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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, to 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 <email@example.com>:
> 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 dark 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. 1985. Patterns of
> temporal change in single morphological characters of a Miocene
> stickleback fish. Paleobiology 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:
> -- 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 just 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/