Histone proteins are among the most highly conserved proteins in eukaryotes. Histone H₄ proteins from a pea and a cow, for example, differ in only 2 of 102 amino acids. Comparison of the gene sequences shows many more differences, but only two change the amino acid sequence. These observations indicate that mutations that change amino acids must have been selected against during evolution. Why do you suppose that amino acid- altering mutations in histone genes are deleterious?