A population of beetles lived in a particular ecosystem Beetles in this population varied in size from small to large and ranged in color from dark green to dark brown Over thousands of years the population was divided into two separate populations by the development of a stream that eventually turned into a small river The west side of the river was drier and had less vegetation Smaller browner beetles were more successful in this ecosystem The east side of the river received more precipitation and had lush green vegetation Larger greener beetles were more successful in this ecosystem Over time the beetles on each side of the river became so different that they could no longer interbreed Which of the following best describes the mechanism that caused the beetles on each side of the river to become better suited to their different environments and eventually diverge into separate species OA Genetic drift occurred as a result of the populations being geographically separated OB Natural selection resulted from differential reproductive success in both populations OC Individuals within each population changed their genes to best fit their environment OD The environment caused genetic mutations in individuals within each population