From: Cliff Lundberg <cliff@cab.com>
>Parsimony seems to be something you look at when you're really
>in the dark and have nothing to go on but general principles. There is
>such a thing as correct explanation and it doesn't necessarily have any
>connection to parsimony.
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.
>>If you think that *some* complexity can evolve gradually, then why not
all?
>>Where do you draw the barrier?
>
>I'm focusing on gross morphology, on number of skeletal segments and
>number of discrete organs. Other kinds of complexity and refinement of
>adaptation can be gradual, no problem.
>
>>I don't use "complex" to mean having many parts. I would consider
>>duplication of an existing part to be only a small increase in complexity.
>
>Fine. And if the duplicated parts have different positions and uses, they
>can gradually differentiate and take on specialized forms. This is what
>happens, and this is Darwinian evolution.
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"?
>But you still have to reckon
>with the fact that this sort of duplication and multiplication doesn't
>seem to be a factor in evolution since the Cambrian; there were
>qualitatively distinct epochs when complexes were formed rapidly
>through processes that seem no longer operative, presumably
>because organisms became too refined and competitive for
>bizarre experiments to be viable.
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.
>>The point being that this can occur through a small genomic change.
>>Consider, for example the way that a small genomic change can result in an
>>extra pair of legs in a fruit fly, or a 6th toe in a human.
>
>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?
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?
Does anyone know whether the extra toe in humans is caused by
duplication/modifcation of an active gene or by reactivation of an atavistic
gene?
>Our limbs evolved gradually from lungfish limbs with many symmetrical
>bones; the expression of most of these bones has been suppressed by
>random mutations, but the ancient pattern still lurks and can show itself
>at times. This is archetypal thinking, anathema to Darwinism, but it seems
>to be the truth.
>
>As for the fruit fly, that 8-legged version is nothing compared to the
>centipede-like ancestor of insects.
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?
>>How do you recognize an incipient organ? We can see creatures with eyes of
>>various stages of complexity, from a pit lined with light-sensitive cells
up
>>to the human eye. Wouldn't the former qualify as an incipient eye?
>
>It might, if you could trace its evolution through the fossils. But what
you
>see is a Cambrian explosion in which complex eyes popped into being,
>along with creatures with simpler eyes, neither of which have changed
>much since then in terms of basic structural complexity.
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.
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.
>>It may well be the case that, as organisms become more complex, it becomes
>>more difficult for them to evolve new organs, because so many
interrelating
>>parts would have to change. That would explain why we don't see changes in
>>basic body-plan in the more complex species. Whereas a small genomic
change
>>may give a fruit-fly an extra pair of legs, it would require many
>>simultaneous changes to give a human being a functional extra pair of
legs.
>>(Nevertheless, it's amazing how flexible the human body plan is. I believe
>>humans are occasionally born with an extra pair of toes. And have you seen
>>pictures of healthy Siamese twins with shared limbs and organs?)
>
>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.
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.
Richard Wein (Tich)
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