Identify the independent and dependent variables in the Hershey-chase experiment

In which situations is the principle of cross-cutting relationship useful in determining relative age?

Asquirrel population lives in an area. over many years, a river in that area grows wider and stronger, eventually forming a canyon. the squirrel populations on the two sides of the canyon can no longer mate with each other and ultimately form distinct species. this is an example of

Which feature would you expect to find in a population in which sexual selection depends on male competition?