gene recruitment
Tim
article on pubmed (link to the article at the end of this post).
Granted I didn’t go through the entire thing, as it slightly over my
head, but parts of the abstract grabbed my attention so to speak. “…
The prevailing opinion among biologists is that gene recruitment
results from random changes in genes or their regulatory regions, but
the supporting evidence is poor and controversial….[the article
presents] a mechanism in which gene recruitment is a neurally
determined event, an adaptive response to changes in environmental
conditions.”
Essentially my question is two fold: (1) what are the controversies
surrounding gene recruitment resulting from DNA changes and (2) is
there a sound argument for gene recruitment to be cognitive-based?
copy/paste: http://onlinelibrary.wiley.com/doi/10.1002/dvdy.22505/abstract
PS There's also a link for the full article somewhere on the bottom of
the page...I didn't include it as it's more advanced than I am.
r norman
wrote:
Thank you for the link and for opening what I hope will be a fruitful
discussion. I have to read the full paper carefully. My first
comment is that the nervous system is most definitely quite capable of
activating or inactivating specific genes as a result of nervous
activity -- that is to say, cognitively based. That is one of the
important mechanisms for establishing permanent memories or learning.
Typically, the change occurs solely within the particular neurons that
are involved. However the paper refers to "transgenerational
developmental plasticity" and that is something I will have to read
carefully about.
el cid
> On Sat, 11 Dec 2010 14:29:51 -0800 (PST), Tim <TimKit...@yahoo.com>
> wrote:
Indeed. I didn't find their mechanism for directed communication
back into the germ-line. I think if they had one the press would
be greater for this.
r norman
My impression is that we have discussed Cabej before, about his book
"Epigenetic Principles of Evolution." However I can't find the
specifics.
This paper presents a number of interesting real situations where
nervous systems control development including at least one
transgenerational effect: the environmental experience -- daylength
-- of a female flesh fly (Sarcophaga sexta) influences the ability of
the pupa in the next generation to undergo a pupal diapause to allow
it to survive the winter. The maternal influence is transmitted by
some substance in the egg.
The paper also rather overelaborates on the well-known ability of the
nervous system to influence gene activity not just in the particular
nerve cells involved. The mechanism acts through the neuroendocrine
system and the well known ability of many hormones to influence gene
expression. From this type of evidence the author speculates that
nervous control of gene expression is a major factor in many aspects
of developmental regulation.
The paper presents examples of developmental switches where some
particular species expresses genes entirely differently from closely
related species even though they both seem to have the same genetic
machinery. Since it is possible for the nervous system to have such a
switching action, then why not conclude that the nervous system is
frequently responsible for such switches. And then why not conclude
that is is actually a major factor in the evolution of the metazoa?
Many things are possible. Where is any actual evidence of significant
nervous system directed control of gene expression to influence
phenotypes that persist over evolutionary time? Not in this paper!
r norman
wrote:
I should emphasize that transgenerational epigenetic effects are known
and that nervous system controlled epigenetic effects are known.
People should not dismiss completely out of hand even the notion that
experience (i.e., the nervous system) can influence subsequent
generations through epigenetic mechanisms. On the other hand, people
should demand solid evidence for any specific example.
John Harshman
Didn't Cabej himself appear here for a while? As I recall, he never
could explain how his mechanism was supposed to work.
> This paper presents a number of interesting real situations where
> nervous systems control development including at least one
> transgenerational effect: the environmental experience -- daylength
> -- of a female flesh fly (Sarcophaga sexta) influences the ability of
> the pupa in the next generation to undergo a pupal diapause to allow
> it to survive the winter. The maternal influence is transmitted by
> some substance in the egg.
>
> The paper also rather overelaborates on the well-known ability of the
> nervous system to influence gene activity not just in the particular
> nerve cells involved. The mechanism acts through the neuroendocrine
> system and the well known ability of many hormones to influence gene
> expression. From this type of evidence the author speculates that
> nervous control of gene expression is a major factor in many aspects
> of developmental regulation.
>
> The paper presents examples of developmental switches where some
> particular species expresses genes entirely differently from closely
> related species even though they both seem to have the same genetic
> machinery. Since it is possible for the nervous system to have such a
> switching action, then why not conclude that the nervous system is
> frequently responsible for such switches. And then why not conclude
> that is is actually a major factor in the evolution of the metazoa?
>
> Many things are possible. Where is any actual evidence of significant
> nervous system directed control of gene expression to influence
> phenotypes that persist over evolutionary time? Not in this paper!
And that, if I recall, was the major sticking point in his discussions here.
John Harshman
> I’m a newbie here. I actually stumbled upon a gene recruitment
> article on pubmed (link to the article at the end of this post).
> Granted I didn’t go through the entire thing, as it slightly over my
> head, but parts of the abstract grabbed my attention so to speak. “…
> The prevailing opinion among biologists is that gene recruitment
> results from random changes in genes or their regulatory regions, but
> the supporting evidence is poor and controversial….[the article
> presents] a mechanism in which gene recruitment is a neurally
> determined event, an adaptive response to changes in environmental
> conditions.”
>
> Essentially my question is two fold: (1) what are the controversies
> surrounding gene recruitment resulting from DNA changes and (2) is
> there a sound argument for gene recruitment to be cognitive-based?
As far as I know there are no controversies. And if gene recruitment is
cognitive-based, how do plants do it?
el cid
> On Sat, 11 Dec 2010 22:32:02 -0600, r norman <r_s_nor...@comcast.net>
I see no problem with nervous system controlled transgenerational
epigenetic effects. Nervous system controlled gene recruitment
into the germ line genome would be something different, and I
think the paper teases on that front, particularly in the section
"ON THE NATURE OF GENE RECRUITMENT".
r norman
How do plants sense the direction of light? How do plants determine
the length of the day (or, rather, of the night)? How do they
determine up from down? How do they respond to physical contact with
things around them? They don't have nervous systems but they still do
it. Your conclusion is good -- your reasoning is flawed.
Having some gene recruitment cognitively based on animals doesn't mean
that it must work that way in all cases in all organisms. Of course,
Cabej wan't to surmise that the nervous system based epigenetics,
including gene recruitment, is a major factor in metazoan evolution
and that part seems clearly outlandish.
John Harshman
If you want to extend the concept of "neurological" to anything involved
in interacting with the environment, sure.
> Having some gene recruitment cognitively based on animals doesn't mean
> that it must work that way in all cases in all organisms. Of course,
> Cabej wan't to surmise that the nervous system based epigenetics,
> including gene recruitment, is a major factor in metazoan evolution
> and that part seems clearly outlandish.
What seems most outlandish is his underlying contention that genetics
and mutation have nothing to do with development or evolution.
Robert Carnegie: Fnord: cc talk-origins@moderators.isc.org
I am surprised to find myself apparently able to make an original
contribution to this discussion, to wit, that perhaps Tim meant to say
"neutrally determined", not "neurally determined". Unfortunately I
can't take that any further myself, but doesn't it fit better?
If this did not occur to anyone else, then I suppose that is partly
due to "Poe's Law", or something like it. In this case, that however
goofy a theory or argument is that somebody is apparently laying out,
we still assume by default that they mean and believe what they say.
John Harshman
Fraid not. You should check out the paper he's referring to.
William Morse
The discussion about Cabej may have been on sbe. I also recall a
previous discussion.
> This paper presents a number of interesting real situations where
> nervous systems control development including at least one
> transgenerational effect: the environmental experience -- daylength
> -- of a female flesh fly (Sarcophaga sexta) influences the ability of
> the pupa in the next generation to undergo a pupal diapause to allow
> it to survive the winter. The maternal influence is transmitted by
> some substance in the egg.
>
> The paper also rather overelaborates on the well-known ability of the
> nervous system to influence gene activity not just in the particular
> nerve cells involved. The mechanism acts through the neuroendocrine
> system and the well known ability of many hormones to influence gene
> expression. From this type of evidence the author speculates that
> nervous control of gene expression is a major factor in many aspects
> of developmental regulation.
And IIRC this was always the problem with Cabej. Not that he wasn't
discussing real effects, just that he was exaggerating their importance.
lucaspa
> The prevailing opinion among biologists is that gene recruitment
> results from random changes in genes or their regulatory regions, but
> the supporting evidence is poor and controversial .[the article
> presents] a mechanism in which gene recruitment is a neurally
> determined event, an adaptive response to changes in environmental
> conditions.
>
> Essentially my question is two fold: (1) what are the controversies
> surrounding gene recruitment resulting from DNA changes and (2) is
> there a sound argument for gene recruitment to be cognitive-based?
As I read the article, Cabej is confusing 2 different things:
expression of a particular gene and recruitment of that gene to a new
function.
We normally use "gene" to refer only to the protein coding section of
DNA. But every gene has, in the DNA in front of it, regions of the
DNA that will cause that gene to be transcribed to mRNA (expressed) or
repressed -- block the expression of that gene. With me so far? This
is the "promoter" region. When particular chemicals -- sometimes
proteins, sometimes small molecules like estrogen -- bind to the
promoter region, the gene is expressed.
The examples Cabej talks about in the paper deal with the promoter
region and the molecules that will bind to the promoter region. The
examples are specifically molecules generated by neurons in the CNS.
All of the examples involve portions of the brain that are not part of
"cognitive" function, such as the hypothalamus and pituitary. Also,
many of the signals -- such as FSH and estradiol -- are synthesized by
one type of cell but then released into the blood stream such that
other types of cells -- like ovarian cells -- will "pick up" that
signal and translate that signal to the promoter region of their own
DNA and turn on genes there.
One of his prime examples is the water flea. Here environmental
stress will switch the female water fleas -- who normally only produce
female offspring -- to produce male and female offspring. The trigger
in the oocyte (cell that will become the new water flea) is a chemical
called methyl farnesoate. This is a promoter and binds to the DNA to
cause the expression of the Y chromosome. Normally the genes that
make the proteins to synthesize of methyl farnesoate are repressed by
neuropeptides MO-IH-1 (mandibular organ-inhibiting hormone-1) and
MOIH-2 synthesized in, and secreted by, the secretory neurons of the X
organ/sinus gland complex. But in times of environmental stress,
those neuropeptides are themselves repressed. Thus the repression of
the genes that make methyl farnesoate are removed, those genes are
expressed and make methyl farnesoate. The methyl farnesoate, in turn,
binds its specific receptor during a critical period of oocyte
maturation and causes expression of the Y-chromosome. Thus, the next
generation is composed of both male and female water fleas.
Now, the trigger is environmental, but the rest of the mechanism is
under control of genetics -- whether the particular promoter or
repressor regions are on the DNA. Cabej then states: "The dramatic
inherited phenotypic changes of transition from the asexual (females
alone) to sexual (female + male individuals) generation occur in whole
populations and within a generation, unambiguously rejecting the
possibility of involvement of genetic mechanisms (changes in genes or
DNA in general) in the phenomenon." That is very wide of the mark.
There is involvement of "genetic mechanisms" that are there by natural
selection. All those promoter and repressor regions of the DNA for
the genes involved.
Basically, Cabej has made a strawman.
Tim
I apologize for not having been involved in this thread after starting
it. It was a pretty busy weekend. But I hope to do better this time
around. Anyway, I’m pleasantly surprised that so many chimed in. At
the same time, I was hoping to read more of a discussion on the
details of the content of the article, which is slightly above my pay
grade. Also after going through the thread I find my second question
really saw the bulk of the discussion.
I say this because, unfortunately, regarding the first question I
poised: only Harshman really answered it. Harshman, essentially,
believes that there are no “controversies surrounding gene recruitment
resulting fromDNA changes”.
I noticed that my second question (Is there a sound argument for gene
recruitment to be cognitive based?) had a greater degree of responses
and variation in answers. Unfortunately, I thought a few of these
answers were contradictory (Norman essentially accepts the occurrence
of cognitively based recruitment of genes but warns against
generalizations and El cid accepts the idea of a neural control, but
Harshman denies the possibility).
Harshman’s answer to the second question was interesting in that it
seemed to come out of the blue. Harshman argues, “if gene recruitment
is cognitive-based, how do plants do it?” . This would be a prudent
question, normally. But, unless I’m missing something, the article in
question is really on metazoans. So I really can’t grasp the
relevance of the question posed.
In terms of article detail, Norman was really the only one that
touched on some of the details regarding the content of the article.
He seems to accept the idea that nervous system controls cases of
transgenerational developmental plasticity presented in the article
and the transmission of the maternal influence in the egg:
” This paper presents a number of interesting real situations where
nervous systems control development including at least one
transgenerational effect: the environmental experience -- daylength
-- of a female fleshfly (Sarcophaga sexta) influences the ability of
the pupa in the next generation to undergo a pupal diapause to allow
it to survive the winter. The maternal influence is transmitted by
some substance in the egg.”
He also admits, however ambiguously, the possibility for nervous
system to be “a major factor in the evolution”:
“The paper presents examples of developmental switches where some
particular species expresses genes entirely differently from closely
related species even though they both seem to have the same genetic
machinery. Since it is possible for the nervous system to have such a
switching action, then why not conclude that the nervous system is
frequently responsible for such switches. And then why not conclude
that is is actually a major factor in the evolution of the metazoa? “
Regardless, the discussion is actually helping me to “piece” my
understanding of the article better, though a better consensus
(especially on the second question) wouldn’t hurt!
PS I apologize in the case I hit the "wrong" reply button. I am
replying to the overall thread, as opposed any one post.
John Harshman
Actually, I believe we are all agreed. We just use different definitions
of "gene recruitment". We are all agreed that gene expression can be
influenced by the actions of nerve cells. And we are all agreed that
this is not a force in evolution, much less the major force. Such
effects can, in a very few cases, reach across generations, but they are
in no cases long-term enough to influence evolution. And they are all
evolved responses -- *genetically* evolved responses -- arising through
mutation and natural selection of the ordinary sort.
Now, what I take "gene recruitment" to mean is explicitly evolutionary:
a change in gene expression that leads that gene to a novel function.
And this happens by the ordinary processes of mutation and selection. I
know of no exceptions.
> Harshman’s answer to the second question was interesting in that it
> seemed to come out of the blue. Harshman argues, “if gene recruitment
> is cognitive-based, how do plants do it?” . This would be a prudent
> question, normally. But, unless I’m missing something, the article in
> question is really on metazoans. So I really can’t grasp the
> relevance of the question posed.
Let me explain. If epigenetic, neurological effects are claimed to be a
(or perhaps the) major mechanism of gene recruitment in metazoans, then
why don't plants show huge differences in evolutionary outcomes? How do
they manage to do such recruitment fully as much as metazoans do, while
lacking that supposedly essential mechanism? I resolve that dilemma by
supposing that there is no such mechanism, and that plants and animals
do gene recruitment in the same, ordinary way.
> In terms of article detail, Norman was really the only one that
> touched on some of the details regarding the content of the article.
> He seems to accept the idea that nervous system controls cases of
> transgenerational developmental plasticity presented in the article
> and the transmission of the maternal influence in the egg:
> ” This paper presents a number of interesting real situations where
> nervous systems control development including at least one
> transgenerational effect: the environmental experience -- daylength
> -- of a female fleshfly (Sarcophaga sexta) influences the ability of
> the pupa in the next generation to undergo a pupal diapause to allow
> it to survive the winter. The maternal influence is transmitted by
> some substance in the egg.”
> He also admits, however ambiguously, the possibility for nervous
> system to be “a major factor in the evolution”:
> “The paper presents examples of developmental switches where some
> particular species expresses genes entirely differently from closely
> related species even though they both seem to have the same genetic
> machinery. Since it is possible for the nervous system to have such a
> switching action, then why not conclude that the nervous system is
> frequently responsible for such switches. And then why not conclude
> that is is actually a major factor in the evolution of the metazoa? “
I doubt he meant that as approval of Cabej's claims, but we'll see. The
idea that the two species have the same genetic machinery, i.e. that
their expression differences are entirely epigenetic, has not been
tested, and Cabej presents no evidence for that claim. I'm highly dubious.
> Regardless, the discussion is actually helping me to “piece” my
> understanding of the article better, though a better consensus
> (especially on the second question) wouldn’t hurt!
>
> PS I apologize in the case I hit the "wrong" reply button. I am
> replying to the overall thread, as opposed any one post.
I don't think you can do that, mechanically speaking. What you actually
did was respond to lucaspa, which I presume was the last post in the
thread at the time.
r norman
John is right -- we have small quibbles among ourselves but agree in
the larger issues.
My own view is that I am more willing than others to at least accept
the conceptual notion of "cognitive" (really "nervous system
mediated") meddling in transgenerational epigenetics but demand
actual hard evidence that such occurs in every case. Others seem to
have a more conservative policy of arguing that, until real hard
evidence is presented of a significant and lasting evolutionary change
produced by the nervous system, then we shouldn't even bother to
discuss it. Doing so is just a waste of time and effort.
As far as the notion that two species can have the same genetic basis
but differ in epigenetics factors, I once presented here a conceptual
mechanism that produced exactly that result. It was loudly criticized
(hooted down, if I recall) as being totally non-biological and
impossible to achieve through any evolutionary mechanism. That was
quite true but was not my objective: I wanted to show that the genome
alone does not "program for" or "produce" any one specific result. The
genome acting as one part of a dynamic system inside a functioning
cell is what is required to produce the result. The same genome inside
different dynamic systems can produce different results. This is a
theoretical possibility. Cabej claims something like this but far
less drastic is a reality and important in evolution. There is no
evidence to support converting that conceptual notion into a reality.
Everyone agrees that Cabej presents good evidence of known biological
processes and then stretches the interpretation out to extremes far
beyond what there is any evidence to support.
As to what "gene recruitment" really means, I will leave that to the
molecular biologists. My own take is that structural gene products
themselves are NOT responsible for the production of complex features
such as the organized structures that result from developmental
morphogenesis. These are, instead, the product of a large array of
genes along with their regulatory elements and factors all interacting
in a complex dynamical system (using that term the way complexity
theorists use it). Exactly the same structural genes can produce
entirely different results if they are used differently in a different
dynamical system. And the switch in dynamical systems can be produced
by changes in the gene regulatory factors or even by changes in
epigenetic factors. In that sense, a reorganization of the dynamic
system in which genes are expressed can cause each gene in the system
to take on "new functions" in the sense that they produce entirely
different results.
John Harshman
to be stable and long-lasting enough to be evolutionarily important.
Their mode of inheritance just isn't good enough. The differences in
gene expresssion between species are usually the result of genetic
differences in regulatory sequences, e.g. transcription factor binding
sites.
r norman
<jhar...@pacbell.net> wrote:
I agree completely. I tried to avoid using the notion of evolutionary
importance in this description of epigenetics. The system is
important, though, in doing things like creating different castes of
social insects or determining whether a particular honey bee will turn
into a worker or a queen or whether certain reptiles will become male
or female. The notion of castes and environmental determination of
phenotype does play an important role in the evolution of these
groups. Of course, the evolution of each caste or sex or reproductive
capability is itself the product of ordinary evolutionary process as
is the evolution of the mechanisms for the switch; whether chemical or
environmental or mediated through the nervous system. That last point
is one I know you repeatedly emphasize and it is worth that emphasis.
Tim
> >>>> the same time, I was hoping to read more of a discussion on the
> >>>> details of the content of the article, which is slightly above my pay
> >>>> grade. Also after going through the thread I find my second question
> >>>> really saw the bulk of the discussion.
> >>>> I say this because, unfortunately, regarding the first question I
> >>>> poised: only Harshman really answered it. Harshman, essentially,
> >>>> resulting fromDNA changes�.
> >>>> I noticed that my second question (Is there a sound argument for gene
> >>>> recruitment to be cognitive based?) had a greater degree of responses
> >>>> and variation in answers. Unfortunately, I thought a few of these
> >>>> answers were contradictory (Norman essentially accepts the occurrence
> >>>> of cognitively based recruitment of genes but warns against
> >>>> generalizations and El cid accepts the idea of a neural control, but
> >>>> Harshman denies the possibility).
> >>> Actually, I believe we are all agreed. We just use different definitions
> >>> of "gene recruitment". We are all agreed that gene expression can be
> >>> influenced by the actions of nerve cells. And we are all agreed that
> >>> this is not a force in evolution, much less the major force. Such
> >>> effects can, in a very few cases, reach across generations, but they are
> >>> in no cases long-term enough to influence evolution. And they are all
> >>> evolved responses -- *genetically* evolved responses -- arising through
> >>> mutation and natural selection of the ordinary sort.
>
> >>> Now, what I take "gene recruitment" to mean is explicitly evolutionary:
> >>> a change in gene expression that leads that gene to a novel function.
> >>> And this happens by the ordinary processes of mutation and selection. I
> >>> know of no exceptions.
>
> >>>> seemed to come out of the blue. Harshman argues, �if gene recruitment
> >>>> is cognitive-based, how do plants do it?� . This would be a prudent
> >>>> question, normally. But, unless I�m missing something, the article in
> >>>> question is really on metazoans. So I really can�t grasp the
> >>>> relevance of the question posed.
> >>> Let me explain. If epigenetic, neurological effects are claimed to be a
> >>> (or perhaps the) major mechanism of gene recruitment in metazoans, then
> >>> why don't plants show huge differences in evolutionary outcomes? How do
> >>> they manage to do such recruitment fully as much as metazoans do, while
> >>> lacking that supposedly essential mechanism? I resolve that dilemma by
> >>> supposing that there is no such mechanism, and that plants and animals
> >>> do gene recruitment in the same, ordinary way.
>
> >>>> In terms of article detail, Norman was really the only one that
> >>>> touched on some of the details regarding the content of the article.
> >>>> He seems to accept the idea that nervous system controls cases of
> >>>> transgenerational developmental plasticity presented in the article
> >>>> and the transmission of the maternal influence in the egg:
> >>>> nervous systems control development including at least one
> >>>> transgenerational effect: the environmental experience -- daylength
> >>>> -- of a female fleshfly (Sarcophaga sexta) influences the ability of
> >>>> the pupa in the next generation to undergo a pupal diapause to allow
> >>>> it to survive the winter. The maternal influence is transmitted by
> >>>> He also admits, however ambiguously, the possibility for nervous
> >>>> �The paper presents examples of developmental switches where some
> >>>> particular species expresses genes entirely differently from closely
> >>>> related species even though they both seem to have the same genetic
> >>>> machinery. Since it is possible for the nervous system to have such a
> >>>> switching action, then why not conclude that the nervous system is
> >>>> frequently responsible for such switches. And then why not conclude
> >>> I doubt he meant that as approval of Cabej's claims, but we'll see. The
> >>> idea that the two species have the same genetic machinery, i.e. that
> >>> their expression differences are entirely epigenetic, has not been
> >>> tested, and Cabej presents no evidence for that claim. I'm highly dubious.
>
> >>>> understanding of the article better, though a better consensus
>
> >>>> PS I apologize in the case I hit the "wrong" reply button. I am
> >>>> replying to the overall thread, as opposed any one post.
> >>> I don't think you can do that, mechanically speaking. What you actually
> >>> did was respond to lucaspa, which I presume was the last post in the
> >>> thread at the time.
>
> >> John is right -- we have small quibbles among ourselves but agree in
> >> the larger issues.
>
> >> My own view is that I am more willing than others to at least accept
> >> the conceptual notion of "cognitive" (really "nervous system
> >> mediated") meddling in transgenerational epigenetics but demand
> >> actual hard evidence that such
>
>
> read more »- Hide quoted text -
>
> - Show quoted text -
Thank you for the lively discussion on the article. But I have to
point out that I still find some disagreement on the subject of
cognitive based evolution among you.
For example, Harshman says “we are all agreed that this is not a
force in evolution, much less the major force. Such effects can, in a
very few cases, reach across generations, but they are in no cases
long-term enough to influence evolution. “
But Norman doesn’t seem to quite agree when he says: “The system is
important, though, in doing things like creating different castes of
social insects or determining whether a particular honey bee will turn
into a worker or a queen or whether certain reptiles will become male
or female. The notion of castes and environmental determination of
phenotype does play an important role in the evolution of these
groups. Of course, the evolution of each caste or sex or reproductive
capability is itself the product of ordinary evolutionary process as
is the evolution of the mechanisms for the switch; whether chemical or
environmental or mediated through the nervous system.”
r norman
wrote:
>On Dec 14, 6:46 pm, r norman <r_s_nor...@comcast.net> wrote:
>> >>>> around. Anyway, I�m pleasantly surprised that so many chimed in. At
>> >>>> the same time, I was hoping to read more of a discussion on the
>> >>>> details of the content of the article, which is slightly above my pay
>> >>>> grade. Also after going through the thread I find my second question
>> >>>> really saw the bulk of the discussion.
>> >>>> I say this because, unfortunately, regarding the first question I
>> >>>> poised: only Harshman really answered it. Harshman, essentially,
>> >>>> believes that there are no �controversies surrounding gene recruitment
>> >>>> resulting fromDNA changesďż˝.
>> >>>> I noticed that my second question (Is there a sound argument for gene
>> >>>> recruitment to be cognitive based?) had a greater degree of responses
>> >>>> and variation in answers. Unfortunately, I thought a few of these
>> >>>> answers were contradictory (Norman essentially accepts the occurrence
>> >>>> of cognitively based recruitment of genes but warns against
>> >>>> generalizations and El cid accepts the idea of a neural control, but
>> >>>> Harshman denies the possibility).
>> >>> Actually, I believe we are all agreed. We just use different definitions
>> >>> of "gene recruitment". We are all agreed that gene expression can be
>> >>> influenced by the actions of nerve cells. And we are all agreed that
>> >>> this is not a force in evolution, much less the major force. Such
>> >>> effects can, in a very few cases, reach across generations, but they are
>> >>> in no cases long-term enough to influence evolution. And they are all
>> >>> evolved responses -- *genetically* evolved responses -- arising through
>> >>> mutation and natural selection of the ordinary sort.
>>
>> >>> Now, what I take "gene recruitment" to mean is explicitly evolutionary:
>> >>> a change in gene expression that leads that gene to a novel function.
>> >>> And this happens by the ordinary processes of mutation and selection. I
>> >>> know of no exceptions.
>>
>> >>>> Harshman�s answer to the second question was interesting in that it
>> >>>> seemed to come out of the blue. Harshman argues, �if gene recruitment
>> >>>> is cognitive-based, how do plants do it?ďż˝ . This would be a prudent
>> >>>> question, normally. But, unless I�m missing something, the article in
>> >>>> question is really on metazoans. So I really can�t grasp the
>> >>>> relevance of the question posed.
>> >>> Let me explain. If epigenetic, neurological effects are claimed to be a
>> >>> (or perhaps the) major mechanism of gene recruitment in metazoans, then
>> >>> why don't plants show huge differences in evolutionary outcomes? How do
>> >>> they manage to do such recruitment fully as much as metazoans do, while
>> >>> lacking that supposedly essential mechanism? I resolve that dilemma by
>> >>> supposing that there is no such mechanism, and that plants and animals
>> >>> do gene recruitment in the same, ordinary way.
>>
>> >>>> In terms of article detail, Norman was really the only one that
>> >>>> touched on some of the details regarding the content of the article.
>> >>>> He seems to accept the idea that nervous system controls cases of
>> >>>> transgenerational developmental plasticity presented in the article
>> >>>> and the transmission of the maternal influence in the egg:
>> >>>> ďż˝ This paper presents a number of interesting real situations where
>> >>>> nervous systems control development including at least one
>> >>>> transgenerational effect: the environmental experience -- daylength
>> >>>> -- of a female fleshfly (Sarcophaga sexta) influences the ability of
>> >>>> the pupa in the next generation to undergo a pupal diapause to allow
>> >>>> it to survive the winter. The maternal influence is transmitted by
>> >>>> some substance in the egg.ďż˝
>> >>>> He also admits, however ambiguously, the possibility for nervous
>> >>>> system to be �a major factor in the evolution�:
>> >>>> �The paper presents examples of developmental switches where some
>> >>>> particular species expresses genes entirely differently from closely
>> >>>> related species even though they both seem to have the same genetic
>> >>>> machinery. Since it is possible for the nervous system to have such a
>> >>>> switching action, then why not conclude that the nervous system is
>> >>>> frequently responsible for such switches. And then why not conclude
>> >>>> that is is actually a major factor in the evolution of the metazoa? ďż˝
>> >>> I doubt he meant that as approval of Cabej's claims, but we'll see. The
>> >>> idea that the two species have the same genetic machinery, i.e. that
>> >>> their expression differences are entirely epigenetic, has not been
>> >>> tested, and Cabej presents no evidence for that claim. I'm highly dubious.
>>
>> >>>> Regardless, the discussion is actually helping me to �piece� my
>> >>>> understanding of the article better, though a better consensus
>> >>>> (especially on the second question) wouldn�t hurt!
The fact that there are different castes is important for the
evolution of these insects. The evolution of the different castes was
done using standard evolutionary mechanisms. That is exactly what
John Harshman and I agree on.
The switch that controls which caste is going to be produced can be
done by epigenetic mechanisms. John Harshman and I agree on this.
That switch might be transgenerational. The flesh fly does that in
producing two kinds of offspring: some which undergo lengthy diapause
to survive the winter, others that do not. John Harshman and I agree
on this.
The switch does not last over many generations. Frankly I don't know
the details about how different castes of termites or ants are
produced. But there is a switch in honeybees that determines whether
an embryo grows into a worker or a queen determined by food fed to the
larva. There is a switch in some reptiles that determines whether an
embryo grows into a male or a female depending on the temperature.
That switch lasts for just that one generation. John Harshman and I
agree on this.
There is no disagreement that I can see. I just pointed out the
classical evolutionary mechanisms produced the package of
possibilities that epigenetic mechanisms can select from. The
epigenetic selection does not last over many generations; I believe
there are cases known of grandparental influences on grandchildren. It
is just that the package of possibilities (the result of ordinary
evolution) and providing the ability for epigenetics to make the
switch (again, the result of ordinary evolution) is important
evolutionarily.
John Harshman
inheritance (whether neurological or otherwise) is not a significant
force in evolution. However, epigenetic processes are crucial, including
the processes that use environmental (in the broad sense) cues to
differentiate insect castes. The evolution here isn't the epigenetic
effect itself, but the genetic changes that gave rise to the variable
epigenetic responses. That is, the castes arise epigenetically, but the
reason those epigenetic responses exist is the underlying genetic
machinery that produces the different responses to different stimuli,
and that machinery arose through standard mutation, selection, and
genetic inheritance. Again, there is no disagreement.