Why would frameshift mutation have a larger impact than a substitution mutation on the organism in which the mutation occurred?
Frameshift mutations completely alter the entire protein sequence that occurs after the mutation, whereas a substitution only alters a single amino acid. See Below.
I did a google search for "3-letter word sentences". Here is one.
"His car was old. Her cat can eat. You are not God" this represents your codon sequence.
A substituion mutation would be to replace the I in His, with a G.
"Hgs car was old. Her cat can eat. You are not God." The His got messed up, but the rest of the sentence makes sense. If this were a mutation in a protein, and His was not too important (stem loop, valine to isoleucine, etc), then the everything still might work.
However, a frameshift mutation shifts the frames of your codons (ribosome only reads 3 nucleotides, then 3 more...and on and on)
here is a frameshift mutation when I completely remove I from His.
"Hsc arw aso ldH erc atc ane atY oua ren otG od...."
You might be able to piece the sentence back together, but inside the ribosome, the wrong tRNA was put down for each frameshifted codon, and the resulting protein is garbage.
Hope that helps.