Overview
Variability is essential for evolution by natural selection and can be attributed to a number of factors including immigration/emigration, sexual reproduction and mutations. Animal vision with retinal rods and cones is a good example of such variability. Different photoreceptor types possess opsins that differ in their absorption capabilities. Consequently, we find dichromatic mammals that have 2 cone types and trichromatic mammals with three cone types.
There are 5 classes of vertebrate opsins with rods expressing only rhodopsin (Rh1). In most birds, fish, and amphibians a cone can express one of four different opsins with peak absorption sensitivities at ultraviolet (UV, SWS1), short (SWS2), medium(Rh2) and long wavelengths (LWS). Placental mammals have lost their SWS2 and Rh2 opsins, so most mammals possess only two pigment cones: S cones with the SWS1 pigment and M cones with the LWS pigment. In primates, the LWS pigment has two variants, such that the M cones sensitive to middle wavelengths and an L cone sensitive to the longer wavelengths. Consequently, we have dichromatic and trichromatic mammalian vision.
In an original, well researched and organized essay (of appropriate academic level: 3000 level University) demonstrate your understanding of evolution (by natural selection) of vision in mammals by making an argument for the origin of rods and cone variability in mammals. (hint: what selection pressures would exist for such selection, what are the cost/benefits of such diversity?)
https://crosstalk.cell.com/blog/5-things-you-didnt....
https://elifesciences.org/articles/07166
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC53120....