A mutation in the population of green beetle caused the creation of the new trait (brown color), that would make the genes for brown beetles become more frequent in the population. That created difference introduced by mutations might be advantageous and may help an organism survive. For example, green beetles are easier for predators to see and consequently eat than brown beetles. Thus, brown beetles are a little more likely to survive to produce offspring.
This is an example of how one evolutionary mechanism (mutation) can cause evolutionary change.