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?

The genetic diversity of new population decreased and this is an example of genetic drift.  

Explanation:

Founders effect is the case of genetic diversity which occurs when a small number of individual from a larger population of species gets isolated from the larger population and occupy a different habitat. In the smaller population, the genetic diversity reduces because they lost many allele which was present in the larger population.

Here a small number of individuals from mainland species moved to the islands of Galapagos therefore these individuals would not be having the total gene pool that was present in the large population of mainland species.  

So the genetic diversity of the new population of finches on the Galapagos  must be decreased concerning the population on the mainland and this is an example of genetic drift(founder's effect).

because the light is a great source of energy for the plants to survive. (i hope this )