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Re: Question on morphological saltation



I was referring to a morphological gradualism in terms of the
old-fashioned orthogenetic series of horses, carried to an infinitum of intermediate forms (which I suppose nobody defended), without referring explicitly to them (and only for simplicity neglecting ramifications).<<<

Of course people defended it; it was an actual hypothesis of decent. The major problem with it was the very limited knowledge geographically as well as poor temporal resolution. That said, it seems to me you are very much talking about Punctuated Equilibrium, as many workers do not think that macroevolution is significantly different than microevolution except for the number of generations involved. You seem to be asking what the smallest amount of change between species is, and as evolutionary biologists are still disagreeing as to what the dominant pattern of selection, stasis, and speciation is, your question cannot even begin to be answered at this time.

For example, suppose a mutation created a white bear from brown
parents. The white variety then progressively displaced the old variety from the population until reaching fixation. That is a gradual replacement. Yet, there was a change from brown fur to white fur, and not between all infinite hues of brown each time lighter. This I called "morphological saltation", because of not knowing another term.<<<

You are using the term correctly, but it's not obvious that this is actually the case. As I mentioned, in some organisms you get very discontinuous variation like this, but in more circumstances you do actually get more continuous variation (like from light to dark). You also can get chances in the pattern of coat colors that are both continuous (say variation in stripe patterns on zebras) or discrete (like the presence or absence of stripes in various white tigers). It _could_ be that polar bears produced a lighter coat thanks to a chance mutation that caused it to be white, but it's not necessary and there's not intrinsic reason why the color could not have gotten significantly lighter over time. Which actually occurred would have to be established by experiment (in this case a better understanding of the gene network that determines color value in ursine fur would probably constrain the options...perhaps this has been done, but if so I am unaware of the study).

This, I suppose, can also apply to the trait, say, development of
"process x", without having to hypothesize infinite stages of the developmental histories for the process.<<<

Right, but again, this is a matter of ongoing debate; almost certainly both types of variation occur, and the question whether one type is predominant, and what roll selection plays on those characters. Gould discusses much this same subject in the latter third of his Magnum-Opus-sized Structure of Evolutionary Theory, although he and I would not see eye to eye on the subject of which pattern dominates in the history of life.

So unless I am greatly misunderstanding your request (and if so I apologize) then you are asking for something that cannot be answered, because there is debate over which types of variation are more common in nature, and to what degree selection acts on them.

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: dinoboygraphics@aol.com
Cc: VRTPALEO@usc.edu; dinosaur@usc.edu
Sent: Mon, 20 Apr 2009 11:48 am
Subject: Re: Question on morphological saltation






Thanks for the responses. I am not speaking in population terms when speaking of morphological saltation, this term is used by me (perhaps incorrectly) in a sense different from that of punctuated equilibrium. I am not sure if the gradualism in morphological change I referred to is necessarily related to the observed variation in one moment of the population history either.