Genetic variation- Variation exists in the mocking birds. The gray can mimic sound and the green ones can't
Overproduction and the struggle for existence- mocking birds over one year will produce more birds than the environment can handle and not will survive.
Differential survival and reproduction- Mockingbirds don't live that much but just enough to mate with others. The gray mocking bird can mate easier than the green because of the ability to mimic sound and sing it attracts more mates.
Differential survival and reproduction- Because the mockingbirds who can mimic sound can attract mates they will produce more offspring.They will be able to live long to keep reproducing.
changes in the gene pool- Over time the genetic gene to mimic sound will be more common and will live longer.Half or more of the gene will be able to mimic sound to create a song for mates.