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.