There are two types of photoreceptors:

Rods 

Cones

TLS Online TPP Program

#Id: 467

#Unit 2. Cellular Organization

Mutations occur at the same rate in both exons and introns, but exon mutations are eliminated more effectively by selection.

TLS Online TPP Program

#Id: 468

#Unit 2. Cellular Organization

Introns may more freely accumulate point substitutions and other changes due to no selection pressure.

TLS Online TPP Program

#Id: 469

#Unit 2. Cellular Organization

Introns evolve or accumulate mutations/changes much more rapidly than exons

TLS Online TPP Program

#Id: 470

#Unit 2. Cellular Organization

Intron do not face selective pressure to produce a polypeptide with a useful sequence hence highly variable.

TLS Online TPP Program

#Id: 471

#Unit 2. Cellular Organization

Exon sequences are conserved by the negative selection of individuals in which the sequences have changed

TLS Online TPP Program

#Id: 472

#Unit 2. Cellular Organization

Most intron mutations are expected to be selectively neutral except branch site, splicing junctions, and sequences influencing splicing