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evo, devo, and neuro (Re: Neurotransmitters)

Hemidactylus at my-dejanews.com Hemidactylus at my-dejanews.com
Sat Feb 20 00:58:21 EST 1999

In article <7ake8t$ok7$1 at fremont.ohsu.edu>,
  Matt Jones <jonesmat at ohsu.edu> wrote:
(snip good stuff)
> As for the "evolutionary garbage" idea, I think what your prof may have
> meant is that there's always an ongoing process of mutation and selection
> among both neurotransmitters and receptors.

One species' garbage might become another's treasure. This is an underlying
theme of evolution. Gould (1991) discusses some of the issues overlooked by
strict Darwinist evolutionary biologists. Co-option could come into play,
where a component (morphological or molecular) previously adapted to another
function could be utilized in a novel manner (an exaptation in Gould's
words). One might want to consider the history of a component, in addition to
its present function. Some evolutionary biologists or evolutionary
psychologists might "reverse engineer" a component all the way back to the
start from a superficial look at its present utility. Daniel Dennett and
Richard Dawkins have been critical of Gould though.

Contingency is one theme among many picked up by Gerhart and Kirschner
(1997). One could look at signal transduction pathways as moldable in the
sense of how they fit into the overall picture (i.e.-context). Has anyone
read this book? Opinions?

Redundancy is another element in evolution where duplication of genes allows
the degrees of freedom facilitating an exploration of the "landscape". And
since I've alluded to Sewall Wright's metaphor, let's not forget neutral
drift (nor Kimura and Lewontin). To what degree have components like
receptors undergone sequence divergence due to drift? Sometimes alleles get
fixed in a population by chance, such a when a "sampling error" occurs in a
reduced population (aka bottleneck).

> Evolution is always trying
> out new structures by accident, and some of these turn out to be useful.

Useless (neutral) change could accrete to a degree at the molecular level
don't you think? Of course when looking at a receptor (e.g._ ACh) you have
functional regions that might be more constrained, but other regions that
might drift, I would assume, not being a molecular biologist :-)

Oh yeah, constraint (or stabilizing selection?) is another possible feature
of evolution. Some developmental processes early on are conserved in the
vertebrates. Slack, Holland and Graham (1993) discuss an early conserved
pattern of gene expression, the "zootype", which has components such as
*orthodenticle*, *empty spiracle*, and the *Hox* cluster(s). Gerhart and
Kirscher (1997) discuss compartmentalization issues regarding Hox clusters. I
recall some work having been done on the rhombomeres. I assume
*orthodenticle* and *empty spiracle* to be important in cephalization. Maybe
I've read too much into this? And with the *Hox* clusters there could have
been one of those duplication/divergence processes. Garcia-Fernandez and
Holland (1994) found that the cephalochordate (sister taxon of vertebrates)
known as amphioxus has one *Hox* cluster as opposed to four in mammals.

> So you may end up with ten different (but closely related) peptides
> acting on twenty different (but closely related) receptors, where they
> are all doing the same job (for example, causing the opening of potassium
> channels).

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