friesen@psivax.UUCP (Stanley Friesen) (05/21/85)
Alright, since there seems to be a lot of misunderstanding
about just what is involved in Punctuated Equilibrium Theory, I
thought I would give a summary of it.
The "canonical" version of the theory, as supported by
Stephen J Gould, goes something like this. Observing that at the
*detail* level new species often appear in the fossil record
"suddenly", it was proposed that speciation occurs relatively
rapidly, in say several dozen to about a thousand generations.
This would cover such a short time as to be invisible at the
resolution of the fossil record. It was further proposed that
large population effects resulted in universal *stabilizing*
selection in established species, resulting in essentially *no*
evolution occuring *within* a species. Thus essentially all real
change is held to occur at speciation. Proponents of this theory
generally feel that pure mutation/selection mechanisms of classical
evolutionary theory are inadequate to explain the rapid speciation
proposed, and that thre must be other, as yet undiscoverd mechanisms
to explain these occurances. In general general these mechanisms
are held to introduce *relativeley* large changes in peripheral
populations of a species leading to the formation of a morphologically
distinct new species. These changes arew essentially considered to
be in essentially "reorganizations" of the genetic structure of
the organism, followed by accomodating selection to fix the changes.
No proposed or imagined mechanism suggests any *radical* changes
taking place, at least not on the order of a lizard giving birth
to a bird. In fact, I suspect even the change from a pheasant to
a peacock would be considered excessive by most reputible theorists.
Other evidence for the theory includes the existance of a number of
species *specialized* for taking advantage of man's agricultural
practices, which have only been in existance a few thousand years.
Thus these species are using adaptions which were meaningless a
few thousand years ago.
Another concept introduced in connection with this theory
is the idea of Species Selection. This is essentially an extension
of the Darwinian concept to selection among reproductively isolated
populations. The idea behind it is that only those species which
resulted from generally useful changes during speciation would be
successful in the long run. To some extent this replaces natural
selection at the population level as the explanation for adaptedness
in the living world, since selection is held to be a minor component
in the speciation process.
There is, however, a more "traditional" variant of the theory.
It is particularly espoused by Ernst Mayr. The main difference is that
the mutation/selection mechanism is held to be a prominant part of the
speciation event. Also the stabilizing effect in established species
is not considered to be *quite* so absolute. The lynch-pin of this
interpretation is the Ernst Mayr theory of speciation by means of
peripheral isolates. Thus change is held to be *relatively* rapid
during speciation, and *relatively* slow in established populations.
In this model speciation is held to take several hundred to several
thousand generations, still essentially instantaneous from the geological
point of view. There is thus, in this view, no reason to hypothesize
unknown mechanisms, since known mechanism are considered to be
adequate. Nonetheless species selection is often accepted, but
it can be considered simply a special of natural selection.
Well, that is probably long enough, so I will leave here.
Coming soon - the Ernst Mayr speciation model.
--
Sarima (Stanley Friesen)
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