A population is a cloud of differences, never a shelf of copies.

Beak depths vary; a child inherits a parent's beak, plus a small change.

More are born than can live: seeds are hard, and only deep beaks crack them.

Here, 200 birds are born; only the ones above the threshold survive.

A drought leaves only the hardest seeds. The threshold moves up.

The birds that survive are the deep-beaked ones, and they breed.

Repeat for generations: the average beak depth moves toward the drought.

In the real Daphne Major finches, birds after the 1977 drought had larger beaks.

This is a model; the one real fact is the 1977 change in the Galápagos finches.