Gene Evolution - Fast or Slow?

The way that mutations affect the survival of a cell depends greatly on the location of the genetic change. In regions of the genome that do not code for genes or regulatory regions, mutations may have little effect, such that the cell can often continue as usual.

The overall effect is that non-coding sequences are free to change fairly rapidly in evolutionary terms, meaning these regions of a genome may be almost unrecognizable even in two closely related species.

However, in coding sequences, mutations are not so freely adopted. While in very rare cases they may be beneficial, such as a mutation in a gene for an enzyme creating a better binding affinity for the substrate; the majority will be detrimental.

Let’s look at the 16s rRNA gene, for example. It encodes a structural RNA forming part of the ribosome. Some of the regions of this RNA are critical to ribosome function, and changes in these segments are exceptionally rare. These highly conserved regions change so slowly that they can be used to examine sequence homology across phyla, kingdoms, and even all living species - making them a valuable tool for studying relationships between even distantly related organisms.

However, there are still sections of the 16s rRNA sequence less critical to function, which may evolve slightly faster. These “variable regions” can be useful for elucidating relationships between more closely related species – such as genera or even strains of bacteria.

Overall, this leads to the phenomenon that different genome regions may evolve at vastly different rates, even within regions encoding a single gene.