Re: The MEANING of "gene flow"
From: Gil Lawton (gillawton_at_earthlink.net)
Date: 03/03/05
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Date: Thu, 3 Mar 2005 13:07:22 -0500 (EST)
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Here is a paragraph containing a term I do not comprehend yet. (The =
FULL
TEXT of your message, from which this is excerpted, is conserved later =
below.)
>> The reason this struck me is that the ability of an individual to
>> adapt to its environment is limited by gene flow. Barlow in "The
>> Cichlid Fishes" gives the example of deep-bodied reef cichlids
>> adapted for feeding by scraping that disperse to a reef where the
>> major food source is free swimming plankton. They will do better by
>> switching to suction feeding and more streamlined bodies. However,
>> if they are still part of the gene pool of the scraping feeders, they
>> will be unable to change in this new direction.
(From GL)
What is the meaning of "gene flow" in the above paragraph?
If you have read my yesterday post, in which I speak of what some
have called "internal constraints" on the next upcoming evolutionary=20
increment (or next few upcoming increments), you know I described
there my fledgling attempts to make a logical picture in my mind of
the impetus in a certain direction (or only slight oblique directions)
from current form/function of an evolving organism. While I am not
sure it is so, I am under the impression that this is what some =
biologists
have referred to by the term "channeling." If that impression is =
correct
then "channeling" would simply allude to the situational imperative of
constraints on what next upcoming stage (or the next few upcoming
stages) are proximate and accessible from current form/function of
an individual. This seems paralleled by your examples in the above
paragraph.
What is NOT CLEAR to me from the above paragraph is whether
we are talking about the same phenomenon when you use the term
"gene flow." If so, I would be grateful for a confirmation of it.
If not, then I would be grateful for further elucidation of what the
mechanism or process is that you refer to as "gene flow."
For me it seems that learning about biology is made much harder
by virtue of the fact that many of the terms for phenomena suggest
to the new student something totally different than they should.
If it is the same phenomenon -- i.e., evolutionary directional =
constraints
imposed by current form/function of an individual organism, upon=20
where it may (and may not) go in the next upcoming step, or steps,
then I think I grasp the meaning.
If it means something OTHER THAN that, then I would be grateful
for a clarification of that, or a text source on the phenomenon of "gene
flow."
Looking forward to your (and/or others')response on this,
g
----- Original Message -----=20
From: Sci.bio.evolution moderation account=20
To: gillawton@earthlink.net=20
Sent: Wednesday, March 02, 2005 10:01 PM
Subject: sci.bio.evolution mailing list
Newsgroups: sci.bio.evolution
Approved: josh@darwin.ediacara.org
Subject: Re: Species Selection Redux
From: William Morse <wdmorse@twcny.rr.com>
References: <cvuaui$11mm$1@darwin.ediacara.org> =
<d00q50$1s19$1@darwin.ediacara.org>
Organization: HieroGraphics
Wirt Atmar <atmar@aics-research.com> wrote in
news:d00q50$1s19$1@darwin.ediacara.org:=20
>> The reason this struck me is that the ability of an individual to
>> adapt to its environment is limited by gene flow. Barlow in "The
>> Cichlid Fishes" gives the example of deep-bodied reef cichlids
>> adapted for feeding by scraping that disperse to a reef where the
>> major food source is free swimming plankton. They will do better by
>> switching to suction feeding and more streamlined bodies. However,
>> if they are still part of the gene pool of the scraping feeders, =
they
>> will be unable to change in this new direction.=20
=20
>> In such a situation, a species that has the ability to speciate=20
>> parapatrically, perhaps by assortative mating favoring divergent
>> selection, would be favored. So there might be a tendency to =
evolve
>> evolvability at the species level, even though there is no benefit =
at
>> the individual level.=20
=20
> Species such as the cichlids have attracted so much attention that
> their stories are being repeated second- and third-hand by now, and =
in
> the process the story of their speciations is becoming more muddled
> and more hyperbolic with each retelling. But there doesn't appear to
> be anything magic about the lineages or their proclivities to
> speciate. Rather the mechanism underlying their speciations appears =
to
> be wholly mechanical: episodic allopatric speciations abetted by
> rising and falling water levels.=20
>=20
> Let me recommend two excellent talks that we've recorded on the
> subject, the first by Axel Meyer (Univ. Konstanz), on cichlids, and
> the second by Doug Futyuma (SUNY Stony Brook), on host-insect
> associations. They were given during the centenary celebration of
> Mayr's 100th birthday at Harvard last year and appear at:
>=20
> http://aics-research.com/lectures/ernstmayr/
>=20
> Both Meyer and Futyuma argue that the speciation events that they =
have
> measured are the result of the simplest form of speciation:
> geographical isolation, and in that regard, both even "out-Mayr" =
Mayr
> in their beliefs that sympatric speciation does not occur.
Thanks for the references. I haven't had time yet to reviiew the =
Futuyma =20
piece, but I enjoyed the talk by Meyer.
To set the record straight, Barlow's "The Cichlid Fishes" is =
definitively=20
_not_ a second or third-hand repeat of the cichlid story. Barlow =
(professor=20
emeritus at Berkeley) has been studying cichlids for over 50 years. =
Much of=20
the work Meyer recounts in the talk is discussed in detail in the =
book. And=20
much of the book has to do with other aspects of the cichlids that =
shed=20
much light on evolution, in particular the variety of social =
arrangements=20
seen in the cichlids, which include essentially the same range seen in =
the=20
primates, including monogamy and assistance in rearing kin. If you =
haven't=20
read the book, you should.=20
And to further clarify, Barlow does not advocate parapatric =
speciation, but=20
notes the influence of changing lake levels in leading to allopatric=20
speciation.
BUT
Reading both the book and Meyers' talk, one is still left with 500 =
cichlid=20
species in Lake Victoria in 500,000 years, with a significant =
bottleneck at=20
only 14,000 years ago. (Barlow, in his book, indicated that the =
bottleneck=20
might be complete, probably based on the article that Meyer refers to =
in=20
his talk. Meyer notes - which I find more believable - that based on=20
current variety in an adjacent shallow lake, it is likely that =
considerable=20
variety survived in pockets of shallow habitat in Lake Victoria.) =
Still -=20
500 species of cichlids, vs. a handful of species in each of the other =
fish=20
families that inhabit the lakes. I will grant you allopatric =
speciation -=20
you still need an explanation for why the cichlids speciate more =
rapidly=20
than other fishes.
And Meyers in his talk does refer to assortive mating and sexual =
selection.=20
Barlow in his book tends to downplay this, preferring the explanation =
that=20
the special jaw structure of the cichlids (the pharyngeal arches) as =
well=20
as their use of mouth brooding and other particulars of their behavior =
leads them to differentiate more easily than other fish. However, =
Meyers=20
noted in his talk that work on South American cichlids showed =
divergence in=20
polymorphism based on color was more positive than that based on jaw=20
structure.Again, assuming strictly allopatric speciation, a mechanism =
that=20
causes divergent selection could still be one of the factors that =
accounts=20
for the high speciation rate of the cichlids.=20
Coelacanths have been around for several hundred million years, with=20
slightly over a hundred documented species. Cichlids in Lake Victoria =
have=20
speciated at a thousand times that rate. I do not think it is =
unreasonable=20
to speculate on a mechanism that can explain this difference at a =
different=20
level than that of individual characteristics, even though careful =
study=20
may eventually show that it is the characteristics of the individuals =
that=20
explains the discrepancy.=20
In summary,while we can discuss the nuances in parapatric vs. =
allopatric=20
speciation, I think Meyers work still supports my conclusion that =
assortive=20
mating with divergent selection is one mechanism to favor =
"evolvability",=20
and that some lineages seem to be more evolvable than others.=20
Yours,
Bill Morse
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<BODY bgColor=3D#ffffff>
<DIV>Here is a paragraph containing a term I do not comprehend =
yet. (The=20
FULL</DIV>
<DIV>TEXT of your message, from which this is excerpted, is =
conserved later=20
below.)</DIV>
<DIV><BR>>> The reason this struck me is that the ability of an =
individual=20
to<BR>>> adapt to its environment is limited by gene flow. Barlow =
in=20
"The<BR>>> Cichlid Fishes" gives the example of deep-bodied =
reef=20
cichlids<BR>>> adapted for feeding by scraping that disperse to a =
reef=20
where the<BR>>> major food source is free swimming plankton. They =
will do=20
better by<BR>>> switching to suction feeding and more =
streamlined=20
bodies. However,<BR>>> if they are still part of the gene pool of =
the=20
scraping feeders, they<BR>>> will be unable to change in this new=20
direction.</DIV>
<DIV> </DIV>
<DIV><FONT face=3D"Bradley Hand ITC" size=3D2>(From GL)</FONT></DIV>
<DIV>What is the meaning of "gene flow" in the above =
paragraph?</DIV>
<DIV> </DIV>
<DIV>If you have read my yesterday post, in which I speak =
of what=20
some</DIV>
<DIV>have called "internal constraints" on the next upcoming =
evolutionary </DIV>
<DIV>increment (or next few upcoming increments), you know I=20
described</DIV>
<DIV>there my fledgling attempts to make a logical picture in my mind =
of</DIV>
<DIV>the impetus in a certain direction (or only slight oblique=20
directions)</DIV>
<DIV>from current form/function of an evolving organism. =
While I am=20
not</DIV>
<DIV>sure it is so, I am under the impression that this is what =
some=20
biologists</DIV>
<DIV>have referred to by the term "channeling." If that impression =
is=20
correct</DIV>
<DIV>then "channeling" would simply allude to the situational imperative =
of</DIV>
<DIV>constraints on what next upcoming stage (or the next few=20
upcoming</DIV>
<DIV>stages) are proximate and accessible from current form/function =
of</DIV>
<DIV>an individual. This seems paralleled by your examples in the=20
above</DIV>
<DIV>paragraph.</DIV>
<DIV> </DIV>
<DIV>What is NOT CLEAR to me from the above paragraph is=20
whether</DIV>
<DIV>we are talking about the same phenomenon when you use the =
term</DIV>
<DIV>"gene flow." If so, I would be grateful for a confirmation of =
it.</DIV>
<DIV>If not, then I would be grateful for further elucidation of =
what=20
the</DIV>
<DIV>mechanism or process is that you refer to as "gene flow."</DIV>
<DIV> </DIV>
<DIV><FONT face=3D"Bradley Hand ITC" size=3D2>For me it seems that =
learning about=20
biology is made much harder</FONT></DIV>
<DIV><FONT face=3D"Bradley Hand ITC" size=3D2>by virtue of the fact that =
many of the=20
terms for phenomena suggest</FONT></DIV>
<DIV><FONT face=3D"Bradley Hand ITC" size=3D2>to the new student =
something totally=20
different than they should.</FONT></DIV>
<DIV><FONT face=3D"Bradley Hand ITC" size=3D2></FONT> </DIV>
<DIV><FONT face=3D"Bradley Hand ITC" size=3D2>If it is the same =
phenomenon=20
-- i.e., evolutionary directional constraints</FONT></DIV>
<DIV><FONT face=3D"Bradley Hand ITC" size=3D2>imposed by current =
form/function of an=20
individual organism, upon </FONT></DIV>
<DIV><FONT face=3D"Bradley Hand ITC" size=3D2>where it may (and may not) =
go in the=20
next upcoming step, or steps,</FONT></DIV>
<DIV><FONT face=3D"Bradley Hand ITC" size=3D2>then I think I grasp the=20
meaning.</FONT></DIV>
<DIV><FONT face=3D"Bradley Hand ITC" size=3D2></FONT> </DIV>
<DIV><FONT face=3D"Bradley Hand ITC" size=3D2>If it means something =
OTHER THAN=20
that, then I would be grateful</FONT></DIV>
<DIV><FONT face=3D"Bradley Hand ITC" size=3D2>for a clarification of =
that, or a text=20
source on the phenomenon of "gene</FONT></DIV>
<DIV><FONT face=3D"Bradley Hand ITC" size=3D2>flow."</FONT></DIV>
<DIV><FONT face=3D"Bradley Hand ITC" size=3D2></FONT> </DIV>
<DIV><FONT face=3D"Bradley Hand ITC" size=3D2>Looking forward to your =
(and/or=20
others')response on this,</FONT></DIV>
<DIV><FONT face=3D"Bradley Hand ITC" size=3D2></FONT> </DIV>
<DIV><FONT face=3D"Bradley Hand ITC" size=3D2>g</FONT></DIV>
<DIV><FONT face=3D"Bradley Hand ITC" size=3D2></FONT> </DIV>
<DIV><FONT face=3D"Bradley Hand ITC" size=3D2></FONT> </DIV>
<DIV> </DIV>
<DIV><FONT face=3D"Bradley Hand ITC" size=3D2></FONT> </DIV>
<DIV><FONT face=3D"Bradley Hand ITC" size=3D2></FONT> </DIV>
<BLOCKQUOTE=20
style=3D"PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; =
BORDER-LEFT: #000000 2px solid; MARGIN-RIGHT: 0px">
<DIV style=3D"FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV=20
style=3D"BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: =
black"><B>From:</B>=20
<A title=3Dsbe@darwin.ediacara.org=20
href=3D"mailto:sbe@darwin.ediacara.org">Sci.bio.evolution moderation =
account</A>=20
</DIV>
<DIV style=3D"FONT: 10pt arial"><B>To:</B> <A =
title=3Dgillawton@earthlink.net=20
href=3D"mailto:gillawton@earthlink.net">gillawton@earthlink.net</A> =
</DIV>
<DIV style=3D"FONT: 10pt arial"><B>Sent:</B> Wednesday, March 02, 2005 =
10:01=20
PM</DIV>
<DIV style=3D"FONT: 10pt arial"><B>Subject:</B> sci.bio.evolution =
mailing=20
list</DIV>
<DIV><BR></DIV>Newsgroups: sci.bio.evolution<BR>Approved: <A=20
=
href=3D"mailto:josh@darwin.ediacara.org">josh@darwin.ediacara.org</A><BR>=
Subject:=20
Re: Species Selection Redux<BR>From: William Morse <<A=20
=
href=3D"mailto:wdmorse@twcny.rr.com">wdmorse@twcny.rr.com</A>><BR>Refe=
rences:=20
<<A=20
=
href=3D"mailto:cvuaui$11mm$1@darwin.ediacara.org">cvuaui$11mm$1@darwin.ed=
iacara.org</A>>=20
<<A=20
=
href=3D"mailto:d00q50$1s19$1@darwin.ediacara.org">d00q50$1s19$1@darwin.ed=
iacara.org</A>><BR>Organization:=20
HieroGraphics<BR><BR>Wirt Atmar <<A=20
=
href=3D"mailto:atmar@aics-research.com">atmar@aics-research.com</A>> =
wrote=20
in<BR><A=20
=
href=3D"news:d00q50$1s19$1@darwin.ediacara.org">news:d00q50$1s19$1@darwin=
.ediacara.org</A>:=20
<BR><BR><BR>>> The reason this struck me is that the ability of =
an=20
individual to<BR>>> adapt to its environment is limited by gene =
flow.=20
Barlow in "The<BR>>> Cichlid Fishes" gives the example of =20
deep-bodied reef cichlids<BR>>> adapted for feeding by scraping =
that=20
disperse to a reef where the<BR>>> major food source is free =
swimming=20
plankton. They will do better by<BR>>> switching to suction =
feeding=20
and more streamlined bodies. However,<BR>>> if they are =
still part=20
of the gene pool of the scraping feeders, they<BR>>> will be =
unable to=20
change in this new direction. <BR> <BR>>> In such a =
situation, a=20
species that has the ability to speciate <BR>>> parapatrically, =
perhaps=20
by assortative mating favoring divergent<BR>>> selection, =
would be=20
favored. So there might be a tendency to evolve<BR>>> =
evolvability at=20
the species level, even though there is no benefit at<BR>>> the=20
individual level. <BR> <BR>> Species such as the cichlids have =
attracted so much attention that<BR>> their stories are being =
repeated=20
second- and third-hand by now, and in<BR>> the process the story of =
their=20
speciations is becoming more muddled<BR>> and more hyperbolic with =
each=20
retelling. But there doesn't appear to<BR>> be anything magic about =
the=20
lineages or their proclivities to<BR>> speciate. Rather the =
mechanism=20
underlying their speciations appears to<BR>> be wholly mechanical: =
episodic=20
allopatric speciations abetted by<BR>> rising and falling water =
levels.=20
<BR>> <BR>> Let me recommend two excellent talks that we've =
recorded on=20
the<BR>> subject, the first by Axel Meyer (Univ. Konstanz), on =
cichlids,=20
and<BR>> the second by Doug Futyuma (SUNY Stony Brook), on=20
host-insect<BR>> associations. They were given during the centenary =
celebration of<BR>> Mayr's 100th birthday at Harvard last year and =
appear=20
at:<BR>> <BR>> <A=20
=
href=3D"http://aics-research.com/lectures/ernstmayr/">http://aics-researc=
h.com/lectures/ernstmayr/</A><BR>>=20
<BR>> Both Meyer and Futyuma argue that the speciation events that =
they=20
have<BR>> measured are the result of the simplest form of=20
speciation:<BR>> geographical isolation, and in that regard, both =
even=20
"out-Mayr" Mayr<BR>> in their beliefs that sympatric speciation =
does not=20
occur.<BR><BR>Thanks for the references. I haven't had time yet to =
reviiew the=20
Futuyma <BR>piece, but I enjoyed the talk by Meyer.<BR><BR>To =
set the=20
record straight, Barlow's "The Cichlid Fishes" is definitively =
<BR>_not_ a=20
second or third-hand repeat of the cichlid story. Barlow (professor=20
<BR>emeritus at Berkeley) has been studying cichlids for over 50 =
years. Much=20
of <BR>the work Meyer recounts in the talk is discussed in detail in =
the book.=20
And <BR>much of the book has to do with other aspects of the cichlids =
that=20
shed <BR>much light on evolution, in particular the variety of social=20
arrangements <BR>seen in the cichlids, which include essentially the =
same=20
range seen in the <BR>primates, including monogamy and assistance in =
rearing=20
kin. If you haven't <BR>read the book, you should. <BR><BR>And to =
further=20
clarify, Barlow does not advocate parapatric speciation, but <BR>notes =
the=20
influence of changing lake levels in leading to allopatric=20
<BR>speciation.<BR><BR>BUT<BR><BR><BR>Reading both the book and =
Meyers' talk,=20
one is still left with 500 cichlid <BR>species in Lake Victoria in =
500,000=20
years, with a significant bottleneck at <BR>only 14,000 years ago. =
(Barlow, in=20
his book, indicated that the bottleneck <BR>might be complete, =
probably based=20
on the article that Meyer refers to in <BR>his talk. Meyer notes - =
which I=20
find more believable - that based on <BR>current variety in an =
adjacent=20
shallow lake, it is likely that considerable <BR>variety survived in =
pockets=20
of shallow habitat in Lake Victoria.) Still - <BR>500 species of =
cichlids, vs.=20
a handful of species in each of the other fish <BR>families that =
inhabit the=20
lakes. I will grant you allopatric speciation - <BR>you still need an=20
explanation for why the cichlids speciate more rapidly <BR>than other=20
fishes.<BR><BR>And Meyers in his talk does refer to assortive mating =
and=20
sexual selection. <BR>Barlow in his book tends to downplay this, =
preferring=20
the explanation that <BR>the special jaw structure of the cichlids =
(the=20
pharyngeal arches) as well <BR>as their use of mouth brooding and =
other=20
particulars of their behavior <BR>leads them to differentiate =
more=20
easily than other fish. However, Meyers <BR>noted in his talk that =
work on=20
South American cichlids showed divergence in <BR>polymorphism based on =
color=20
was more positive than that based on jaw <BR>structure.Again, assuming =
strictly allopatric speciation, a mechanism that <BR>causes divergent=20
selection could still be one of the factors that accounts <BR>for the =
high=20
speciation rate of the cichlids. <BR><BR>Coelacanths have been around =
for=20
several hundred million years, with <BR>slightly over a hundred =
documented=20
species. Cichlids in Lake Victoria have <BR>speciated at a thousand =
times that=20
rate. I do not think it is unreasonable <BR>to speculate on a =
mechanism that=20
can explain this difference at a different <BR>level than that of =
individual=20
characteristics, even though careful study <BR>may eventually =
show that=20
it is the characteristics of the individuals that <BR>explains the=20
discrepancy. <BR><BR>In summary,while we can discuss the nuances in =
parapatric=20
vs. allopatric <BR>speciation, I think Meyers work still supports my=20
conclusion that assortive <BR>mating with divergent selection is one =
mechanism=20
to favor "evolvability", <BR>and that some lineages seem to be more =
evolvable=20
than others. <BR><BR>Yours,<BR><BR>Bill =
Morse<BR><BR></BLOCKQUOTE></BODY></HTML>
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