Re: Genetic or Epigenetic: The Causal Basis of Evolutionary Change

On Feb 20, 9:49 pm, "rnor...@xxxxxxxxx" <rnor...@xxxxxxxxx> wrote:
On Feb 20, 7:15 pm, "rnor...@xxxxxxxxx" <rnor...@xxxxxxxxx> wrote:



On Feb 20, 7:09 pm, CNCa...@xxxxxxx wrote:

On Feb 20, 8:15 pm, "rnor...@xxxxxxxxx" <rnor...@xxxxxxxxx> wrote:

On Feb 20, 5:42 pm, hersheyh <hershe...@xxxxxxxxx> wrote:

On Feb 20, 3:14 pm, CNCa...@xxxxxxx wrote:

<snip all except this one point>

Evolution of the body size in Manduca sexta.

In the course of 30 years, or about 220 generations, this insect
evolved a 50% increase in body size and investigators  came to the
conclusion that no mutational changes but changes in three epigenetic
factors: growth rate, critical weight and timing of secretion of
neurohormone PTTH “almost completely account for the evolutionary
increase in body size observed.” (D’Amico et al., 2001;  Davidowitz et
al., 2003; Davidowitz et al., 2004).

Inadequate reference.  This is not the first time we have pointed out
that you have not adequately referenced this claim.  BTW, the mere
claim that the increase in body size is due to "growth rate, critical
weight, and timing of secretion of...PTTH" does not prove anything
about whether or not the cause of these changes were genetic (genomic)
or epigenetic.   <snip>

I know this is a little old now, but here is the real dope on body
size in Manduca sexta, the Tobacco Hornworm.  Davidowitz has a nice
web site describing his work at
 http://eebweb.arizona.edu/faculty/davidowitz/research.html

Here is what Davidowitz himself claims, taken verbatim from his web
site:

--------------Davidowitz claims---------------------------
In spite of the interest in the ecology and evolution of body size,
remarkably little is known about the developmental and physiological
mechanisms that determine body size, or how these developmental
mechanisms change to result in plasticity of body size. The goal of
our research is to understand these mechanisms. Our main findings so
far show:

That there are three physiological factors that regulate body size and
development time in the tobacco hornworm (Manduca sexta): (i) the
timing of the cessation of juvenile hormone secretion in the last
larval instar, (ii) the timing of ecdysone secretion and (iii) growth
rate (Davidowitz and Nijhout, 2004).

These three factors can explain 95% of the variation in body size
(D'Amico, Davidowitz and Nijhout, 2001).

These three factors also regulate development time (Davidowitz and
Nijhout, 2004; Davidowitz, Roff and Nijhout, 2005).

The mechanism regulating phenotypic plasticity of size depends on the
environmental signal: regulation of plasticity of size in response to
diet differed from the mechanism regulating plasticity of size in
response to temperature (Davidowitz, D'Amico and Nijhout, 2003;
Davidowitz, D'Amico and Nijhout, 2004).

The same three factors that regulate plasticity in body size also
regulate the evolution of body size (D'Amico, Davidowitz and Nijhout,
2001).

----------end Davidowitz claims-----------------

So the mechanisms regulating body size are physiological and these
same factors also "regulate the evolution of body size".  There is
nothing to indicate that genetic factors are not responsible for the
physiology or that epigenetic factors must be responsible.  Genes
build the body to work in a certain way (physiology) and that explains
it completely.

Thank you Mr Norman. All of them are epigenetic factors. That is
elementary biology.-

Physiology is epigenetics?  That is not elementary biology.  I have
taught elementary biology for more than 30 years and physiology has
never been considered to be epigenetics.  I have  taught physiology
for more than 30 years and physiology has never been considered to be
epigenetics

Let me be rather more pointed and direct in my response.  Davidowitz,
Roff, and Nihout published a paper in 2005 titled "A Physiological
Perspective on the Response of Body Size and Development Time to
Simultaneous Directional Selection"
  INTEGR. COMP. BIOL., 45:525–531 (2005)
In that paper, they use the word "gbenetic" 15 times.  The subject of
the paper relates clearly to the physiological mechanisms underlying
evolution of body size in these insects and they did selection
experiments to work out some of the details about how the
physiological mechanisms interact.  Their introduction states: "Here
we demonstrate how an understanding of the underlying physiological
mechanisms of life history traits, can provide important insight into
their response to simultaneous selection."

Good for you. Physiological, not genetic mechanisms.

Their abstract (synopsis) starts: "Natural selection typically acts on
multiple traits simultaneously. Quantitative genetics provides the
theory for predicting the response to selection of multiple traits".

What I read in the synopsis is a little different. You have halved
the sentence and, unwillingly, distorted its meaning. Here is the
unabbridged sentence:

"Quantitative genetics provides the theory for predicting the response
to selection of multiple traits and predicts symmetrical responses to
selection (the response to upward selection on both traits is equal to
their response to downward selection). In reality, however, the
response to simultaneous selection on two traits is often
asymmetrical. We provide a physiology-based framework to explain the
asymmetrical response to simultaneous selection on two important life
history traits: body size and development time."

They also add:

"The observation of asymmetric responses to selection highlights the
insufficiency of quantitative genetic analyses based solely on
estimates of genetic variances and covariances (Nordskog, 1977Go).

The quote shows that the case rejects the prediction of quantitative
genetics and/hence authors provide a physiological framework, not a
genetic framework, to explain the asymmetrical response.

In the body of the paper they state: "In Fig. 3 we assume a positive
genetic correlation
between development time and body size (a test of this assumption will
be reported elsewhere)."

The paper goes into details about "symmetric" vs "asymmetric"
responses, a detail we need not concern ourselves about here.  The
final two sentences in the paper are: "The degree of the
asymmetrical response to selection will depend on the relative
strengths of the phenotypic and genetic correlations represented in
the P and G matrices, respectively. Thus, this model emphasizes the
importance of the underlying physiological processes in understanding
evolutionary responses to selection and the control of complex
traits."
It seems quite clear that Davidowitz and company are not at afraid of
discussing all of their work on factors controlling body size in
Manduca on an evolutionary basis using classical genetics tools.
Note:  these are ecologists and evolutionary biologists, not molecular
biologists.  They are not at all interested in looking for specific
genes controlling the physiology.  The important point is that they
simply rely on the tools of classical genetics to do their work and
interpret their results.

Do you really believe that in this article authors reject their
previous conclusions on the nature of the evolutionary change in body
size?

In the same article you will read also:

"This model emphasizes the importance of physiological processes in
understanding evolutionary responses to selection and the control of
complex traits."
"Here we demonstrate how an understanding of the underlying
physiological mechanisms of life history traits, can provide important
insight into their response to simultaneous selection."

"It is not surprising then, that the regulation of body size and
development time, two important life history traits that affect the
whole organism, are coordinated through the hormone system"
The hormonal system is an extension of the nervous system and is under
strict control and regulation by the central nervous system. Look what
a distinguished biologist, H.F. Nijhout, one of the authors of the
article says:

"The central nervous system can integrate information about the
animal's internal and external environment and use this information to
regulate the secretion of hormones.... Perhaps the most interesting
thing about having a hormonal regulation of development is that
development comes under the control of the central nervous system."

Regards,

N.C.

.

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