Observing Evolution is full of moths. One hundred six tiny moth images (body length under 3 mm) are scattered through the text. When I ran into the first one on page 16, I thought: what a great way to demarcate subsections of chapters in a book about the peppered moth! The second moth turned up on page 25, again at the head of a subsection. Soon I found myself marking their incidence, looking for a pattern. Their range: page 16 to page 294. (There are 6 additional moths in the table of contents, marking each of the book’s six parts.)

Author Bruce Grant, professor emeritus of biology at the College of William & Mary, researched Drosophila population genetics in his lab for many years. Grant’s interest in the peppered moth blossomed as a result of a 1982 conversation on the William & Mary campus, after a talk by University of...

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