I'm Curious About Symmetry In Creatures
Cyde Weys
example: the heart is very obviously asymmetrical, and most people will have
feet of varying sizes, or fingers of slightly different lengths, or maybe
one arm longer than the other, or something. Heck, in males, the Vas
deferens is decidedly assymetrical - that's why the left ball hangs lower
than the right ball in about 99% of men! Men out there - if you don't
believe me, make sure you're locked upped in an office somewhere and unzip -
sure enough, you should be "hanging to the left."
Well, anyway, at these low levels, creatures are decidely asymmetric. But
externally, we look vertically symmetrical! Two legs, two arms, one breast
on each side ... in fact, it seems that the symmetry line goes right down
the middle of the body and perfectly separates two externally identical
parts - one hemisphere each of the brain, for guys, one testical and half of
a penis, for women, one vagina lip and half of a clitoris, etc.
So, I'm wondering, is there any animal that is largely externally
asymmetric? Like if it has breasts on its side instead of symmetrically
across the front, or maybe genitals decidely further in the direction of one
leg than the other, or something? All I can think of are two creatures: the
fiddler crab, which doesn't really count, because the females have two equal
sized arms and it's only the males who will grow bigger claws for mating
shows and fighting off competitors. And the flounder: you know, how one eye
has migrated across to the other side of its head. That's about all I can
think of. Anyone know of anything else? And if there really aren't many
examples, why does evolution tend to favor symmetry so much?
mel turner
vze2...@verizon.net wrote...
>
>Obviously, animals aren't really symmetrical.
Some may be, or at least come very close.
>I'll use humans as an
>example: the heart is very obviously asymmetrical,
And the major blood vessels, and various internal organs....
[snip]
>Well, anyway, at these low levels, creatures are decidely asymmetric. But
>externally, we look vertically symmetrical! Two legs, two arms, one breast
>on each side ... in fact, it seems that the symmetry line goes right down
>the middle of the body and perfectly separates two externally identical
>parts - one hemisphere each of the brain, for guys, one testical and half of
>a penis, for women, one vagina lip and half of a clitoris, etc.
Yes, we vertebrates belong to a large group of animals
called the Bilateria.
>So, I'm wondering, is there any animal that is largely externally
>asymmetric? Like if it has breasts on its side instead of symmetrically
>across the front, or maybe genitals decidely further in the direction of one
>leg than the other, or something? All I can think of are two creatures: the
>fiddler crab, which doesn't really count, because the females have two equal
>sized arms and it's only the males who will grow bigger claws for mating
>shows and fighting off competitors.
But the males do have one greatly enlarged claw.
How about lobsters, with one claw enlarged for crushing,
the other one smaller and sharper...
>And the flounder: you know, how one eye
>has migrated across to the other side of its head.
Actually, there are a great many species [in many genera and several
families] of flatfishes, all in one order [Pleuronectiformes] of the
bony fish. This is a case of a major modification of an ancestral
bilateral symmetry. Interestingly, some species [and whole families]
lie on the right side, others on the left.
http://aol.bartleby.com/65/fl/flatfish.html
http://people.clemson.edu/~jwfoltz/WFB300/subjects/Perco/FLATFISH/FLATFISH.HTM
http://cichlidresearch.com/fish_html/opleuron.html
http://www.biology.ualberta.ca/jackson.hp/IWR/Taxa/Pleuronectiformes/index.php
That's about all I can
>think of. Anyone know of anything else?
Your flatfish example is one of a change [both developmental
and evolutionary] away from more bilaterally-symmetrical
ancestry.
Another familiar example might be the Gastropoda, a huge
group of molluscs with asymmetric modifications of their bodies
related to their spirally-twisted shells. [They're understood
to have evolved from more bilaterally-symmetrical ancestors].
Hermit crabs IIRC also have added some asymmetry to their soft
abdomens, to better fit their borrowed snail shells.
>And if there really aren't many
>examples, why does evolution tend to favor symmetry so much?
Well a lot of it may be a reflection of common ancestry of
the very large animal group "Bilateria". But some animal
groups differ.
Sponges are pretty asymmetrical as adults. Cnidarians tend to
show radial symmetry, as do echinoderms [but some early
echinoderm-like organisms were more asymmetic or distorted-bilateral]
Searching for animal-symmetry-related keywords finds a few discussions
that may be worthwhile, and lots of introductory classroom stuff...
http://bluebullets.knox.k12.il.us/teacher_pages/winey/bio1/biowk15/banintro.htm
http://www.utm.edu/~rirwin/symmetry.htm
http://biology.uoregon.edu/classes/bi355f00/topics/topic%2012%2000.html
http://www.uwinnipeg.ca/~simmons/1116/16anim5.htm
<http://www.biologie.uni-hamburg.de/b-online/library/onlinebio/BioBookDiversity_7.html>
http://www.tyler.net/coop/echinodermata.html
http://www.webwerx.co.za/patb/invertorigins.htm
etc.
And of course plants have their symmetry issues as well. Flowers can
be radially symmetric [actinomorphic] or bilaterally symmetric
[zygomorphy] or asymmetric, and you have things like the handedness
of twining vines and plants with unequally symmetric shoots and
leaves...
fun topics.
cheers
Sverker Johansson
Snails. Their shells are coiled one way or the other, not symmetrically.
Oysters.
And of course, cnidarians and their relatives are radially symmetric
rather than bilaterally. (But note that echinoderms are bilateral,
despite their superficially radial structure.)
IIRC there are quite a few animals that mate side by side, with
asymmetric genitals. "Lefty" males have to find "rightie"
females, and vice versa.
> And if there really aren't many
> examples, why does evolution tend to favor symmetry so much?
Embryologically simpler. Lay down the basic body structure
along the main body axis, and let controlling signals spread
in both directions from the axis -- much simpler than sending one
signal to the left side and another to the right. Even a lot of our single
organs are grown as two halves, which then join along the midline
-- heart, brain, jaws...
And of course, in lots of cases (limbs etc) it makes sense to
have stuff in pairs, whereas it is harder to think of something
where a pair would be a disadvantage.
--
Best regards, HLK, Physics
Sverker Johansson U of Jonkoping
----------------------------------------------
Definitions:
Micro-evolution: evolution for which the evidence is so
overwhelming that even the ICR can't deny it.
Macro-evolution: evolution which is only proven beyond
reasonable doubt, not beyond unreasonable doubt.
John Harshman
<vze2...@verizon.net> wrote:
Almost all gastropods, for a big example.
Birds are internally asymmetrical, since most birds have only one
functional gonad. There are several externally asymmetrical birds: the
wrybill, so named because its beak swerves to the left, and crossbills, so
named because their upper and lower beaks are twisted in opposite
directions.
> And if there really aren't many
> examples, why does evolution tend to favor symmetry so much?
There are many examples, but your question still holds. Because it's
developmentally easy and is therefore the default condition? Because it's
easier to operate a symmetrical body? Because, all things being equal, you
find the same stuff useful on your left as on your right? I don't know
that this has ever been seriously addressed.
--
*Note the obvious spam-defeating modification
to my address if you reply by email.
Ethan Young
"mel turner" <mtu...@snipthis.acpub.duke.edu> wrote in message
news:ae42gp$2rf$1...@news.duke.edu...
| In article <GSdN8.35017$fM....@nwrddc01.gnilink.net>,
| vze2...@verizon.net wrote...
| >
| >Obviously, animals aren't really symmetrical.
|
| Some may be, or at least come very close.
|
| >I'll use humans as an
| >example: the heart is very obviously asymmetrical,
|
| And the major blood vessels, and various internal organs....
The blood vessels are placed where they are needed, correct? The body then
can develop larger ones if more oxygen is needed in certain parts of the
body, correct?
I'm curious, would these assymetrical internal organs possibly have started
out symetrically placed, then perhaps "migrating" over time to optimal
locations? I.e., could the heart have started out in the center, but have
migrated over time (or generations, or perhaps natural selection) to the
left side of the body for functionality purposes?
Ethan
|
<snip>
|
| cheers
|
|
|
Cyde Weys
"Sverker Johansson" <l...@hlk.no.hj.spam.se> wrote in message
news:3D05BA78...@hlk.no.hj.spam.se...
> And of course, cnidarians and their relatives are radially symmetric
> rather than bilaterally. (But note that echinoderms are bilateral,
> despite their superficially radial structure.)
>
> IIRC there are quite a few animals that mate side by side, with
> asymmetric genitals. "Lefty" males have to find "rightie"
> females, and vice versa.
Are these mammals? Can you get some examples?
Chas. 'Mark' Bee
You should check out the protozoa. Some are symmetrical, some are
most definitely not; the needs of the environment rule.
Anyone know of anything else? And if there really aren't many
> examples, why does evolution tend to favor symmetry so much?
Probably because it gives a complete organism with fewer
instructions. But that's a guess. Also, above a certain level,
attractiveness becomes a factor in reproduction.
Chas. 'Mark' Bee
Much more likely in my view is that the internal asymmetry is the
real pattern of life, and the external symettry came about later in
evolution. The main reason I say this is my studies of protozoa.
GKlein112473633
>instructions. But that's a guess. Also, above a certain level,
>attractiveness becomes a factor in reproduction.
This begs the question though. Why is symetry so attractive that sexual
selection can operate?
Gene
My candle burns at both ends; It will not last the night;But ah, my foes, and
oh, my friends--It gives a lovely light!
Ethan Young
"Chas. 'Mark' Bee" <c-b...@uiuc.edu> wrote in message
news:3D0665B8...@uiuc.edu...
So would it be possible that the internal symmetry eventually "catch up"
with the external symmetry, and future generations of the evolved could
possibly be truly symmetrical, in all accounts, inside and out? This would
have to offer some sort of survival benefit, I guess, in order to happen...
Thanks,
Ethan
Cyde Weys
"GKlein112473633" <gklein1...@aol.com> wrote in message
news:20020611182621...@mb-cm.aol.com...
> > Probably because it gives a complete organism with fewer
> >instructions. But that's a guess. Also, above a certain level,
> >attractiveness becomes a factor in reproduction.
>
> This begs the question though. Why is symetry so attractive that sexual
> selection can operate?
If we weren't symmetric, asymmetric would be sexy, so whatever ...
Wayne D. Hoxsie Jr.
[SNIP]
>So, I'm wondering, is there any animal that is largely externally
>asymmetric? Like if it has breasts on its side instead of symmetrically
>across the front, or maybe genitals decidely further in the direction of one
>leg than the other, or something? All I can think of are two creatures: the
>fiddler crab, which doesn't really count, because the females have two equal
>sized arms and it's only the males who will grow bigger claws for mating
>shows and fighting off competitors. And the flounder: you know, how one eye
>has migrated across to the other side of its head. That's about all I can
>think of. Anyone know of anything else? And if there really aren't many
>examples, why does evolution tend to favor symmetry so much?
>
I've seen a lot of good examples, but serendipitously, I just happened
to be paging through Futuyma's _Evolutionary Biology_ (third edition)
and found a very striking example. The male Narwhal (Monodon
monoceros). One upper incisor (the left one WRT the animal) forms a
long spiral tusk. The opposite incisor remains vestigial and never even
protrudes from the skull.
--
Wayne D. Hoxsie Jr. KG9ME
Southern Illinois University Edwardsville
College of Arts and Sciences
Dept. of Biological Sciences
wa...@hoxnet.com
http://www.hoxnet.com
PGP Key ID 138BCEE1
Sverker Johansson
Not that simple. Within the range of normal human variation in symmetry,
more symmetric people are judged more attractive than less symmetric ones.
Frequency-dependent selection won't explain that. The most common
explanation is instead in terms of health and developmental stability.
Given "symmetric" genes, you can still get an asymmetric body through
various environmentally-induced birth defects, or malnutrition or
disease during infancy. A symmetric body is a sign of health.
Sverker Johansson
Not any that I know of.
I suspect, however, that a web search for lefty genitals
and asymmetric mating positions will turn up a lot of
decidedly mammalian links :->
> Can you get some examples?
Fish and insects. Some guppy relative, but I don't remember the details.
xyzzy
>And if there really aren't many
>examples, why does evolution tend to favor symmetry so much?
Environment? For moving things there is one preferred axis, ie the
direction of motion. Gravity provides the other direction. With two
directions mostly perpendicular to each other, all you have left is
mirror symmetry. For plants the two preferred directions happen to be
collinear so we see (approximate) radial symmetry (Slow moving animals
can be said to lack one axis and can be radially symmetric as well).
Finally, symmetry is cheaper than non-symmetry so we can probably
expect that there is a tendency for all possible symmetries to be
exploited.
Snorkelboy
left, although #i believe there's the odd right hander, and even
double tusker (one from each side) around.
Drearash
>From: snorke...@hotmail.com (Snorkelboy)
>Date: 6/12/02 5:34 AM Eastern Daylight Time
>Message-id: <7f1f28a5.02061...@posting.google.com>
>
>Narwhal tusks are the extended teeth of the animal - usually from the
>left, although #i believe there's the odd right hander, and even
>double tusker (one from each side) around.
>
Von Smith
Fortuna nimis dat multis, satis nulli.
Derek Stevenson
> Not any that I know of.
> I suspect, however, that a web search for lefty genitals
> and asymmetric mating positions will turn up a lot of
> decidedly mammalian links :->
Oddly enough, Google doesn't turn up any hits for either phrase.
(Oh, like nobody *else* wondered what a search would find!)
Cyde Weys
"Wayne D. Hoxsie Jr." <postm...@hoxnet.com> wrote in message
news:slrnagdgq4.r...@hoxnet.com...
> In article <GSdN8.35017$fM....@nwrddc01.gnilink.net>, Cyde Weys wrote:
>
> [SNIP]
>
> >So, I'm wondering, is there any animal that is largely externally
> >asymmetric? Like if it has breasts on its side instead of symmetrically
> >across the front, or maybe genitals decidely further in the direction of
one
> >leg than the other, or something? All I can think of are two creatures:
the
> >fiddler crab, which doesn't really count, because the females have two
equal
> >sized arms and it's only the males who will grow bigger claws for mating
> >shows and fighting off competitors. And the flounder: you know, how one
eye
> >has migrated across to the other side of its head. That's about all I
can
> >think of. Anyone know of anything else? And if there really aren't many
> >examples, why does evolution tend to favor symmetry so much?
> >
>
> I've seen a lot of good examples, but serendipitously, I just happened
> to be paging through Futuyma's _Evolutionary Biology_ (third edition)
> and found a very striking example. The male Narwhal (Monodon
> monoceros). One upper incisor (the left one WRT the animal) forms a
> long spiral tusk. The opposite incisor remains vestigial and never even
> protrudes from the skull.
This begs the question: what in the heck is a narwhal?!?! A mammal? A
whale? An insect? What? Excuse my ignorance regarding this animal.
For all I know, it's like a naga, of nagahide fame ...
Florian
> "Wayne D. Hoxsie Jr." <postm...@hoxnet.com> wrote in message
> news:slrnagdgq4.r...@hoxnet.com...
(snip)
> > I've seen a lot of good examples, but serendipitously, I just
> > happened to be paging through Futuyma's _Evolutionary Biology_
> > (third edition) and found a very striking example. The male
> > Narwhal (Monodon monoceros). One upper incisor (the left one WRT
> > the animal) forms a long spiral tusk. The opposite incisor
> > remains vestigial and never even protrudes from the skull.
>
> This begs the question: what in the heck is a narwhal?!?! A mammal? A
> whale? An insect? What? Excuse my ignorance regarding this animal.
>
> For all I know, it's like a naga, of nagahide fame ...
Here's link some may find interesting:
http://groups.google.com/groups?selm=rdUJ8.4083%24b73.313%40nwrddc01.gnilink.net
--
odoratusque est Dominus odorem suavitatis
mel turner
wrote...
>"Wayne D. Hoxsie Jr." <postm...@hoxnet.com> wrote in message
>news:slrnagdgq4.r...@hoxnet.com...
>> In article <GSdN8.35017$fM....@nwrddc01.gnilink.net>, Cyde Weys wrote:
[snip]
>> I've seen a lot of good examples, but serendipitously, I just happened
>> to be paging through Futuyma's _Evolutionary Biology_ (third edition)
>> and found a very striking example. The male Narwhal (Monodon
>> monoceros). One upper incisor (the left one WRT the animal) forms a
>> long spiral tusk. The opposite incisor remains vestigial and never even
>> protrudes from the skull.
>
>This begs the question: what in the heck is a narwhal?!?! A mammal? A
>whale? An insect? What? Excuse my ignorance regarding this animal.
>
>For all I know, it's like a naga, of nagahide fame ...
It's a cetacean, part of the toothed whale/dolphin group [its
closest kin is the beluga or white whale]. It's the one in which
the males have one long straight spirally-twisted tusk sticking
straight out in front [the narwhal tusk was a major contributor
to the common picture of unicorn horns]
http://hometown.aol.com/puffindog/narwhal.html
http://whales.magna.com.au/DISCOVER/NARWHAL/index.html
http://www.allaboutnature.com/subjects/whales/species/Narwhal.shtml
http://www.cetacea.org/narwhal.htm
cheers
Tim Tyler
: And if there really aren't many examples, why does evolution tend to
: favor symmetry so much?
"Kaleidoscopic Embryos" in "Climbing Mount Improbable" is the best answer
I have seen.
Why is there asymmetry at all?
Eat/excrete often creates one asymmetry. It is the first to appear in the
developing embryo.
Sometimes gravity creates another.
Only more rarely are there further breaches in symmetry.
--
__________
|im |yler http://timtyler.org/ t...@tt1.org