Apopulation of squirrels is preyed on by small hawks. the smaller squirrels can escape into burrows. the larger squirrels can fight off the hawks. after several generations, the squirrels in the area tend to be very small or very large. what process is responsible for this outcome?

Which of the following must be true for a fossil organism to be useful as an index fossil? a. the fossil organism must be widespread geographically. b. the fossil organism must be abundant. c. the fossil organism must be limited to a short span of geologic time. d. all of the above.