Darwin's Finches Came from a Mainland Species Where Some Individuals

Darwin's finches came from a mainland species where some individuals were able to move to the islands and then spread from one island to another.Is the genetic diversity of the new population of finches on the Galápagos increased or decreased in relation to the population on the mainland,and what is this an example of?

A) It had no effect on their genetic diversity because the colonists have the same diversity as the mainland population.
B) It increased their genetic diversity because this represents an example of gene flow.
C) It decreased their genetic diversity because this represents an example of genetic drift.
D) It increased their genetic diversity because this would introduce new mutations into the population.
E) It decreased their genetic diversity because this represents an example of natural selection.