In a phylogenetic tree of primates, a gene involved in the production of brain peptides shows 25 silent substitutions but only 4 missense substitutions.From these results, we can infer that

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The presence of 25 silent substitutions indicates that the sequence of the gene has been conserved over time, as these mutations did not result in changes to the amino acid sequence of the peptides. The fact that there are only 4 missense substitutions suggests that positive selection has not been operating on the gene, as we would expect more changes to the amino acid sequence if this were the case. Therefore, the most likely explanation is that strong purifying selection has been operating on the gene, which is consistent with the conservation of its sequence.

In a Phylogenetic Tree of Primates, a Gene Involved in the Production