Evolution of Eyes in

Mammals : Dichromatic Vision

The Mammal Eye

Color vision allows animals to reliably distinguish differences in the distributions of spectral energies reaching the eye Jacobs G. H. (2009). Dichromatic vision is not present in all species, the possession of color vision across various mammals strongly correlates its significance as a key trait in how animals flourish and are able to thrive in their given ecological niches.

Mammals have had dichromatic vision , evolved 200 million of years ago! The mammal dichromatic vision is an evolutionary adaptation stemming from their ancestry, ancestors which are nocturnal. Nocturnal mammals are those that sleep during the day, and awake at night. This means their vision needs to be successful in seeing in dim lit, dark conditions in order for them to survive, hunt and mate! The dichromatic vision aids in this.

Dichromatic Advantages: Night Vision

Many mammals have gained night vision from their ancestors overtime, nocturnal animals specifically. The dichromacy, is two pigment vision. In animals that have dichromatic vision, they have an easier time seeing dim, darker conditions than bright, daylight conditions. This is seen as strong fitness among nocturnal animals who thrive at night and rely on this time to do tasks to survive.

Evolution & Ancestry of Dichromacy

Based on what we know about these mammals and fossil evidence, they most likely were nocturnal, and indication they possess dichromatic vision. The animals in the photo above are a array of a few nocturnal mammals. The mammal ancestors which these animals have evolutionary adaptive traits of are hypothesized to be nocturnal, burrowing and small in stature. The early mammal ancestors relied on their dichromatic vision as strength in their fitness.

Dichromacy & Trichromacy photos: Jacobs, Gerald H., and Jeremy Nathans. (2009)

Dichromacy

Dichromacy describes two pigment vision, allowing for the vision of only 2 colors. Most mammals possess dichromatic vision. The two pigments have different jobs. One absorbs short-wavelength light, and one that is more sensitive to longer wavelengths. The photo above is showing these two pigments in tandem to create the dichromatic vision.

Trichromacy

Trichromacy describes 3-D color vision, allowing for the vision of multiple colors. This vision is what humans possess. The vision involves 3 pigments. A short wavelength pigment and two longer wavelength pigments. The photo above is showing these 3 pigments in tandem to create the Trichromatic vision.

An understanding of night vision in animals!

The following TED-ED talk video, discusses night vision across animals. And how human vs mammals adaptive night vision are different. similarly how the size of the eye, relates to function in successful night vision across mammals.

(TED-Ed)

CONE PIGMENTS & COLOR VISION

Cone pigments are the visual opsins responsible for photopic (color), vision in mammals. The variation in the arrangement of these opsins is analyzed through Jacobs G. H (2009) study concerning the effects on color vision in monotremes, marsupials and eutherian mammals. Primates specifically have evolved overtime as has their vision! Primates have 3-D color vision known as trichromacy which was an evolved vision of the original Dichromacy, a two color vision!

Figure 1. from the study conducted by Jacobs, Gerald H., and Jeremy Nathans. (2009), displays the roles of rod and cone photopigments whose ^max drawn from spectral ranges shown in horizontal line notation. 11-cis-retinal chromophore was utilized. Cone photopigment opsin genes SWS and LWS, rod photopigments opsin gene Rh.

Gene Families & Evolution

The two cone opsin gene families (SWS1 and LWS; figure 1) are both present in modern day, eutherian mammals. A eutherian mammal are placental mammals. Also, excluding a few primates, none of these animals derive more than a single photopigment type from each of their two gene families Jacobs, Gerald H., and Jeremy Nathans. (2009) . The information we know about relative sensitivity and wavelength in tandem with modern day eutherian gene families helps us to identify evolutionary adaptations in the early mammals visual mechanisms.

Evolutionary Connections

It can be seen that modern day eutherian mammals possess two of the cone gene families , SWS1 and LWS. It was also noted the retinas of the early eutherian mammals can be similar with also having both cone gene families! Because these cones are found to be in both modern, and past eutherians = dichromatic vision. Because this trait is seen to uphold acorss generations, decades this shows the impact on the fitness of the animal. The trait of dichromatic vision, helps aid in the survival of the species, how they thrive in their envirnments and overall success of the species, and in turn are seen across time in natural selection. Where this trait is continuously passed onto offspring over time.

Jacobs, Gerald H., and Jeremy Nathans. (2009)

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