely supported theories of evolution.  One was Darwin's theory that
evolution occurs by natural selection.  Another was a variant of the
old idea of orthogenesis - that evolution is driven by an intrinsic
force within living things that drives life "upwards".

Later, after the rediscovery of Mendelian genetics, when it appeared
as if Mendelian genetics was inconsistant with Darwin's theory,
several researchers proposed a *mutationalist* theory of evolution.
This theory proposes that new species arise in a *single* *generation*,
by means of macromutations. While many people have confused this
saltational theory with punctuated equilibrium - they are NOT even
remotely similar. P.E. is a Darwinian, that is a selectionist, theory,
saltationalism is not Darwinian at all.


This last point is critical - P.E. in no way denies the importance of
natural selection. The mechanism of speciation proposed in P.E. is
the peripatric speciation process described by Ernst Mayr, one of the
founders of the Modern Synthesis (aka Neodarwinism). The idea here is
that the reorginzation of the genome in a way that produces reproductive
isolation is most likely to occur in small, isolated population near the
periphery of the range of the parent species. Since genetic drift and
selection can both act rapidly and efficiently in a small population,
this process probably occurs in a geologically short period of time.

What Eldridge and Gould relaized was that a species that originates
in less than 100,000 years in an isolated patch of environment is
likely to appear fully formed in the fossil record - when it finally
expands out of its initial enclave. Note - this is merely a *likelyhood*,
not a certainty, and capturing an occasional speciation event in
progress is quite possible. This may be what Achelousaurus is. In
one of the early books on P.E. E&G suggest that they may have found
an instance of speciation in progress in the trilobites that Eldridge
specializes in.

swf@elsegundoca.attgis.com              sarima@netcom.com

The peace of God be with you.