Minutes of the 1468th Meeting of the Connecticut Academy of Arts and Sciences (CAAS)
September 12, 2018
The Whitney Center in Hamden, Connecticut
The speaker began with a reminder that evolution does not work by design; it tinkers.
The vertebrate eye is a good example of a structure no human would design, where the retinal neurons lie if front of the photoreceptors.
Then followed a survey of image-forming eyes. Anatomically they fall into two groups based on their optical features: “camera” eyes and compound eyes. These, however, do not represent basic evolutionary categories.
For example, the eyes of humans and squid are both “camera” eyes, but their gross anatomical features present an example of convergent evolution because their photoreceptors are fundamentally different, as described below.
There are three features of eyes with deep evolutionary roots. First, in animals with bilateral symmetry there is a gene that directs the early development of eyes. Second, all eyes use 11-cis retinal (or a variant thereof) bound to an opsin protein to form their visual pigment(s). Third, on the great branch of the evolutionary tree leading to vertebrates, the photoreceptors are modified cilia, whereas among the remaining invertebrates the photoreceptors are rhabdomeric, formed by masses of microvilli of the cell membrane.
Superimposed on these basic themes, evolution has produced a myriad of eyes different in functional capacities such a detecting the plane of polarization of light, presence or absence of color vision, capacity to see near uv wavelengths, and using reflection rather than refraction to form images.