Certain DNA sequences show minimal levels of change over long periods of time

• A sequence that is identical (or very similar) across all members of a species or a group of species is called a conserved sequence 

• A sequence that remains identical (or very similar) over long periods of evolution is called a highly conserved sequence

There are two main hypotheses to account for the presence of conserved or highly conserved sequences in genes

1. The gene serves an essential functional requirement within the cell

   • The removal of an essential gene would prevent organism survival – hence any mutations to the conserved sequence will not be maintained within a gene pool

   • Examples of essential genes would include those encoding enzymes involved in cell respiration, transcription, translation, etc.

2. The gene has a slower rate of mutation

   • Certain sequences may be more likely to mutate – for example, methylated cytosines can deaminate to form thymine

   • The locus of a gene may affect the rate of mutation – for example, the proximity to the telomere may influence mutation rate

   • Genes with higher expression levels also have more active DNA repair and proofreading mechanisms – resulting in fewer mutations remaining unchanged