The rods are photoreceptor cells that respond to the longest wavelengths of light, ranging from approximately 500-700 nm. Rods are responsible for vision at low light levels (scotopic vision).
The cones, on the otehr hand are the other <span>photoreceptors in the human retina that </span>are active at higher light levels (photopic vision).
I'm not sure if I will answer the question you asked, as I many have interpreted it incorrectly. In natural selection, there must be variance in the gene pool, the total frequency of alleles in a population. Now, one of the organisms may have a gene that somehow helps them survive from the selective force much better compared to the others. If this does happen, over time the gene pool will narrow down to become just the genes of that organism that survived better in the first place because the rest would be taken care of by the selective force. So, the alleles and trait come from the first organisms that had the advantage over the others of its species.
