http:www.ivis.orgadvancesAnesthesia_Gleedludders2chapter_frm.asp

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http//www.sci.sdsu.edu/multimedia/birdlungs
Web site for avian lung animation above
http//www.ivis.org/advances/Anesthesia_Gleed/ludd
ers2/chapter_frm.asp
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The Biology of Seeing
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Darwin, on the eye
To suppose that the eye, with all its inimitable
contrivances for adjusting the focus to different
distances, for admitting different amounts of
light, and for the correction of spherical and
chromatic aberration, could have been formed by
natural selection, seems, I freely confess,
absurd in the highest possible degree.
C. Darwin, The Origin of Species
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Why a problem?
Darwins theory focused on gradual change. How
could intermediate stages in eye evolution be of
benefit?
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(2 sentences later)
Reason tells me, that if numerous gradations from
a simple and imperfect eye to one complex and
perfect can be shown to exist, each grade being
useful to its possessor, as is certainly the
case if further, the eye ever varies and the
variations be inherited, as is likewise certainly
the case and if such variations should be useful
to any animal under changing conditions of life,
then the difficulty of believing that a perfect
and complex eye could be formed by natural
selection, though insuperable by our imagination,
should not be considered as subversive of the
theory.
-- C. Darwin, The Origin of Species
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How sensory stimuli lead to behavior

• Information, in the form of energy, is
• detected by sensory receptor cells.
• transmitted by sensory neurons.
• processed by nervous system which activates
  muscles.
• modified by hormones

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Energy Sensation

• Energy can take the form of
• sound
• light
• mechanical force
• chemicals
• magnetic force
• electrical force

hearing seeing touch proprioception smell
taste magnetoreception electroreception
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Seeing, or Vision

• Vision involves assessment of
• Presence of light
• Color and other properties
• Images
• Location motion of objects

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A. Detecting Light
Long ?s
Short ?s
Light is electromagnetic radiation. Light has
properties of waves and of particles, called
photons. Detecting light detecting photons
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Photoreceptor cell is basic unit of light
detection. Contains photopigment molecule
rhodopsin.
rod cell in vertebrate eye
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Detecting photons requires rhodopsin,
a protein complex of subunits opsin and retinol.
Opsin
Retinol
Cell membrane
Figure 45.12
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When retinol absorbs photon, it changes shape.
light
cis form
trans form
Figure 45.12
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Figure 45.12
Opsin
Retinol
Rhodopsin

• Change in retinol shape
• closes Na channels.
• changes membranes electric properties.

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B. Seeing in Color
Color provides more information than brightness
only.
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Color Has Importance to Animals
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Color and ?
Long ?s
Short ?s
human-visible spectrum

• Light has properties of waves.
• Color is about wavelength (?).

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Rhodopsin response to ? determined by amino acid
composition of opsin
Opsin
Retinol
Figure 45.12
Different genes code for different opsins.
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Human color vision involves 3 opsins.
Human-visible spectrum
L opsin 560
M opsin 530
S opsin 420
1.0
0.8
Relative proportion of light absorbed
0.6
0.4
0.2
0.0
500
600
400
Wavelength (nm)
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As a rule, discriminating colors requires 2 or
more opsins

• Dichromatic vision 2 opsins
• -- many insects, most mammals, incl. dogs
• 2. Trichromatic vision 3 opsins
• -- primates (humans), bees
• 3. Tetrachromatic vision 4 opsins
• -- birds, some butterflies

Dogs are NOT colorblind!
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Mantis shrimp crustaceans common in warm ocean
waters.
cursorial, highly visual predators
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Mantis shrimp possess twelve-channel color vision!
Color-sensitive band in compound eye
humans 10,000 colors mantis shrimp 110,000
colors
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Range of color sensitivity

• Why dont humans see in ultraviolet (UV) or
  infrared (IR)?
• physics does not allow it. OR
• it does not benefit us enough.

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Physics allows UV vision. Occurs in Insects,
Birds, Fish, Reptiles.
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Flowers reflect UV which bees use as a cue.
Visible light image
UV light image
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Males of some butterflies reflect UV. UV
patterns used in courtship.
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Predator at 340 nm
Predator at 550 nm
Many reef fish see in the UV. May assist
predator detection.
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zebra finches have UV vision
due to opsin gene duplication with 1 amino acid
change ( 1 nucleotide substitution). UV vision
in fish, reptiles and mammals involves other
evolutionary changes.
UV vision independently evolved example of
convergent evolution.
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Range of color sensitivity

• Why dont humans see in ultraviolet (UV) or
  infrared (IR)?
• physics does not allow it. OR
• it does not benefit us enough.

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Seeing in Infrared (IR)

• IR vision is possible too! But rare. Just two
  animals
• Pit vipers use IR to detect warm-blooded prey at
  night.
• 2. One beetle species, whose larvae feed in tree
  trunks killed by fire, uses IR to locate trunks.

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Warm animals emit infrared radiation
What we see in the light
What pit vipers see in the dark
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pit organ
nostril
Pit vipers do NOT use eyes to detect IR, Instead
use opsin-free pit organs.
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C. Detecting Images
Detecting images requires an eye. An eye
involves array of photoreceptors onto which image
is projected and detected.
Eyes in just 6 of 38 phyla, yet gt95 of all
species have eyes.
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Pinhole Camera Eye
Nautilus eye works like a pinhole camera.
s simple no need to focus -s image
dim image not sharp
nautilus
retina array of photoreceptors
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Lens Camera Eye
The vertebrate and squid/octopus eyes have a
focusing lens.
Retina
s bright image image sharp -s image needs
focusing
Lens
Optic nerve (to brain)
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Eye of human and eye of octopus or squid are
similar. But independently evolved.
Example of convergent evolution
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Compound Eye
Insect compound eye consists of ommatidia. Each
ommatidium has lens.
Ommatidia
Lens
Receptor cells
Ommatidium
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Lens camera eye vs. Compound eye
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s no need to focus can see in all
directions can be subdivided! -s requires
bright light modest acuity
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D. Detecting Distance, Location, and Motion
trigonometry solves for distance
Depth perception may involve binocular vision
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Binocular vision and depth perception
cat
rabbit
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Darwin, on the eye
To suppose that the eye ... could have been
formed by natural selection, seems, I freely
confess, absurd in the highest degree.
-- C. Darwin, The Origin of Species
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Why a problem for Darwin?
Darwins theory focused on gradual change. How
could intermediate stages be of benefit?
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Gradual evolution of the lens eye
array
cup
pinhole
lens
All stages occur in living forms. (are therefore
possible as ancestors)
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Possible Evolutionary Pathways
flatworms, mollusks, some annelids
nautilus
some annelids starfish
some insects
vertebrates cephalopods
some insects
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Design Flaws in Vertebrate Eye

• Retina oriented backwards. Light must pass
  through nervous tissue.

Nesse and Williams, Sci. Am. 1998
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Design Flaws in Vertebrate Eye

• Blood supply gets in way, creating a blind spot.

Light
nervous tissue
Squid/octopus eye has neither problem.
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Insect eyes
After a given eye type arose, there evolved
considerable diversification.
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butterfly
caterpillar
Stages within the same animal can have very
different eyes!
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The end