[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

RE: Reversals



Jonathan,
 
Firstly, apologize for misspelling you're name (my senior bio tech was a John Wagner and it just kinda stuck in my mind)
>>As long as we are splitting hairs...
        It seems that everything, especially everything classificatory,
comes down to this eventually. :)
The point of my entire last two posts on this subject is that this
is a semantic nightmare.
>>>George has further pointed out that determining whether a reveral
(sensu cladistica) is "genetic" or "convergent" may be impossible. I agree
strongly with this assertion. George says that some "reversals" are actually
convergences. This results from his acceptance of either the definition
given above, or the definition of a "genetic reversal" as the true defintion
of reversal. This is indeed why the "cladistic" definition of reversal is an
operational defintion, not an explicit invocation of an evolutionary
process. We cannot know always what processes were afoot (sorry) in the
re-evolution of the four-toed therizinosaur pes.
>Well, using the operational definition, "reversal" is the correct
term regardless
 
Seems I have done us both a diservice. The intention was an apparently poor attempt at support for you're conclusions re: the need to understand the operation definition of "reversal", while attempting to illuminate some reasons why the definition must be operational (i.e." we would hardly expect a complete return to the "original" form of the character... with a different starting position in overall morphospace...and is under different selection pressures"). You correctly summed up Georges points re: limitations of the term reversal. And, as we both pointed out (I just did a really poor job), irregardless of the limitations of the term, the operational use of reversal, must and will continue.
 
>>We cannot know always what processes were afoot (sorry) in the
re-evolution of the four-toed therizinosaur pes
Where we differ...
Perhaps one day as more is understood of the genetic mechanisms that drive evolution within extant populations, we will be able to ascribe either "convergence" or "reversal" to particular speciation events in the fossil record. If the fossil record of a particlar clade is particulary complete, we should be even to hazzard an "educated" guess as to the magnitude of the selection pressure (in genetics breeding programs known as the Intensity of Selection).
>>If you wish to examine this question further, by all means do so.
I truly believe that some of the puzzles in paleontology will only be done so employing information gleaned from Neontology ("downward transfer of information").
>>Indeed, our catagorization
of these processes seems to be largely a construct of our imagination rather
than recognition of real phenomena. >>
Never said better. Even our defintion of a species is somewhat of a construct.  In my opinion, this is one of the attractions of paleontolgy (somewhat like techie cryptozoologists!)  Given the constraints of the fossil record, this is why I respect paleontolgists so much- the ultimate in puzzles!
 
Sorry about the long snips and for the confusion,
 
 
David.Lessin@Walgreens.com
David.Lessin@EMC.Walgreens.com
David.Lessin@accesschicago.net
Walgreens Telecommunications Dept.
Network Analyst/Database Design