Re: Question about "punctuated equilibrium"

On Wed, 28 Oct 2009 21:52:58 -0700, John Harshman
<jharshman@xxxxxxxxxxx> wrote:

r norman wrote:
On Thu, 29 Oct 2009 00:37:46 +0000 (UTC), nospam@xxxxxxxxxx (Paul
Ciszek) wrote:

I have frequently encountered the belief that "punctuated equilibrium"
means radical changes appearing in a single generation. Not just
among creationists, either. I'm pretty sure that that is *not* what
it means, but I was wondering if the following would be a fair statement:

Evolution always proceeds gradually, on a time scale measured in
thousands of generations. However, sometimes things reach a stable
situation (a "local optimum") where any further small changes do not
improve genetic survival. When this happens, a species may appear
(at least as far as can be seen from fossils) to remain unchanged
for a period of time. Once a new factor is introduced, however,
the process of change and the spinning off of new species resumes.
Thus, if you look at the entire fossil record, it might look like
alternating periods of fast and slow change, but what is really
happening is alternating periods of slow change and really *really*
slow change.

Is that more or less correct?

My impression is, first of all, that punctuated equilibrium describes
how the fossil record changes over time and not necessarily how
organisms change over time. There are several possible scenarious.

Most important, of course, is the tendency towards stasis which is
much as you describe. You can give it a fancy term -- stabilizing
selection -- but basically an organism is quite well adapted to its
environment and, barring any major change in that environment, will
not change.

Though possibly true, that isn't PE.

Then there are several scenarios that can cause a relatively sudden
shift in the population. One of these is that somewhere else, next
door perhaps, a somewhat different form of the same species or a
somewhat different species has developed and this could have happened
gradually. However these aliens invade your territory and replace
you in relatively short time. So what you see in the fossil record is
a replacement of one species by another, not a change from one into
another. The replacing species most probably started out as a
relatively small population and therefore is not seen in the fossil
record even in its original homeland. It was just opportunistic
enough to discover your habitat where is could flourish and grow in
number.

This has considerable relationship to PE, in that it involves peripheral
isolates being the engine of speciation and thus of morphological
change, and that what you describe is used by Eldredge & Gould to
explain apparent sudden transformations.

Another scenario involves factors that would actually produce stasis
and sudden change in the phenotype of an organism even though the
underlying evolutionary factors, the changes in the genotype, occur
gradually and cumulatively.

That's outside of the classical explanation for PE, which is a
paleontological expression of Ernst Mayrs ideas on speciation.

This involves developmental and
physiological and biochemical negative feedback loops that tend to
hold things at a relatively constant level even in the face of changes
in the equipment. This is commonplace in Mendelian genetics with, for
example, the distinction between complete dominance and intermediate
inheritance. In some plants, a hybrid between red flowers and white
flowers always produces, say, red, whereas in others the hybrid is
pink. If you have a double dose of the genes producing red color, do
you produce twice as much color (the white-pink-red case) or does a
regulatory system mean you produce the same total amount of color
unless you don't have any genes to produce the pigment at all (the
white-red case? Much more complex examples of genetic regulation can
easily produce a final organism, a phenotype, that is relatively
insensitive to many variations in the genetic basis until the changes
push you over a threshold, in which case the system quickly
reorganizes around a different stable product. This can be expressed
in fancy systems theory language, as in the jumping of a dynamic
system from one stable "basin of attraction" to another or, in
Waddington's epigenetic landscape metaphor, to jump out of one groove
in the landscape into another.


You, of course, are perfectly correct in defining what PE historically
meant. I would argue though that evolutionary theory should have
moved on by now beyond Mayrism or EldredgeGouldism. The evo-devo and
molecular biology notions of how genomes work as tightly integrated
systems rather than collections of characters or traits is just an
integral part of our thinking about how organisms work and got to be
the way we are.


.

Relevant Pages

  • Re: Question about "punctuated equilibrium"
    ... Once a new factor is introduced, however, the process of change and the spinning off of new species resumes. ... Thus, if you look at the entire fossil record, it might look like ... selection -- but basically an organism is quite well adapted to its ... This is commonplace in Mendelian genetics with, ...
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  • Re: Question about "punctuated equilibrium"
    ... Once a new factor is introduced, however, the process of change and the spinning off of new species resumes. ... Thus, if you look at the entire fossil record, it might look like ... selection -- but basically an organism is quite well adapted to its ... This is commonplace in Mendelian genetics with, ...
    (talk.origins)
  • Re: Question about "punctuated equilibrium"
    ... the process of change and the spinning off of new species resumes. ... Thus, if you look at the entire fossil record, it might look like ... selection -- but basically an organism is quite well adapted to its ... Another scenario involves factors that would actually produce stasis ...
    (talk.origins)
  • Re: Question about "punctuated equilibrium"
    ... the process of change and the spinning off of new species resumes. ... Thus, if you look at the entire fossil record, it might look like ... selection -- but basically an organism is quite well adapted to its ... Then there are several scenarios that can cause a relatively sudden ...
    (talk.origins)
  • Re: Question about "punctuated equilibrium"
    ... Once a new factor is introduced, however, the process of change and the spinning off of new species resumes. ... Thus, if you look at the entire fossil record, it might look like ... selection -- but basically an organism is quite well adapted to its ... This is commonplace in Mendelian genetics with, ...
    (talk.origins)