Below is a diagram depicting the beak depth of two species o…

Below is a diagram depicting the beak depth of two species of Galapagos finches.  The first two graphs show the beak depth when the birds exist alone on an island and the final graph shows the beak depth when the two species occur together on the same island.  What ecological term describes this shift in beak depth when the species occur together?  Explain how this shift would occur.