Mechanisms of microevolution (2 of 4) Migration
Some beetles with brown genes immigrated from another population, or some beetles carrying green genes emigrated.
Mechanisms of microevolution (3 of 4) Genetic drift
When the beetles reproduced, just by random luck more brown genes than green genes ended up in the offspring. In the diagram at right, brown genes occur slightly more frequently in the offspring (29%) than in the parent generation (25%).
Mechanisms of microevolution (4 of 4) Natural selection
Beetles with brown genes escaped predation and survived to reproduce more frequently than beetles with green genes, so that more brown genes got into the next generation.