Richard Wein wrote:
>So how do you determine which is the "correct" explanation, without
>considering parsimony? You can't. For any set of observations, there are an
>infinite number of theories that could explain them. For example, consider
>fitting a curve to a set of data points. There are an infinite number of
>different polynomials that will fit, not matter how many data points you
>have. But, we would tend to reject higher order polynomials on the grounds
>of parsimony. In fact, knowing that there are likely to be random errors in
>the data, we would probably accept a simple curve which gives an imperfect
>fit, in preference to a high order polynomial that gives a perfect fit,
>because the latter seems ad hoc.
I would think when it comes to explaining the real world, a perfect fit would
always be the goal, a perfect fit between the description and the reality.
>So it seems you *are* allowing that new parts can evolve by duplication and
>specialization. Why can't these new parts include "skeletal segments and
>discrete organs"?
I think the duplication does not take place within an organism, except in
the case of atavism, which is not creating anything new. Duplication is
a crude Siamese-twinning mechanism that affects organisms as a whole
and which was operative in the early Cambrian explosion. Many-segmented
complexes were formed, whose subsequent evolution has been a matter of
gradual reduction and distortion.
>Yes, that may well be the reason why we don't see duplication of skeletal
>segments and organs in more recent times (though I'm not sure that there
>have been *no* such duplications in post-Cambrian times). But let's suppose
>you're right about that. Then we're in agreement as far as post-Cambrian
>evolution is concerned. The question is why you rule out such duplications
>for pre-Cambrian species? Or, if you don't, why you think that this is
>insufficient to explain new skeletal segments and organs in pre-Cambrian
>species.
I don't rule out anything for pre-Cambrian species. I think things were
wilder then, things are more limited now, due to the establishment of
a dominant fauna.
>>Such supernumerary structures are atavistic expressions of structures
>>that are normally suppressed in development in the modern forms, but
>>which were normal in an ancestor. The point being that they are not
>>newly evolved structures. Though rare in nature, they are actually part
>>of the range of variation of the species in question.
>
>Do you have any evidence for this?
The alternative is to believe that brand new structures can just pop into
being in the right places with all the right cell types etc through random
mutation, but that magically this doesn't involve a lot of false starts where
incredibly malformed weird structures pop up all over the body.
>Correct me if I'm wrong, but weren't the extra legs in fruit-flies caused by
>*duplication* of a gene, not by reactivation of an atavistic gene?
They can identify and label such mutations, as they recur regularly,
but I don't think they can tell whether a gene is new. The fact that
there is a regular catalog of recurring mutations suggests to me
that these are ancient, that they have been recurring as long as
there have been fruit flies.
>So how do you think those centipedes got all their legs? One-legged
>symbionts merged to form two-legged ones, then two-legged symbionts merged
>to form four-legged ones, etc? And how did the one-legged symbionts get
>their legs?
Note that the legs are segmented, they are composed of (originally)
identical segments which could have formed through a Siamese-twinning
process.
1. Unsegmented organism.
2. Simple train of segments, forming something that can wiggle around.
3. Combinations of trains of segments, forming the arthropod plan.
4. Further combining, forming the more complex vertebrate skeleton.
At each stage, complexes are formed through a sudden crude multiplicative
process; subsequent evolution is Darwinian, distorting (including eliminating)
segments gradually.
>Well I have to ask you again for *some* sort of scenario for how a complex
>organ such as the eye could be formed by merging of symbionts.
This merging of symbionts is a crude macroevolutionary mechanism
forming complexes of parts which are then gradually modified.
Most of the wonders of evolution are the results of gradual Darwinian
evolution.
>We *do* have a scenario for gradual evolution of the eye, based on the range
>of different eyes that we see today (see Climbing Mount Improbable if you're
>not familiar with this argument), and Nilsson and Pelger's computer
>simulation of the evolution of an eye.
I'm suspicious of the practice of simply lining up organisms in order of
apparent complexity and calling it evolution.
>>It's amazing, how adaptable are the parts of the developing body. When
>>the early embryo is greatly disturbed or disrupted, the various parts *try*
>>to fill their roles, seeking out their proper neighbors and counterparts,
>>trying to adapt to the new configuration, as if they were free symbiont
>>organisms. If the Darwinian model of gradually refined shape and position
>>is true, then there should be no such adaptability; a compromised start
>>should be an immediate disaster for the developmental process.
>
>I'm no expert on embryology. But it seems to me that the embryo
>develops through parts growing out of the basic form, not by parts growing
>separately and then "seeking out" their neighbours.
Then how do you explain the phenomenon I referred to? If development
were simply a matter of filling out the predetermined correct adult form
this capability of making adjustments should not be there.
>By the way, is this all your own theory? If you can direct me to a web page
>that describes the theory, I wouldn't have to ask you so many questions.
At <http://www.cab.com/segment/tablecon.html> there's an article which
is about a theory of this kind discussing the evolution of the vertebrate
skeleton. It's pretty long, and it avoids the more speculative business
about organs. At one point I specify what points I think are original.
Odd as these ideas may seem, I'm confident that if they were ever
to gain some currency, many would say they are old stuff.
--Cliff Lundberg ~ San Francisco ~ 415-648-0208 ~ cliff@cab.com
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