Which type of mutation is most likely to be neutral?

A synonymous mutation is most likely to be neutral because it involves a change in the nucleotide sequence of a gene that does not alter the amino acid sequence of the protein produced. This type of mutation occurs due to the redundancy in the genetic code, where multiple codons can code for the same amino acid. Because the protein's structure and function remain unchanged, synonymous mutations tend not to have any phenotypic effect on the organism, making them neutral with regard to natural selection.

In contrast, insertion mutations and nonsense mutations typically result in significant changes to the protein produced. Insertion mutations can lead to frameshifts that alter the entire amino acid sequence downstream, potentially creating nonfunctional proteins. Nonsense mutations introduce premature stop codons, resulting in truncated proteins that are usually nonfunctional. Nonsynonymous mutations change one amino acid for another in a protein, which can impact the protein's stability, function, or interaction with other molecules, leading to possible phenotypic consequences.

Therefore, synonymous mutations provide a clearer example of neutrality in the context of evolution and natural selection, as they don't result in observable changes in the phenotype of the organism.