Here is a great summary of what a ‘random mutation’ means done by @T_aquaticus at Peaceful Science- What "Random Mutation" Means in Science - Peaceful Science
There are different types of selection pressure that I was thinking of but the basic idea is the genome changes each generation and these changes will (from our perspective) be random. These changes can be neutral, negative or positive.
Negative selection pressure/purifying selection:
Here is an example shared by @sfmatheson a little while back where some researchers found regions of the genome unaffected by mutation which could indicate they are under purifying selection:
Survival bias, WWII planes, and human genetics
Purifying selection can be defined as:
Purifying selection prevents the change of an amino acid residue at a given position in a multiple alignment, thus favoring an excess of synonymous versus nonsynonymous substitutions.
For reference:
Synonymous vs. nonsynonymous mutations which are briefly discussed here-
Positive selection:
Here’s a neat example of looking for parts of the Dolphin genome that are under positive selection pressure that can help learn what makes a dolphin unique:
Note: I probably made at least one error in a nuanced definition so please let me know if that’s the case.