Re: Punk eq vs gradualism

[geojbb@vaxc.hofstra.edu (Bret Bennington)]

Robert.J.Meyerson writes:

>What if both views [punctuated equilibrium and gradualism] simply describe
>>different aspects of the evolutionary=
> process.  Punk eq covers the realm of what is usually thought when people=
> hear the word "evolution,"  being the process that results in profound=
> changes in a population of animals.  Gradualism would describe the realm of=
> adaptation; in other words, the process that deals with subtle changes in a=
> population.  My point being that since animals are constantly adapting to=
> their environment, gradualism would be the best model for this process.

If one is going to discuss punctuated equilibrium and gradualism without a
lot of confusion and misapprehension, then it is very important that the
terms be defined as they were originally used by Eldredge and Gould (the
authors of the original paper defining p.e. Eldredge, N. and Gould, S. J.
1972. Punctuated equilibria: an alternative pyletic gradualism. In Schopf,
T. J. M., ed. Models in Paleobiology. Freeman, Cooper, and Co.).  In this
paper, E and G present punctuated equilibrium as a model to describe the
pattern of evolutionary change AS SEEN IN THE FOSSIL RECORD.  They argue
that most well studied stratigraphic sequences of fossils do not show the
gradual accumulation of evolutionary change through the temporal span of
the species (Phyletic gradualism).  In other words, the fossil record does
not support the hypothesis that a large population of individuals of a
species can accumulate adaptive change over time and slowly transform from
one species into a daughter species.  Rather, E and G argue that in most
cases the fossil record shows change in a species population to be a random
walk - sometimes in one direction, later in another - that does not
accumulate over time to produce speciation.  Instead, the fossil record
shows the GEOLOGICALLY rapid appearance of new species with a jump or
punctuation in morphological change.  In other words, species appear to
acquire their particular novelties during the speciation event and
henceforth remain relatively unchanged until they go extinct.  The
speciation event itself is not usually preserved in the fossil record
because 1) it probably occurs in a small population inhabiting a restricted
geographic range and 2) it occurs over a geologically short period of time
- perhaps 1000 to 10,000 years.  Both of these factors would greatly
diminish the probability of preserving the speciation event relative to the
probability of preserving the expanded, established population of the new
species over the million years (average for marine invertebrates) of its
existance.

With this in mind, I think that Robert is essentially correct.  Gradual,
i.e. Neo-Darwinian evolutionary processes based on natural selection and
adaptation certainly are the cause of speciation over the 1000 - 10,000
years that it happens (ecological time).  But on the much longer time scale
of the life of a species (geological time), there does not appear to be
directional, gradual change.  Rather, the pattern at this scale is
punctuational (in most cases).

J Bret Bennington
Department of Geology
114 Hofstra University
Hempstead, NY 11550-1090
516-463-5568
FAX: 516-463-6010
E-mail: geojbb@vaxc.hofstra.edu