Every time I read about insect trachea, I think of grasshoppers. I vaguely recall having read something about the efficiency with which oxygen delivers to the cells that power flight.

We know this specifically for the fruit fly Drosophila melanogaster.

It blew me away when I first learned this too. The next amazing thing is that transcription factor concentration gradients cause each region of nuclei to specialize into different cells, thereby dividing the insect into segments. I'm still trying to understand how gradients accomplish this.

I had a dog who had three mast cell tumors removed over two years. Visits to the vet were so stressful for her that I decided I wouldn't put her through that again -- and she never had another mast cell tumor. She died years later at 16.5 from some autoimmune condition acquired from a dead Opossum.

This is Google Docs. I still have tremendous trouble with Word even for more common words.

A is my clear fav. I'd rank B as the least readable.

LOL! The book isn't arguing for random chance though. It lists six ways nucleotides can change.

Your point is valid, but I don't believe the quote make any claim about what persists.

Right. The text talks much about conserved regions. The claim isn't that the base pairs we have now have all changed in that time, only that replication has failed for each base pair during that time during some attempt at replication.

It sounds like you're saying that although these base pair changes may have been "tried", many have not likely survived. Does that change the reported stats?

From Alberts et al., Molecular Biology of the Cell, 7th ed. (2022)