The following figure is presented here at reduced scale to accommodate a browser. A larger scale version is available in Chapter 1 in the Download Files area reached from the Site navigation bar.
Evolutionary Tree of the Animal Visual System [from Section 1.2.1]
The diagram shows a variety of species and families. The tree is formed based on the presence of retinol (Vitamin A) in their systems. Three distinct types of Vitamin A have found that relate to the visual system. Common retinol, Vitamin A1, is found in the majority of species, those living in salt water or derived from those living in salt water (most of the terrestrial chordates). It is derived from beta carotene.
3,4-dehydroretinol, Vitamin A2, is found in the retinas of those species that have evolved to live in fresh water. 3-hydroxylretinol, Vitamin A3, is found in the retinas of Diptera, a major order of Arthropoda. These are the two-winged flies.
The type of Vitamin A used to form the chromophores of vision in these Phyla and Orders appear to depend primarily on their environment. Those fish and reptiles that live in fresh water are known to rely on retinol2, Vitamin A2 for their metabolism and vision. This appears to be due to a shortage of beta carotene in their environment. Those fish that move back and forth between fresh and salt water are known to change their basic concentrations of Vitamin A from A1 to A2, or vice versa, as part of their migration.
The flies have apparently adopted Vitamin A3 because they feed on decaying animal and plant matter that is subject to the rapid decay of beta carotene. Zeaxanthin, the source of Vitamin A3 is apparently more stable than carotene in this environment.
These features are discussed in Chapters 1 & 2.
Return to the Phylogenic Tree page describing the species frequently used in vision research and the primates in particular.
Return to the website home page