Intron sequences mcq from mcat preo?

Question

the intron sequences of identical genes in closely related species are often very different. which of the following is most strongly suggested by this evidence?

a. identical genes in closely related species may code for different proteins.

b. changes in the amino acid sequence of a protein do not necessarily change protein function.

c. intron sequences are heavily characterized by selective pressure.

d. selective pressure has little or no role in the development of intron sequences.

the answer is d....but the mcat prep stuff did not fully explain selective pressure...can anyone provide insight? i am a frequent answerer!

Answer 1

If this gene, which exists in two species, is really important to the health of the organisms in question, then any mutation in this really important gene is likely to cause a large decrease in fitness;  over time, the action of natural selection will weed out such mutants, in effect keeping the exon sequences of this gene unchanged.

However, intron sequences are not part of the final product of that gene - if a mutation occurs in an intron, there is no reason why natural selection should act to remove it, because that intron mutation won't affect the fitness of the organism.  So changes in intron sequence are 'free' to build up because there is no selection pressure against them.