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 (2 of 4) Migration
Some beetles with brown genes immigrated from another population, or some beetles carrying green genes emigrated.
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.