Richard Wein wrote:
>Perhaps I'm mixing up "parsimony" and "explanatoriness". But I think the two
>concepts are quite intertwined, and it's not easy to separate them. The
>point still remains that there's no need to appeal to a personal prejudice
>to reject non-natural explanations. There are good reasons to reject
>non-natural explanations based on the rational principles of parsimony and
>explanatoriness.
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.
>Taking this to the extreme, those who are satisified by such explanations
>can simply explain everything with the 3 words "God did it"! Or even shorten
>that to the single word "God!". ;-)
They can shorten it further yet, as far as I'm concerned.
>But if these abilities can evolve in organisms living in tight symbiosis,
>why can't they all evolve in a single organism?
It seems much less likely to me, that all these different tissue types
could evolve from nothing and perfect themselves within a complex
organism when selection pressure is bearing upon the organism as a
whole. During their independence they could evolve more efficiently.
Within an organism there are so many levels, so many cell types,
so many things going on, the crude process of selection upon the
organism as a whole doesn't seem up to the job, especially within
the limited Cambrian-explosion time frame.
>If you don't think that *any* complexity can evolve gradually, then there
>must have been a vast number of cases of symbionts merging, in order to
>account for today's huge complexity.
I don't see complexity increasing. Complexity seems to be diminishing
since the Cambrian explosion. I see vestigial organs, I don't see any
incipient organs; I see loss of skeletal segments, I don't see new
segments being added, except for occasional atavisms, which are
only throwbacks, not innovations.
>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. 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.
>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.
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.
>Likewise, the merging of two symbionts may have required only a relatively
>small genetic change, which is why I don't necessarily consider it to be a
>big increase in complexity.
Well, if phenotypes mean little to you, you can think that.
>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.
>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.
--Cliff Lundberg ~ San Francisco ~ 415-648-0208 ~ cliff@cab.com
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