Re: Evolution, "adaptation", and what's currently adaptive
Bryant (mycol1@unm.edu)
5 Sep 1996 21:34:31 -0600
In article <50ncrt$l8c@news.sdd.hp.com>,
Gerold Firl <geroldf@sdd.hp.com> patiently explained his comments about
nature's "complete gamut" of eye structures:
>I say "complete" because it ranges from a very simple ability to sense
>light and dark, with no structural "eye" at all, to the sophistication
>of humans and raptors, who can discriminate thousands of colors and
>extremely fine details, with all the intermediates between.
Which opens an opportunity for a nice adaptive tale (heh)... one that's
been tested, though. This was published in nature or science not too
many years ago (1? 2?) and I'll chase down a ref for anybody who actually
wants to read the sucker. I don't recall the exact device used by the
biologists or the biologists' names.
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)?
Somebody was bold or foolish enough to hypothesize the latter after it
was discovered that vole poop and/or urine ('poop' for our purposes here)
absorbs UV light.
Kites have this uncanny ability to "forgage" 'optimally'...
they avoid fields which have few voles, and circle patiently around fields
which biologists count many voles in. The ecologists involved
hypothesized that their UV detecting eye pigments are, in part, vole poop
detectors--and indirectly, vole density estimators.
So an honest-to-God experiment was performed, in which UV-absorbing
substance that mimics vole poop was spread around a vole-free field.
Another, near-by field was left poop-free. Guess where kites, and their
UV-detecting pigments, preferred to hunt? Ah, yes: the "poop" filled
field, with its trails of UV-absorbing counterfeit vole wastes.
As far as I know, the next logical step hasn't yet been undertaken. That
would be a comparison of the eye biochemistry of a wide suite of raptors
that live on prey which produce UV-absorbant wastes vs. those who live on
"invisible" poop. An extreme adaptationist would expect that only birds
living on voles and other UV-absorbant poop-producers would have the eye
pigments.
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.
>|> 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_).
It is through morphological (and increasingly, genetic) homology that
cladists build modern taxonomic phylogenies.
Hey, since we're on the topic, there's a nice paper floating around out
there which describes exquisitely preserved trilobite fossils. An
analysis of their eye lense suggests a correction for light defraction on
the ocean floor. (I didn't know that light defraction was a problem in
water unless you were looking into it from above the surface, but
apparently there is a correction to be made).
>|> ? 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_.)
>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?
>|> 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. :)
Bryant
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