Complete the statement below, using the word bank provided.
Originally only one squirrel species inhabited the Ponderosa pine forest around the rim of the Gran Canyon, the Abert squirrels. About 10,000 years ago when Arizona's Grand Canyon was formed, the squirrels were separated into two species - the Abert squirrels, who inhabit the south rim; and the Kaibab squirrels, who inhabit the north rim. One population of squirrels had become two separate populations that could no longer reproduce. These squirrels are an excellent example of ______________
wordbank: selection on single-gene traits, stabilizing selection, genetic equilibrium, speciation