No, that’s not what I’m saying. I’m saying why can’t the accumulation in the same protein continue? There’s an antibody, it has some sequence. It mutates in the hypervariable region in a stretch of 6-10 amino acids, and over the course of a few weeks 50 FI (or whatever) is generated. Why could a larger portion of the antibody not continue mutating?
Why could the same thing not happen to 200 residues, out of a 600 amino acid protein over the course of 500 million years? In fact, isn’t this exactly what phylogenetics reveals to us has happened? We create a large tree of homologous proteins, and we see that along some lineages lots of mutations have accumulated in the protein during this time period?
Natural selection fixed the mutations in the antibody. Why can’t natural selection have fixed the mutations in the larger protein?