Out of all the lectures in the course, this was one of my favorites to put together. I return to the topic of evolutionary developmental biology, first raised in the “Vertebrae” lecture, by extending from the Hox genes to toolkit genes, focusing on the role of Pax6 in eye development. Again, we see how model organisms like fruit flies and zebrafish are relevant to understanding human biology.
Then, we zoom closer into the phylogeny of primates, considering the superfamilies and reminding students that New World monkeys, Old World monkeys and hominoids are all anthropoid primates. The anthropoids have a tremendously interesting difference with respect to color vision. Many New World monkey species have trichromacy in some individuals but many remain able only to see two colors. This is because one of the genes that codes for color-detecting pigments has different alleles. Heterozygotes can see three colors, homozygotes can see only two. By contrast, Old World monkeys and hominoids have trichromatic vision by virtue of a gene duplication in our ancestry, which generated two different genes that diverged in sequence to be sensitive to different wavelengths of light.
The convergence of trichromatic vision reflects its adaptive value in anthropoids, which emerged from diurnal activity pattern, the need to detect young leaves for their protein content and low toxicity, and a coevolution of color vision with mating displays. At the same time, owl monkeys lost two-color vision in parallel with lorises and galagos, in this case reflecting the low adaptive value of color vision in nocturnal primates.
Last, I discuss the polymorphism of eye color in living humans, which emerges due to the regulation of OCA2 in the surface layers of the iris.