Re: Genetic or Epigenetic: The Causal Basis of Evolutionary Change
- From: "rnorman@xxxxxxxxx" <rnorman@xxxxxxxxx>
- Date: Fri, 20 Feb 2009 11:01:59 -0800 (PST)
On Feb 20, 10:24 am, hersheyh <hershe...@xxxxxxxxx> wrote:
On Feb 19, 5:52 pm, CNCa...@xxxxxxx wrote:
[snip]
Given that changes in DNA sequences are excluded as factors of
producing and transmitting these characters in the offspring and that
the characters are maternally, via eggs, transmitted it is reasonable
to conclude that maternal cytoplasmic factors somehow influence the
maternal nervous system of the next generation to produce and deposit
the same factor in its eggs in repeating cycles for many generations..
No. It is most definitely NOT reasonable to *conclude* that maternal
cytoplasmic factors somehow influence the maternal nervous system of
the next generation to produce the *same* factor in its eggs. It is
not even reasonable to *assume* that the trait is a heritable
*difference* existing outside the gene rather than a regulatory
difference determined by the maternal *environment* acting on the
existing genome. Such a regulatory difference *can* be epigenetic,
but there is no reason at all to think that it isn't the genetic
capacity that exists and that that capacity is *reset* each
generation. That is certainly what happens with maternal effect genes
and with the epigenetic shut down of extra X chromosomes in mammals.
Again, among the *known* mechanisms of epigenetic phenomenon that do
get transmitted between generations, the vast majority reset each
generation and are genetic (DNA-based) at root. In unicellular
organisms, such epigenetic effects as prion-like switches in yeast can
persist for several generations of cell growth. But I don't know of
one that does in metazoans.
You keep ignoring how genes work by pretending that epigenetic
phenomena do not involve genetic bases (e.g., maternal effect genes,
the X-chromosome inactivation sites), have proposed that microtubules
and centrioles play a role without any evidence whatsoever that I can
see, ignored the *fact* that a metazoan body is a genome's way of
producing another genome, ignored epigenetic events that do not meet
your criteria, focus on metazoans because you have some bug up your
rectum about the role of the CNS, thus ignoring most of biology, fail
to even mention the difference between metazoans that have mosaic
development and those that have regulatory development, and ignore the
role of 'resetting' during gametogenesis.
I haven't bothered responding to these new series of posts partly
out of exasperation at his lack of understanding (or willingness to
accept) genetic mechanisms underlying all his examples and partly out
of his misinterpretation of my criticisms. For example, my claim of
"physiological mechanism" is not at all a denial of epigenetics in
total, it is merely a claim that factors that influence gene
expression during the life of a cell are not at all epipgenetic.
The notion of epigenetics can only be applied to changes that
propagate through cell divisions. I have long ago repented of my
earlier claim that the changes had to propagate across generations,
although only these latter types of changes can relate to evolution.
Cabej's claim about the nervous sytem is, of course, quite
outrageous. That "there is no gene for flying" can be replaced by the
notion that "there is no gene for predation", yet the whole
developmental process (controlled by standard genetics and molecular
biology) produces an organism capable of flying or practicing
predation as a life style. Behavior, of couse, is mediated by the
nervous system but rather complex behavior can be produced by nervous
circuits which are genetically determined. Many animals have highly
specific individual neurons that can be identified in all individuals
of a species with related species showing some differences in the
structure of the individual neuron, the "wiring diagram" between
cells, and the behavior resulting from the activity of such circuits.
There are known behavioral mutations in Caenorhabditis and Drosophila
especially, but also in other arthropods and gastropod molluscs. It
is true that the details by which genes specify neural circuits is
still murky and very incomplete, but this is a difficult area to
investigate. Certainly there is no reason to suspect that somehow the
organization of molecules withing the unfertilized egg carries the
information necessary to determine these circuits and behaviors.
.
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