Can someone please explain what types of mutations that affect chromosomes are least likely to be important in evolution and why?
The mutation you are referring to is a neutral mutation. This type of mutation does not affect the fitness of an organism. An example is when a DNA triplet is changed but still produces that same amino acid in the protein chain. Because the amino acids can be coded by more than one triplet, even if a nitrogenous base changes, but it still produces the same amino acid, this mutation will not affect the function of the protein. Another definition is that a neutral mutation has no selective advantage or disadvantage, therefore, it does not harm the organism.