Answer:
The genetic diversity of new population decreased and this is an example of genetic drift.
Step-by-step 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).