Some green beetles appear among the red beetle because:
Beetles become green by accumulating chlorophyll from the green leaves that they eat
Natural variations occur during sexual reproduction.
Red beetles mimic green colour of leaves whenever they see crows.
Beetles change colour from red to green with change of season.
Natural variations occur during sexual reproduction.
In this case natural selection is directing evolution in the beetle population. Natural selection brings about improved adaptive relations between organisms and environment by favouring the reproduction and survival of more suited organism to the given environment.