The answer to this can be found in Chapter 7, page 123 in the paperback edition. According to Diamond, the Industrial Revolution led to pollution, which made trees in England darker because of all the soot and such that landed on them. This affected moth populations because it changed the color of the bark that they would sit on. When the bark was light, light moths would blend in and be camouflaged. As the trees got darker, light moths were at a disadvantage and dark moths were camouflaged. This led to moths getting darker because natural selection preferred those that were dark.
Diamond tells this story to illustrate the ways in which human activities can affect the ways that animals (and plants) evolve.