Mechanisms of microevolution (1 of 4) Mutation
Some green genes randomly mutated to brown genes, causing brown coloration. However, since any particular mutation is rare, this process alone cannot account for a big change in allele frequency over one generation.
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.