Gradualism vs' Punctuated Equilibrium
conrad
so far a great read, and I'm wondering about gradualism
vs' punctuated equilibrium; specifically, if we have
a set of fossils from one species to an ancestor
such that the transitional fossils that exist within
the set are such that it can be viewed as being
virtually continuous, then would this not
counter the claims of punctuated equilibrium?
I'm sure there is some subtle point I'm missing
here because Stephen J. Gould was a rational
fellow who devoted a life time of studying and
investigating evolution and so is not likely to
put forward an idea that seemingly runs counter
to gradualism while having likely been
aware of rather complete sets of fossils like
that of the whale and its ancestor, horses, or
from the therapsid reptiles to the mammals.
Thus, if someone could shed some light on
this point I would much appreciate it.
--
conrad
r norman
wrote:
Gould and, more properly, Eldredge, proposed punctuated equilibrium
most specifically because of their familiarity with long stretches of
fossil record of specific organisms. Wikipedia has a very nice essay
on the subject
http://en.wikipedia.org/wiki/Punctuated_equilibrium
which carefully distinguishes between punc-eq and "saltational"
evolution or macromutation where large jumps are made by single
mutational events. That is not what punc-eq really refers to and it
is not at all antagonistic to gradual change. It is just that a
relatively large population of organisms is rather stable although at
the geographic fringes of the range, gradually accumulated changes may
result in populations rather different from the norm, new species in
fact. Some type of ecological event may then cause the population to
suddenly change with the formerly isolated group invading and taking
over the habitat previously inhabited by the parent group. The fossil
record taken from a single location would show a sudden change in
phenotype even if the changes occurred gradually.
With the advent of evo-devo (the application of new data on
developmental biology and gene regulation and expression to
evolutionary biology) we now know that it is possible both for
seemingly small mutational events to cause a rather large phenotypic
effect and that it is also possible for the gradual accumulation of
mutations to first produce no noticeable effect until the regulatory
system becomes overwhelmed and suddenly reorganizes to produce a very
large change. In this sense, saltations and macromutations are also
quite possible mechanisms to produce the punc-eq effect.
It is also true that just how "smooth" vs. "step-wise" the fossil
record appears depends strongly on how closely you look. The examples
you mention, the evolution of the horse or of the whale involve such
major and drastic changes that a series of small step-wise changes
in each detail would look like gradual change when viewed over the
long stretch.
John Harshman
> I'm currently reading "What Evolution Is" by Ernst Mayr,
> so far a great read, and I'm wondering about gradualism
> vs' punctuated equilibrium; specifically, if we have
> a set of fossils from one species to an ancestor
> such that the transitional fossils that exist within
> the set are such that it can be viewed as being
> virtually continuous, then would this not
> counter the claims of punctuated equilibrium?
No. But Gould never argued for saltation. In classical PE, transitions
are gradual, with all the intermediates anyone would expect. In fact,
the initial PE paper contained a claimed example of gradual change, in
the evolution of Phacops trilobites. The claim is that such gradual
changes are rare in the fossil record because they usually happen
quickly in geological time, and in a restricted geographical area, such
that we are unlikely to have a good series to show them.
But let's suppose you are referring to a series showing gradual change
on a geological time-scale, i.e. a smooth transition over millions of
years. Would this disprove PE? Sure, in that one case. But Gould never
(hmmm, better make that "seldom", just in case) claimed that PE was the
sole means of speciation, just the predominant one. You need to assemble
a whole bunch of transitions and see how many of them match gradualism
(however you want to define that) and how many match PE.
A bigger problem is that it's impossible to tell if one fossil species
(really, a morphotype) is ancestral to another. An even bigger problem
is that you can't really identify species in the fossil record, only
morphotypes.
> I'm sure there is some subtle point I'm missing
> here because Stephen J. Gould was a rational
> fellow who devoted a life time of studying and
> investigating evolution and so is not likely to
> put forward an idea that seemingly runs counter
> to gradualism while having likely been
> aware of rather complete sets of fossils like
> that of the whale and its ancestor, horses, or
> from the therapsid reptiles to the mammals.
Gould would say that these series are not gradual transitions at the
level of species. Instead they're a series of species showing a
long-term trend. Nothing to do with PE vs. gradualism. Remember, PE is a
theory about speciation.
> Thus, if someone could shed some light on
> this point I would much appreciate it.
Sure.
conrad
wrote:
I may be misremembering but having read
the Blind Watchmaker, in retrospect, it
would seem like Richard Dawkins misunderstood
Gould's position then? I don't know if you have read
this particular book, but from what I recall, Richard
Dawkins took great pains in showing that PE was
encapsulated by gradualism.
--
conrad
conrad
Do you know of good books or research articles
that discuss the cumulation of mutations not
taking effect until a "regulatory system becomes
overwhelmed"?
It sounds like great stuff, I'd like to read more
on it.
> It is also true that just how "smooth" vs. "step-wise" the fossil
> record appears depends strongly on how closely you look. The examples
> you mention, the evolution of the horse or of the whale involve such
> major and drastic changes that a series of small step-wise changes
> in each detail would look like gradual change when viewed over the
> long stretch.
>
Good point.
--
conrad
John Harshman
This all depends on what you mean by "gradualism". Gould never claimed
that PE was instantaneous; according to him, speciation would generally
take somewhere around 10-100 thousand years. This would seldom be
visible in the fossil record.
I don't remember what Dawkins said, but PE is really a theory about the
relationship between speciation and morphological change. Under PE,
species spend most of their time in stasis, and change only during
speciation. Unfortunately, in the fossil record the only way to attempt
to recognize speciation is by observing morphological change, and thus
PE reduces to a theory that morphological change is coincident with
morphological change.
Tim Tyler
> I'm currently reading "What Evolution Is" by Ernst Mayr,
> so far a great read, and I'm wondering about gradualism
> vs' punctuated equilibrium; specifically, if we have
> a set of fossils from one species to an ancestor
> such that the transitional fossils that exist within
> the set are such that it can be viewed as being
> virtually continuous, then would this not
> counter the claims of punctuated equilibrium?
It doesn't counter the claim that stasis is commonplace.
--
__________
|im |yler http://timtyler.org/ t...@tt1lock.org Remove lock to
reply.
r norman
wrote:
>On Jul 5, 9:44 pm, r norman <r_s_norman@_comcast.net> wrote:
<snip>
>> On Sat, 5 Jul 2008 19:15:06 -0700 (PDT), conrad <con...@lawyer.com>
>> With the advent of evo-devo (the application of new data on
>> developmental biology and gene regulation and expression to
>> evolutionary biology) we now know that it is possible both for
>> seemingly small mutational events to cause a rather large phenotypic
>> effect and that it is also possible for the gradual accumulation of
>> mutations to first produce no noticeable effect until the regulatory
>> system becomes overwhelmed and suddenly reorganizes to produce a very
>> large change. In this sense, saltations and macromutations are also
>> quite possible mechanisms to produce the punc-eq effect.
>>
>
>Do you know of good books or research articles
>that discuss the cumulation of mutations not
>taking effect until a "regulatory system becomes
>overwhelmed"?
>
>It sounds like great stuff, I'd like to read more
>on it.
My comments were based mostly on my knowledge of complex systems
theory and neurobiology where such things are commonplace. For
example, a series of receptors with analog or graded behavior can act
as a digital switch (on/off only) by interacting cooperatively or with
positive feedback. Each component is analog, graded, or gradual but
the ensemble acts digitally producing no effect until a threshold is
reached and then suddenly switching to a new pattern of activity. The
all-or-none action potential in neurons is a result of such positive
feedback interaction. Biochemistry has many examples of such
cooperativity: oxygen binding by hemoglobin is the classic example
although the end product is not so extreme as to be all-or-none but
rather shows a "sigmoidal" relationship instead of a linear one. The
basic electronic digital storage unit, the flip-flop circuit, is
another example.
If you want genetic types of examples, at the trivial level showing
the effect of regulation there is the phenomenon of dominant alleles
that produce the same effect whether one or two copies are present. So
some flowers given either one or two copies of a functional gene that
produces, say, red color, will always produce the same final amount of
redness. Other species with different regulatory systems will vary
the intensity of the color with gene dosage so that crossing red with
white does not produce all red offspring but rather will produce an
intermediate pink. That simply shows the phenomenon of gene
regulation, not its "being overwhelmed" and "suddenly reorganizing".
To show a rather sudden change in phenotype with a gradation of
genetic changes, consider Huntington's Disease. This is caused by
mutations in the Huntingtin gene (yes, the spelling is different).
This gene contains a repeating segment CAGCAGCAG... a so-called
trinucleotide repeat that codes for a series of glutamine. If the
series contains up to 36 glutamines in a row, there is no problem.
However 36 to 39 produces a late onset and slow progression of
symptoms of the disease and 40 or more produces severe disease.
There are a number of diseases caused by variation in this
trinucleotide repeat sequence. See
http://en.wikipedia.org/wiki/Trinucleotide_repeat_disorders
As to true "genetic reorganization" on the basis of cumulative
mutations, that was primarily a surmise on my part based, as I
indicated earlier, on standard systems theory considerations, purely
theoretical. I am not familiar enough with the molecular biological
literature to give you better examples.
r norman
wrote:
For starters, though, you could google "genetic regulatory
reorganization".
Augray
<jharshman....@pacbell.net> wrote in
<kNWbk.13174$uE5....@flpi144.ffdc.sbc.com> :
>conrad wrote:
>> I'm currently reading "What Evolution Is" by Ernst Mayr,
>> so far a great read, and I'm wondering about gradualism
>> vs' punctuated equilibrium; specifically, if we have
>> a set of fossils from one species to an ancestor
>> such that the transitional fossils that exist within
>> the set are such that it can be viewed as being
>> virtually continuous, then would this not
>> counter the claims of punctuated equilibrium?
>
>No. But Gould never argued for saltation. In classical PE, transitions
>are gradual, with all the intermediates anyone would expect. In fact,
>the initial PE paper contained a claimed example of gradual change, in
>the evolution of Phacops trilobites. The claim is that such gradual
>changes are rare in the fossil record because they usually happen
>quickly in geological time, and in a restricted geographical area, such
>that we are unlikely to have a good series to show them.
>
>But let's suppose you are referring to a series showing gradual change
>on a geological time-scale, i.e. a smooth transition over millions of
>years. Would this disprove PE? Sure, in that one case. But Gould never
>(hmmm, better make that "seldom", just in case) claimed that PE was the
>sole means of speciation, just the predominant one.
We never claimed either that gradualism could not occur in
theory, or did not occur in fact. Nature is far to varied and
complex for such absolutes; Captain Corcoran's "hardly ever" is
the strongest statement that a natural historian can hope to
make.
- Gould, S. J., & Eldredge, N. 1977. Punctuated equilibria: the
tempo and mode of evolution reconsidered. Paleobiology
3:115-151.:
>You need to assemble
>a whole bunch of transitions and see how many of them match gradualism
>(however you want to define that) and how many match PE.
[snip]
Friar Broccoli
Hmmm, in addition to this, above you said "Remember, PE is a
theory about speciation." I also know that it is your view (or just
a plain fact) that speciation and morphological change in
populations are separate processes without any necessary linkages,
which appears to render nonsensical the idea that: [Species] change
[morphologically] only during speciation.
Putting all this together, I gather that you think PE isn't worth much?
John Harshman
Not quite. It might be that, though there is no necessary logical
linkage, there is in fact a biological linkage. PE isn't nonsensical on
its face. I just think it's wrong. And it's hard to test with
paleontological data.
> Putting all this together, I gather that you think PE isn't worth much?
Yes. If we can salvage anything, it's the study of stasis, for which
paleo data is potentially useful.