Re: Evolution, "adaptation", and what's currently adaptive
Len Piotrowski (lpiotrow@magnus.acs.ohio-state.edu)
Fri, 6 Sep 1996 21:17:41 GMT
In article <50o647$u7m@argo.unm.edu> mycol1@unm.edu (Bryant) writes:
>[snip of portions of the tale]
>At some point during the evolution of kites (~falcons) or their
>geologically recent ancestors, there emerged a pigment (or several) which
>allow the detection by kite eyes of UV light. Why would such a trait be
>retained in the species? Random drift, perhaps? Or might it serve a
>function (i.e., make life and/or reproduction easier on kites)?
I only wish to make note of the difference between emergence and retention
alluded to here. More later.
>[snip]
>An extreme adaptationist would expect that only birds
>living on voles and other UV-absorbant poop-producers would have the eye
>pigments.
Appears from my experience with extreme adaptationists that many more purposes
associated with the detection of UV light would do as well. This is an
especially surprising conclusion given the fact that vole poop/urine was
apparently only recently discovered to absorb UV light. What other
environmental materials may have this same property?
>Nice, testable prediction number two.
>Now, what I want to know is this: how in the world would somebody who
>wants to convince us that that pigment is not in kite eyes because of any
>fitness benefit, but say, due to random genetic drift alone. In small
>populations, drift can drive alleles to fixation, after all. But how
>would one test such a notion without first falsifying the adaptationist
>alternatives?? There's *got* to be a way, but to my knowledge, nobody
>has identified it yet.
Perhaps there is a differential distribution of the pigment among different
kite populations that could be compared against their food sources. Perhaps
there are kites with the pigment who do not feed on UV absorbent poop
producing voles. Perhaps you can experiment with kites in controlled
situations where UV absorbent poop voles are not a food source, observing if
there are behavioral changes, differences, or shifts that are retained or
abandoned when they are returned to their natural state.
>>|> Interesting mapping function here: where does this "evolutionary pathway"
>>|> exist?
>>
>>In the history of living organisms.
>Boy, this is ironic. But I recommend that Lenny and others who do not
>understand how evolutionary pathways can be identified read SJ Gould's
>essay on homology, entitled "Why History Matters." (American Scientist
>published it as a part of the collected works in _Exploring Evolutionary
>Biology_).
Boy, this sure is ironic. I ask for a clarification and end up once again with
a not so subtle request by Bryant to get myself educated. I guess it doesn't
do any good to question you on your home turf, eh Bryant? But thanks anyway
for the recommendation. I think I can handle the terminology from here on.
>It is through morphological (and increasingly, genetic) homology that
>cladists build modern taxonomic phylogenies.
You mean to say that Firl's "evolutionary pathway" refers to taxonomy?
Never encountered the phylogenetic taxonomy of "eyes" before, at least not
so vividly portrayed as in Firl's zoological garden.
>[trilobite fossils eye lense snipped).
>>|> ? Vision systems continue to evolve *because* of the adaptive benefits that
>>|> vision accrues? How come a mollusc can't see then?
>>
>>Some molluscs can't see, just like some vertebrates can't see. Fish
>>which have lived in underground caves for many generations will lose
>>their eyes; it isn't worth it for them to make the investment in
>>growing eyes which are useless anyway.
>Some molluscs can (I think octopi were mentioned). But eyes are
>expensive. Without real fitness benefits to maintain their presence, all
>the "phylogenetic constraint" in the world won't keep them around, as the
>study of cave vertebrates shows: mutation accumulation destroys them.
>(This too is described in _Exploring Evolutionary Biology_.)
I thought the hypothesis was that "sight" was the purpose of "eyes" which has
obvious fitness benefits. I concede that in constant, ultimate darkness (the
absence of any detectable emission of the electro-magnetic spectrum) the
property of "sight" and thus an organ such as "eyes" would be anachronistic
(historical constraint and all that). But a recognition of the fact that
"eyeness" isn't universal despite situations where many generations existed
under the exposure to the electro-magnetic spectrum, flies in the face of such
broad generalizations like - "vision systems continue to evolve *because* of
the adaptive benefits that vision accrues."
I would also note at this point the rather unsystematic use of language
epitomized by such statements as: " it isn't worth it for them to make the
investment in growing eyes." If these words are meant to be direct, explicit,
and concise presentations of argument from the discipline, they leave much to
be desired. Unless there is some esoteric meaning attached to "worth," it is
beyond my fathom to envision the agent responsible. "Selection" as one of the
"forces of evolution" is a poor choice as an agent of change in the sense of
an entity balancing alternatives. Even so, selection isn't identified as the
agent of change in this case; "them" make the investment, which I can only
surmise is meant to be the organism itself!?!
>>Now, before you start strumming your teleological harp, lets get this
>>straight: the genes of the fish do not "know" that they're in a cave,
>>or that it's dark. What happens is that eventually a mutation occurs
>>which produces an eyeless fish, and not only does this new
>>configuration prove to be non-deleterious, it is actually advantageous
>>- "adaptive", one might even say.
>Because of the reduced metabolic investment in a pointless trait, you mean?
Firl said a mutation produced eyeless fish. "Sightlessness" as a purpose is
just as teleological as "worth" to this hypothetical cave dweller. I guess you
can account for even non-existent traits in a functional apatationist
framework, eh?
>>|> What it (eyeball) can be used for (to see) says nothing about where it came
>>|> from. Is this getting too taxing for you, Firl?
>To the contrary, Lenny. Eyes are so specialized for the task of vision
>that it is absurd to declare them an exaptation for some other task.
>Name one viable alternative to the theory that eyes have been designed by
>selection for sight.
>C'mon: just one hypothesis. :)
Thanks for the invitation to analyze another one of your tropes. However, I
don't think you'll yet be satisfied, at least no until I capitulate, eyeballs
and sugar craving, into your rhetorical camp. Let me try out some suggestive
notions though.
I notice both conceptual and definitional problems with your trope, as well as
a fuzziness (perhaps intentional) to your systematics. I think the first
problem with your statement is the declaration that "exaptation for some other
task" is at the core of much of the past debate. All "eyes" are not
specialized, nor do they perform all the "tasks" that "eyes" have been
classified to do. This does not, in my understanding, reduce itself to a simple
matter of exaptation and task alternatives as explanations for the occurrence
of "eyes."
Where "eyes" came from versus why "eyes" are retained appears to me
to be a more important consideration in disentangling your trope. The problem
of emergence versus retention are usually undetailed in the presentation of
the functional paradigm - where in the process does purpose become a part?
Kite eyes, you would say, were designed by selection for viewing UV absorbent
vole poop/urine. When this eye pigment first emerged, did it emerge by design?
Kite eyes apparently retain eye pigment sensitive to UV detection. Was this
retention by design? A design implies a designer, an agent of implementation,
an actor that achieves some act. You do not intend to suggest an existential
entity or the organisms themselves designed this pigment, do you? If you claim
this is not what you meant by the word "design," then what worth is a "theory"
that doesn't explicitly mean what it says?
You may claim this is semantic quibbling, but that's what theory building is
about, conveying meaning. In as much as your theory makes claim to a science,
than the meaning should ostensibly attempt to reflect a real world invariance.
In as much as "design" and "functional purpose" presuppose objects, classes,
and motives outside the subject of study, they are detractors from a clear
presentation of ideas, and open to misinterpretation.
My other quibble is over the two broad classes being mapped to one another,
and their use as proof of completeness, as if this justified the methodology.
"Eyes" have been used a number of times to include all manner of organs for
detecting aspects of the electro-magnetic spectrum. Although visible light has
been alluded to, we have just read about the UV detectors of kites. This class
of organs include the dispersed property of plants to detect sun light,
so-called 'unstructured "eyes"", "sophisticated" eyes, and the remarkable UV
detectors mentioned above. However, as a class designed to represent all
possible organs for detecting electro-magnetic radiation, it is incomplete.
"Sight" has been mapped to this class of organs, but this too is a broad,
substantial category, including the "seeing" of different things depending on
the nature and kind of illumination. The seeming simplicity of the mapping
function is deceptive. You soon find a lack of a one-to-one correspondence
between "sight" and "eyes," as you notice that some organs share aspects of
"sight" with other organs, yet not all share the same capabilities. The
functional, mapping relationship is an illusion when you resolve the
categories into their constituent parts.
So, what does it mean to say that all eyes were selected by design to see.
Well not very much. Are eyes designed at all? Are plant photo-receptors
equivalent to kite eyes? Are objects illuminated by visible light seen by eyes
the same way when illuminated by UV light? Of what usefulness is such a
narrow and ambiguous "theory" to understanding anything about eyes or sight or
evolution? Very little of significance in my estimate.
The entire "theory" cannot be said to be true as a logical statement. By
definition, if it's an eye it see's something. If it's designed, we have to
search for a designer. If selection designed it, it's emergence must have been
undesigned. The only argument for a final design is if it is used for
some purpose. If it's used by the organism for that purpose we must allow for
self-design, as the organism takes advantage of a new found property. So, from
my point of view, as an entity of change we can just as well postulate a
self-reorganizing system of behavior as the agent of ultimate change and
retention.
At any rate, such is my feeble attempt at answering your question. Probably
what you expected. But then again, I'm not a member of your camp either. : )
Cheers,
--Lenny__
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