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Re: Question on morphological saltation



I suppose this completely gradual analog, with infinite
morphological intermediate stages, is not defended by anyone. I suppose the question is how many steps there are between the two morphologies, and so, how much of gradual and how much of saltational there is in a morphological transition. But at some degree, I think some saltation has to be admitted, no mattering how small it is, to pass from one intermediary stage (step) to the other.<<<

You are not really referring to saltatorial evolution then (which is when a phenotypic character moves from one adaptive peak to another without crossing non-adaptive morphospace) but rather the issue of how many discrete intermediate "morphologies" exist that selection may act on. The problem with this question is that there is no single answer. Using your example of fur coloration shows just how different coloration developmental pathways can be. A quick survey of extant organisms shows several cases where color variation is very discontinuous (e.g. the famous peppered moths, or any of the range pseudo-albino white large cats varieties) while other organisms show a high degree of continuity in color spectrum (human hair color, for example).

Large discrete gaps tend to occur when a mutation happens further "upstream" in a gene network (resulting in specific protein products just not being produced) while more subtle continuous variation tends to result from changes "downstream"20(i.e. closer to the final protein product) and thus changes in the conformation (shape) of the protein, or the relative amount that is produced.

As a result, the question of how continuous a character can be must be determined by observation and/or experiement rather than falling under a simple rule of thumb. You are certainly correct that there are not an infinite amount of variations, but how many exist depend on several genetic and epigenetic factors. I would recommend investigating the "evo-devo" literature to find answers on specific character variation issues.

Cheers,

Scott


Scott Hartman Science Director Wyoming Dinosaur Center 110 Carter Ranch Rd. Thermopolis, WY 82443 (800) 455-3466 ext. 230 Cell: (307) 921-8333

www.skeletaldrawing.com

-----Original Message-----
From: Augusto Haro <augustoharo@gmail.com>
To: vrtpaleo <VRTPALEO@usc.edu>; dinosaur mailing list <dinosaur@usc.edu>
Sent: Mon, 20 Apr 2009 10:17 am
Subject: Re: Question on morphological saltation







Thank you very much Judith and Michael for the help and effort! I will search for these. However, I did not want to look for trouble regarding the tempo of evolution, or the punctuated equilibrium/phyletic gradualism controversy, which as you say, is affected by the incompletitude of the fossil record.

I do not try to criticize all morphological gradualism. There are
intermediate morphologies which suggests some degrees morphological

gradualism. But I suppose at some level there would be some
morphological saltation, to avoid the problem of the infinity.

I think I may perhaps express better my doubt in this metaphor:
suppose two evolutionary stages (of morphology/developmental history)
represented by the top and the base of an staircase. These stages are
defined by their height. You can have more or less steps in between,
representing intermediate morphologies/developmental histories, which
imply a kind of coarse gradualism. These steps in the example
represent morphologies or developmental histories, and not
intermediate stages in replacement in the population of some variety
by another. But between steps, there has to be saltation to reach the
step immediately above. The more intermediate stages there are
(steps), the smaller the saltation, and the more gradual the process
can be seen (although the steps do not have to be evenly spaced). The
purely gradual analog of this would be a single inclined platform
connecting the stages that represent the base and top of the
staircase, with infinite intermediary steps.

I suppose this completely gradual analog, with infinite morphological
intermediate stages, is not defended by anyone. I suppose the question
is how many steps there are between the two morphologies, and so, how
much of gradual and how much of saltational there is in a
morphological transition. But at some degree, I think some saltation
has to be admitted, no mattering how small it is, t
o pass from one
intermediary stage (step) to the other.

I would like to know if somebody previously stated that at least, not
all morphological transitions have to resemble the "inclined
platform". In Gould and Eldredge's papers these viewpoints on
morphological change gradualism are oppossed but not much treated, as
these authors were more interested on intermediate stages of
replacement within a population.

2009/4/20 Michael A. Bell <mabell@life.bio.sunysb.edu>:
Augusto -- Interesting question, but the fundamental problem with
answering it is that the geological record is generally too coarse to
capture the gradual steps that might have been present in the
transition
between species. There are some cases of gradual transitions at a
coarse
tme scale, and lots of punctuations. However, limited temporal
precision
generally prevents us from knowing what went on inside a punctuation.
Are
they ecological replacements, as originally proposed by Eldredge and
Gould
(1972, and surprisingly by Darwin, 1859, as well!), or are they
gradual
evolutionary events that took place in less than the >5000-year
resolution
time that most sedimenatry sequences provide? I suggest that you look
at
two reviews (there may be others):

Geary, D. H. 2009. The legacy of punctuated equilibrium. In:Allmon,
W. D.,
P. H. Kelly and R. M. Ross(eds.), Stephen J. Gould: Reflections on his
view of life, pp. 127-145. Oxford: Oxford
University Press.

Erwin, D. h. and R. Anstey (eds.) New Approaches to Specition in the Fossil Record. New York: Columbia Univ. Press.

In the 1980's there were a few computer simulation studies that showed
that assuming reasonable heritabilities and selection coefficients,
species-level phenotypic transitions could occur in a few hundred
generations. Papers by Sadler (1981) and Schindel (1980 in
Paleobiology)
also showed that the fossil record is almost always too coarse to
observe
changes that take place within a few hundred years(~ few hundred
generations). More recently, there has been a flood of information
showing
that major evolutionary changes can occur within a few tens of
generations, even in populations exposed to reasonably natural
enviornmental changes (Hendrey and Kinnison 1999 review in Evolution,
2001
special issue of Genetica). The fossil record is unlikely to allow
one to
distinguish between transitions that resulted from truly saltational
evolution based on genes with major effects and pure gradualism based
on
many genes of small effect.

The hypothetical example you happened to pick concerning coat color
is not
a good one, though I am sure you did not intend to be taken literally.
That trait is strongly influenced by the malanin synthesis pathway,
which
can be disrupted by a single mutation in one of several genes. Thus,
the
phenotype can go from dar
k to light by means of the action of a single
gene, and evolution can be somewhat saltational. I don't think we
have the
time series for coat color evolution because it does not leave a
fossil
record. A review in early 2008 by Arendt and Reznick in Trends in
Ecology
and Evolution includes evolution of coat color in several mammals,
and it
will give you entry into evo-devo of coat color.

Arendt and Reznick (2008) also cite a paper by Shapiro et al. (2004)
which
discusses the role of a gene of major effect (Pitx1) plus others of
minor
effect in the evolution of pelvic skeleton reduction in modern
stickleback
fish. There is a fossil record for that trait(Bell et al. 1985;
2006a),
and Pitx1 is implicated in the fossil case (2006b):

Bell, M.A., J .V. Baumgartner and E.C. Olson. Â1985. ÂPatterns of
temporal change in single morphological characters of a Miocene
stickleback fish. ÂPaleobiology 11:258 271.

Bell, M. A., M. P. Travis, and D. M. Blouw. 2006a. Inferring natural
selection in the fossil record. Paleobiology 32:562-576.

Bell, M. A., V. Khalef and M. P. Travis. 2006b. Directional asymmetry
of
pelvic vestiges in threespine stickleback. Journal of Experimental
Zoology
(Molecular and Developmental Evolution) 306B:189-199. DOI:
10.1002/jez.b.21132.

-- Mike Bell

Hi, and sorry the bother and ignorance, but I have a question
for the
evolution-knowing people. I am searching for Recent published
statements against the necessity of morphological gradualism.

I searched in Gould's papers on punctuated equilibria, but he is
mostly interested in phyletic gradualism, and on assessing the
problem
if the new varieties gradually replaces the old ones, at a constant
rate, or not. Sadly, I passed my Gould (2002) book to a friend and
can
not check for some days.

What I am sarching is, putting an example, not if a white bear
variety
gradually replaced the brown bear variety in the North Pole along the
time or whether there was stasis or not. I am searching for a
published statement against the necessity that lots of hues of
progressively lighter fur have to be hypothesized to account for the
appearance of the white bears. And, if you know, of refutations or
attempts of refutation to this.

I think an exaggerated morphological gradualism of this kind is
perhaps not defended by anybody (I did not find citations), for it
would imply infinite intermediate morphologies progressively more
similar to the descendant one (there are infinite hues between brown
and white, and between any colors), and there can not be infinite
individuals in any lapse of time involved in an evolutionary
transformation. This also applies when we see morphology as a dynamic
entity, as a developmental history, and not=2
0just an arbitrarily
defined morphology at a stage of their ontogenies. If we define the
dynamical morphology in terms of its developmental history, there
would be no more different developmental histories as there are
individuals, so there would not be possibility of complete
morphological gradualism either.

Note that I am not defending or referring to "hopeful monsters", or
messing into the macroevolutionary debate, we may just be referring
to
change in a single body part, in a single respect.



Michael A. Bell, Professor
Department of Ecology and Evolution
Stony Brook University
Stony Brook, NY 11794-5245, USA
Phone:1-631-632-8574. Fax: 1-631-689-6682
Bell Lab: http://life.bio.sunysb.edu/ee/belllab/