All 5 Uses of

molecule
in
Turtles All the Way Down

He then explained to me in rather excruciating detail that Tua (it had a name) was not a lizard at all, but a genetically distinct creature that dated back to the Mesozoic Era 200 million years ago, and that it was basically a living dinosaur, and that tuatara can live to be at least 150 years old, and that the plural of tuatara is tuatara, and that they are the only extant species from the order Rhynchocephalia, and that they were endangered in their native New Zealand, and that he'd written his PhD thesis on tuatara molecular evolution rates, and on and on until the door opened again, and Lyle said, "Dr Peppers, boss."†
p. 36.5

molecular = relating to the smallest amount of a chemical substance that can exist by itself

But what fascinates me most, and is the focus of my research, is their molecular evolution rate.†
p. 176.6

But despite doing everything slowly and having not changed much in two hundred million years, tuatara have a faster rate of molecular mutation than any other known animal.†
p. 177.4 *

Definitions:

(1) (molecule)
the smallest amount of a chemical substance that can exist by itself -- usually consisting of 2 or more atoms
(2) (meaning too rare to warrant focus)
meaning too rare to warrant focus:

Less commonly, molecule is used informally to refer to anything that is very small.