Section 36–5 Other eyes

(Sexual selection / Convergent evolution / Extreme adaptation)

 

In this section, Feynman discusses the evolution of sexual selection that influences peacock’s color vision and convergent evolution through the development of the octopus’s eye, as well as mentions the giant squid's enormous eyes, but it could be related to extreme adaptation. Simply phrased, the section is about the eyes of peacock, octopus, and giant squid that are related to sexual selection, convergent evolution, and extreme adaptation.

 

1. Peacock’s eye:

“Fish, butterflies, birds, and reptiles can see color, but it is believed that most mammals cannot. The primates can see color. The birds certainly see color, and that accounts for the colors of birds. There would be no point in having such brilliantly colored males if the females could not notice it! That is, the evolution of the sexual “whatever it is” that the birds have is a result of the female being able to see color (Feynman et al., 1963, p. 36–9).”

 

Feynman says that the evolution of the sexual “whatever it is” that the birds have is a result of the female being able to see color. On the other hand, Darwin (1859) writes: “when the males and females of any animal have the same general habits… but differ in structure, color, or ornament, such differences have been mainly caused by sexual selection (p. 89).” Thus, one may explain that the evolution of sexual selection influences the color vision in animals. In other words, many animals can see color, which plays a crucial role in sexual selection. In peacocks, for example, the vivid colors of males have evolved to attract females, indicating that female birds’ ability to see color drives the development of bright plumage.

 

“So next time we look at a peacock and think of what a brilliant display of gorgeous color it is, and how delicate all the colors are, and what a wonderful aesthetic sense it takes to appreciate all that, we should not compliment the peacock, but should compliment the visual acuity and aesthetic sense of the peahen, because that is what has generated the beautiful scene! (Feynman et al., 1963, p. 36–9).”

 

In his lectures titled QED delivered at UCLA for the public, Feynman explains the iridescent feathers as a phenomenon of colors produced by the partial reflection of white light by two surfaces. Furthermore, he adds, “[P]erhaps you have wondered how the brilliant colors of hummingbirds and peacocks are produced. Now you know. How those brilliant colors evolved is also an interesting question. When we admire a peacock, we should give credit to the generations of lackluster females for being selective about their mates (p. 35).” However, in a letter to Asa Gray, Darwin (1860) writes, “the sight of a feather in a peacock’s tail, whenever I gaze at it, makes me sick!” Darwin thought that the peacock’s feather did not support his theory of evolution because the feather could make it difficult for the peacock to escape predators.

 

2. Octopus’ eye:

“It is very interesting that, besides the development of its brain and its reactions and so on, which are rather good for an invertebrate, it has also developed, independently, a different eye. It is not a compound eye or an eye spot—it has a cornea, it has lids, it has an iris, it has a lens, it has two regions of water, it has a retina behind. It is essentially the same as the eye of the vertebrates! It is a remarkable example of a coincidence in evolution where nature has twice discovered the same solution to a problem, with one slight improvement (Feynman et al., 1963, p. 36–9).”

 

Convergent evolution refers to the evolution of very similar traits independently in different organisms that are not closely related (Roberts, 1986). In short, Feynman describes convergent evolution through the development of the octopus’ eye.  The octopus’ eye is an example of convergent evolution, where similar eye structures have independently evolved in both vertebrates and invertebrate (or cephalopods). Despite being an invertebrate, the octopus has developed a pair of eyes with a cornea, iris, lens, and retina, similar to vertebrate eyes. However, the claim “nature has twice discovered the same solution to a problem” is a simplification. While convergent evolution has led to similar structures, the developmental pathways and genetic underpinnings of the eye differ significantly.

 

“In the octopus it also turns out, amazingly, that the retina is a piece of the brain that has come out in the same way in its embryonic development as is true for vertebrates, but the interesting thing which is different is that the cells which are sensitive to light are on the inside, and the cells which do the calculation are in back of them, rather than “inside out,” as in our eye (Feynman et al., 1963, p. 36–9).”

 

Despite the similarities in function, the eyes of octopus and human differ in their developmental origins, structural organization, and focusing mechanisms. There are at least three important differences: (1) Retina orientation: The retina of the octopus is directly oriented towards incoming light and thus, the photoreceptor cells are in front of the nerve fibers. On the contrary, the human retina is oriented in the opposite direction as shown below. (2) Blind spots: The octopus’ eye does not have a blind spot because the nerve fibers are behind the photoreceptors; human eye has a blind spot due to the optic nerve exiting the back of the eye, where there are no photoreceptors. (3) Photoreceptors: Octopus typically have one type of photoreceptor, and some species are sensitive to polarized light, which aids in navigation and detecting prey. On the other hand, humans have three types of cones sensitive to red, green, and blue.

Source: Roberts, 1986

3. Squid’s eye:

“The biggest eyes in the world are those of the giant squid; they have been found up to 15 inches in diameter! (Feynman et al., 1963, p. 36–9).”

 

Feynman only mentions that the biggest eyes in the world are those of the giant squid that have been found up to 15 inches in diameter. However, it is worthwhile to include the term extreme adaptation because the giant squid has adapted to living in the deep sea by developing a large body size and enormous eyes that allow it to see in low-light conditions. Specifically, the Atlantic giant squid (Architeuthis dux) has the largest eyes in the world. It has been estimated that the record example from Thimble Tickle Bay, Newfoundland, Canada, in 1878 had eyes measuring 40 cm (15.75 in) in diameter (Breverton, 2013). Interestingly, every octopus has eight arms – limbs with suckers dotted all the way along, whereas a squid has not just eight arms but also two tentacles, with suckers just at the end, which it uses to hunt fish and shrimp.

 

Octopus and squid eyes share some similarities due to their common cephalopod lineage, but have distinct differences reflective of their specific adaptations and lifestyles. Similarities: Both octopus and squid eyes function like a camera, with a single lens focusing light onto a retina, and have pupils that can change shape. Furthermore, neither the octopus nor the squid eye has a blind spot because the optic nerve does not pass through the retina. Differences: Squid often have larger eyes relative to their body size compared to octopuses. For example, the colossal squid has some of the largest eyes in the animal kingdom, which helps detect faint light in the dark ocean depths. These differences manifest in aspects like habitat-specific adaptations, eye size, light sensitivity, and behavioral use of vision.

 

Review Questions:

1. How would you explain the evolution of sexual selection influences the color vision of peacocks?

2. How would you explain the concept of convergent evolution through the octopus’ eye and human eyes?

3. How would you explain the biggest eyes in the world are those of the giant squid due to extreme adaptation?

 

The moral of the lesson: this section highlights how sexual selection influences color vision in animals, illustrates convergent evolution through the development of the octopus eye, and demonstrates extreme adaptation of the giant squid's enormous eyes.

 

References:

1. Breverton, T. (2013). Breverton's Nautical Curiosities: A Book of the Sea. London: Quercus.

2. Darwin, C. (1859). On the origin of species: facsimile of the first edition.

3. Darwin, C. (1993). The correspondence of Charles Darwin. 8. 1860 (Vol. 8). Cambridge University Press.

4. Feynman, R. P. (1985). QED: The strange theory of light and matter. Princeton: Princeton University Press.

5. Feynman, R. P., Leighton, R. B., & Sands, M. (1963). The Feynman Lectures on Physics, Vol I: Mainly mechanics, radiation, and heat. Reading, MA: Addison-Wesley.

6. Roberts, M. B. V. (1986). Biology: a functional approach. Cheltenham: Nelson Thornes.

Section 36–4 The compound (insect) eye

(Color vision / Polarization vision / Angular resolution)

 

In this section, Feynman discusses the color vision, polarization vision, and angular resolution of the compound eye of bees. Alternatively, the section could be titled as “the compound eye of bees.”

 

1. Color vision:

“In this way it was discovered that the bee’s eye is sensitive over a wider range of the spectrum than is our own. Our eye works from 7000 angstroms to 4000 angstroms, from red to violet, but the bee’s can see down to 3000 angstroms into the ultraviolet!... It has been shown, however, that there are a few red flowers which do not reflect in the blue or in the ultraviolet, and would, therefore, appear black to the bee! (Feynman et al., 1963, p. 36–7).”

 

According to Feynman, there are a few red flowers which do not reflect blue or ultraviolet, and would appear black to the bee! To be precise, bees can see from approximately 300 nm to 650 nm because they have three photoreceptors that are maximally sensitive to ultraviolet, blue, and green light (Hempel de Ibarra et al., 2014). Although it is commonly reported that bees cannot see red, but bees are seen visiting some red flowers (as shown below). For example, there are some plants pollinated by bees that produce red flowers such as Onosma confertum (Chen et al., 2020). In short, the color-deficient bees would perceive other colors instead of red, and thus red flowers do not necessarily appear black to the bees. However, bees can be attracted to flowers because of their scent.

Source: Can Bees See Red Flowers? Do Bees Visit Red Flowers? (buzzaboutbees.net)

 

Bee’s inability to see red color is commonly attributed to Karl von Frisch who was awarded the Nobel Prize for discoveries related to “bee dance.”  In Frisch’s experiments, bees were trained to associate colored cards (e.g., red) with a dish of sugar water. Importantly, the word ‘red’ has been defined based on human perception, i.e., the perceptual result of light incident upon the human retina in the visible region of the spectrum, having wavelengths in the region of 400 nm to 700 nm. On the other hand, we should not expect bees to have the same perception of red flowers, but it depends on the light spectra reflected by the so-called red flowers. Perhaps the red cards used in Frisch’s experiments appear to be almost black or dull color, but it should be different from the red flowers.

2. Polarization vision:

“The bee can tell, because the bee is quite sensitive to the polarization of light, and the scattered light of the sky is polarized. There is still some debate about how this sensitivity operates. Whether it is because the reflections of the light are different in different circumstances, or the bee’s eye is directly sensitive, is not yet known (Feynman et al., 1963, p. 36–7).”

 

Feynman explains that the bee is quite sensitive to the polarization of light and the scattered light of the sky is polarized. This could be attributed to Frisch’s research on how bees use the sun as a compass. If the sunlight is blocked by a cloud, the bees can make use of the polarized light scattered by the Earth’s atmosphere. Essentially, bees have polarization vision that is related to the Rayleigh scattering phenomenon, which occurs when sunlight interacts with molecules in the sky (or Earth’s atmosphere). Rayleigh scattering is responsible for the blue color of the sky and contributes to the polarization of scattered light. However, the bees’ flight can be related to the polarization pattern of the blue sky and the magnet field of the Earth (Von Frisch, 1974).

 

Feynman says that it is not yet known whether the bee is directly sensitive to the polarization of light. In another study, a simple “maze” of four tunnels arranged in a cross (with sugar reward at its end) was used to show that bees are able to follow polarized light (Evangelista et al., 2014). When the light was transversely polarized in the “correct” tunnels, bees tended to dance vertically (or perpendicular) with waggles in the same perpendicular direction to the tunnels. When the light was polarized parallel to the tunnels, bees tended to dance horizontally with waggles either to the left or right. This experiment is more conclusive on the polarization vision of the bees.

 

3. Angular resolution:

“The book says the diameter is 30 μm, so that is rather good agreement! So, apparently, it really works, and we can understand what determines the size of the bee’s eye! It is also easy to put the above number back in and find out how good the bee’s eye actually is in angular resolution; it is very poor relative to our own (Feynman et al., 1963, p. 36–8).”

 

Feynman’s estimation of angular resolution of the bee’s eye could be improved by at least three ways. Firstly, we may include the factor 1.22 (Abbe's sine condition), which is due to the diffraction-limits of the eye. Secondly, the size of the bee’s eye could be specified as r = 1.22 mm based on measurements (Varela & Wiitanen, 1970), but Feynman guesses that r = 3 mm. Thirdly, bees can see light that vary from about 300 to 650 nm, and thus, one may guess another wavelength instead of 400 nm (suggested by Feynman*). Using r = 1.22 mm, λ_ave = (300 + 650)/2 = 475 nm » 500 nm, and δ=√(1.22λr), we can obtain δ = 27.3 mm, but it is different from Feynman’s estimate that δ = 35 μm. However, this may suggest a refinement of the mathematical model or a change of the wavelength used for the model.

 

In the Audio Recordings* [37 min: 00 sec] of this lecture, Feynman initially guessed λ = 500 nm and estimated δ = 40 μm, but later explained that λ could be shorter.

The Feynman Lectures Audio Collection: https://www.feynmanlectures.caltech.edu/flptapes.html

 

“The book says the diameter is 30 μm, so that is rather good agreement! So, apparently, it really works, and we can understand what determines the size of the bee’s eye! (Feynman et al., 1963, p. 36–8).”

 

According to Feynman, the book says the diameter is 30 μm and thus, the percentage error is about (35 – 30)/30 » 17%. Perhaps some may not conclude that it is a rather good agreement with the book. If we compare the estimated value with an observed value of δ = 32 μm (Varela & Wiitanen, 1970), the percentage error would be (35 – 32)/32 » 9% and this is a better agreement with the observed value. However, Feynman might guess different values of λ and r if he knew a different value of observed δ (e.g., 32 μm) by working backward. The method of estimation could be refined if we take into account of the maximal sensitivity of three photoreceptors of the bee’s eye at their peak wavelength (S-344 nm, M-436 nm, L-556 nm).

 

Review Questions:

1. Do red flowers appear black to the bees?

2. How would you explain the polarization vision of bees?

3. How would you determine the angular resolution of the bee’s eye?

 

The moral of the lesson: Bees have complex visual systems that include: (1) Color vision (three photoreceptors sensitive to UV, blue, and green light), (2) Polarization vision (e.g., navigate using scattered light in blue sky), and (3) Angular resolution (optimized through evolution to balance acuity and diffraction limits).

 

References:

1. Chen, Z., Liu, C. Q., Sun, H., & Niu, Y. (2020). The ultraviolet colour component enhances the attractiveness of red flowers of a bee-pollinated plant. Journal of Plant Ecology, 13(3), 354-360.

2 Evangelista, C., Kraft, P., Dacke, M., Labhart, T., & Srinivasan, M. V. (2014). Honeybee navigation: critically examining the role of the polarization compass. Philosophical Transactions of the Royal Society B: Biological Sciences, 369(1636), 20130037.

3. Feynman, R. P., Leighton, R. B., & Sands, M. (1963). The Feynman Lectures on Physics, Vol I: Mainly mechanics, radiation, and heat. Reading, MA: Addison-Wesley.

4. Hempel de Ibarra, N., Vorobyev, M., & Menzel, R. (2014). Mechanisms, functions and ecology of colour vision in the honeybee. Journal of Comparative Physiology A, 200, 411-433.

5. Varela, F. G., & Wiitanen, W. (1970). The optics of the compound eye of the honeybee (Apis mellifera). The Journal of general physiology, 55(3), 336-358.

6. Von Frisch, K. (1974). Decoding the language of the bee. Science, 185(4152), 663-668.

 

Section 36–3 The rod cells

(Plane structures / Chemical bond / Light absorption)

 

In this section, Feynman discusses plane structures, chemical bond, and light absorption of retinal in rod cells. Thus, the section could be titled as “the retinal of  rhodopsin (or photoreceptor cells).”

 

1. Plane structures:

“The rhodopsin, which is the pigment, is a big protein which contains a special group called retinene, which can be taken off the protein, and which is, undoubtedly, the main cause of the absorption of light (Feynman et al., 1963, p. 36–6).”

 

In his Nobel lecture, Wald (1968) mentions: “… Ball, Goodwin and Morton in Liverpool showed that retinene is vitamin A aldehyde. At Morton’s suggestion the names of all these molecules have recently been changed, in honor of the retina, still the only place where their function is understood. Vitamin A is now retinol, retinene is retinal; there is also retinoic acid.” That is, the term retinene is outdated and had been replaced by retinal, which is more commonly used in current literature. Retinal also refers to the aldehyde form of Vitamin A, which is an essential component of rhodopsin and plays a crucial role in visual phototransduction. Visual phototransduction is the photochemical reaction where light is converted to an electrical signal in the retina.

 

“There are layer after layer of plane structures, shown magnified at the right, which contain the substance rhodopsin (visual purple), the dye, or pigment, which produces the effects of vision in the rods (Feynman et al., 1963, p. 36–6).”

 

Feynman says there are layers of plane structures, which contain the rhodopsin molecules and adds that there is some reason for holding all the rhodopsin molecules parallel. Specifically, rod cells contain a light-sensitive pigment called rhodopsin, which consists of a protein called opsin and a light-absorbing molecule called retinal. However, the retinal molecule within rhodopsin is non-planar. In Wald’s (1968) words, “A cis-linkage always represents a bend in the chain; but because of this steric hindrance the 11-cis molecule is not only bent but twisted at the cis linkage. This departure from planarity, by interfering with resonance, was expected to make the molecule so unstable that one hardly expected to find it.” That is, the non-planarity is essential for the molecule’s light-absorption properties and the structural changes upon absorbing light.

 

2. Chemical bond:

“This kind of a structure, with layers, appears in other circumstances where light is important, for example in the chloroplast in plants, where the light causes photosynthesis. If we magnify those, we find the same thing with almost the same kind of layers, but there we have chlorophyll, of course, instead of retinene. The chemical form of retinene is shown in Fig. 36–6. It has a series of alternate double bonds along the side chain… (Feynman et al., 1963, p. 36–6).”

 

Feynman explains that retinene has a series of alternate double bonds in the side chain. The term “alternate double bonds” refers to a sequence of double bonds that is separated by single bonds (-C=C-C=C-). Double bond is a chemical bond between two atoms involving two pairs of electrons instead of a pair of electrons in a single bond. He also uses the term “conjugated double bonds,” which is extended to the analogous interaction involving a p-orbital containing an unshared electron pair (IUPAC, 2014). It means that the electrons can move freely across alternate double bonds allowing for delocalization of electrons in the retinal. While both terms describe the arrangement of double bonds in retinal, conjugated double bond emphasizes the electronic delocalization and unique optical properties of the molecule.

 

This substance is impossible for human beings to manufacture in their own cells—we have to eat it. So we eat it in the form of a special substance, which is exactly the same as retinene except that there is a hydrogen tied on the right end; it is called vitamin A, and if we do not eat enough of it, we do not get a supply of retinene, and the eye becomes what we call night blind, because there is then not enough pigment in the rhodopsin to see with the rods at night (Feynman et al., 1963, p. 36–6).”

 

Eating retinol (vitamin A) or beta-carotene (provitamin A) is crucial for preventing night blindness because these compounds are essential for the production of retinal, a key component of rhodopsin (Grune et al., 2010). Rhodopsin is responsible for vision in low-light conditions, such as those encountered during nighttime. Retinol and beta-carotene are precursors of retinal: Retinol can be directly converted into retinal in the body, while beta-carotene is converted into retinol and then retinal through metabolic processes. Thus, consuming foods rich in retinol (such as liver, fish and dairy) or beta-carotene (plant-based foods such as carrots, spinach, and lettuce) ensures that the body has a supply of retinal for the production of rhodopsin. However, it is incorrect for Feynman to say that human cannot manufacture retinene (or retinal) because they can convert beta-carotene into retinol and then retinal.

 

3. Light absorption:

“When light strikes this molecule, the electron of each double bond is shifted over by one step. All the electrons in the whole chain shift, like a string of dominoes falling over, and though each one moves only a little distance (we would expect that, in a single atom, we could move the electron only a little distance), the net effect is the same as though the one at the end was moved over to the other end! It is the same as though one electron went the whole distance back and forth, and so, in this manner, we get a much stronger absorption under the influence of the electric field, than if we could only move the electron a distance which is associated with one atom (Feynman et al., 1963, p. 36–6).”

 

Retinal is a conjugated chromophore (a molecule which absorbs light at a particular wavelength and emits color subsequently) that is responsible for our ability to see. When light strikes rhodopsin, retinal undergoes a transformation from its 11-cis form (rhodopsin) to its all-trans form (metarhodopsin), initiating a cascade of biochemical events that result in the generation of electrical signals in the rod cell, enabling vision in low-light conditions. The key to the absorption of light by retinal lies in the conjugated double bonds that are essential for the molecule’s light-absorbing properties. However, electrons do not move in a coordinated manner along a chain of atoms as dominoes do. The behavior of electrons in a conjugated system is more complex and involves delocalization, where electrons are spread out over the entire molecule.  

 

In its native form within visual pigments, retinal exists in the 11-cis configuration and all-trans configuration. The 11-cis prefix is due to the double bond at the 11^th carbon atom is connected to the two largest substituents whereby the largest chains are on the same side. The all-trans prefix (trans is a Latin prefix meaning across or on the other side) refers to the double bonds with the bulky substituents (e.g., hydrogens) are positioned on opposite sides. When light enters the eye and strikes the retina, it breaks the C=C double bond between C₁₁ and C₁₂ atom and causes a rotation to form the all-trans configuration (as shown below). After the change, the all-trans-retinal is later converted back into 11-cis-retinal through a series of enzymatic reactions and allowing the visual pigment for further light absorption.

Source: Das et al., 2024.

 

Review Questions:

1. Is the retinal molecule within rhodopsin planar or non-planar?

2. How would you explain the chemical bond of retinene (retinal)?

3. How would you explain the absorption of light by retinal?

 

The moral of the lesson: retinal molecule within rhodopsin has a non-planar structure and conjugated double bonds where electrons can move across the alternating single and double bonds, and thus, play a crucial role in light absorption.

 

References:

1. Das, U., Das, A., Das, R., & Das, A. K. (2024). Photochemistry of the retinal chromophore in the process of seeing (vision). ChemTexts, 10(2), 3.

2. Grune, T., Lietz, G., Palou, A., Ross, A. C., Stahl, W., Tang, G., ... & Biesalski, H. K. (2010). β-Carotene is an important vitamin A source for humans. The Journal of nutrition, 140(12), 2268S-2285S.

3. Feynman, R. P., Leighton, R. B., & Sands, M. (1963). The Feynman Lectures on Physics, Vol I: Mainly mechanics, radiation, and heat. Reading, MA: Addison-Wesley.

4. IUPAC (2014). Compendium of chemical terminology. Compiled by A. D. McNaught & A. Wilkinson. Oxford: Blackwell Scientific Publications. Retrieved October 8, 2014 from http://goldbook.iupac.org/.

5. Wald, G. (1968). Molecular basis of visual excitation. Science, 162(3850), 230-239.