Re: Evolution, "adaptation", and what's currently adaptive
Len Piotrowski (lpiotrow@magnus.acs.ohio-state.edu)
Thu, 29 Aug 1996 13:36:24 GMT
In article <502e7h$3gje@argo.unm.edu> mycol1@unm.edu (Bryant) writes:
>[snip]
>Perhaps I should backtrack and clarify. Maybe I've been unusually sloppy
>in explaining myself.
>Gould et al say that the adaptationist program (theoretical biology,
>basically) suffers from a Panglossian outlook: every single trait is
>optimally designed to enhance fitness. Most adaptationists I know would
>agree to the contrary, that most traits you can imagine measuring on an
>organism do not constitute traits retained for their fitness benefits.
>That is, most traits are not adaptations.
This is all well and good, but as I've made clear, the problem that arose was
with your "sugar craving" and "jealousy" traits as apparent counter examples.
>[snip] The male nipple has no apparent likely fitness effects.
>It is probably a developmental side-effect of some kind. Baggage.
Then I guess you would agree that there are other possible processes
accountable for the existence of these "traits."
>Where I did suggest functional design with likely fitness benefits were
>the appetite for sugar (not shared by all species, interestingly) and
>sexual jealousy (which is sexually dimorphic in humans and seems to serve
>male fitness via motivating mate guarding and thus paternity assurance).
These were juxtaposed in context with the Gould & Lewontin critique, not,
interestingly, as examples of "functional design," but as an apparent counter
to Gould & Lewontin's claim. I merely pointed out that they *were* examples of
"functional design," something to which you took some exception.
>I am still not sure exactly what the problem is, from your perspective.
>I gather that suggesting any adaptationist explanations is sinfully
>Panglossian, since you equated my two speculations with Gould's
>charactiture of seeing adaptation in every single trait.
That's a misrepresentation of my appraisal. I've made explicit at several
points my true position. (see above)
>Here's the rub: if a trait ain't selectively favored, it's either baggage
>(pleiotropically maintained or with such low fitness effects that it
>simply isn't selected against, which seems unlikely to me given the
>accumulation of mutations which would degrade the more complex traits in
>this group), or it's a recent mutation, or it's a result of genetic
>drift (accident). The early part of this century saw evolutionary theory
>prematurely dismiss trait after trait as "pleiotropic" or the result of
>drift, both very difficult notions to test. Only when adaptationist
>alternatives were tested were the presumed functionlessness natures of
>these traits abandoned.
No real argument there!
>Whereas mutation and drift are random events, natural selection is
>non-random, as are its evolutionary effects. You seem to posit random
>forces of evolution as viable explanations for highly non-random
>phenomenon, such as taste buds which detect sugar.
You slip the noose once again. Your claim was for a "sugar craving" gene, not
for the appearance of taste buds on a tongue. Regardless, as I've stated
elsewhere, the appearance of sugar receptors on the tongue in the ancestors of
human and non-human primates was an apparently ancient event, and may perhaps
have had adaptive value in the past. I've not seen an argument one way or the
other on this origination. But the fact that sugar receptors are retained in
modern human and non-human primates does not alone argue for their current
fitness value. This point of argument is still problematic, even if resolved,
with respect to the purported "sugar craving" gene.
>This is possible, if
>sugar detection involves only one or two loci, I guess. Sure seems
>likely, though, that detecting sugar might effect eating behaviors, and
>that this would have had fitness effects. Maybe I'm mistaken.]
Perhaps you are, who can tell! But it seems to me more likely that
sugar/glucose has more value in motor function than brain evolution or
development. Seems to me once you take the step onto the slippery
functionalist slope of limiting resources, sugar receptors, brain sensory
transmitters, message decoders, "sugar craving" sensations, brain
development, and ultimately brain evolution, the need for a functional
adaptationist explanation at each of the intervening steps/pieces of
the puzzle belies the all-at-once nature of the original hypothesis.
>Finally, although I may have been pretty clumsy with my examples, I
>believe it is fair to describe the brain (and thus the emotions and
>cravings generated within it!) as vulnerable to natural selection.
>Behaviors themselves have fitness effects, and the physical origin of
>those behaviors, the nervous system, is therefore subject to natural
>selection.
You may be right! But most meaningful human behavior witnessed in my everyday
experience has no fitness effects in the manner that you suggest.
Cheers,
--Lenny__
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