Agroup of 50 sparrows (small birds) were blown off course by a storm during their winter migration. the birds were stranded on an island in the middle of the pacific ocean. because this island was too far from the mainland for a yearly migration, the sparrows only mated with the other sparrows stranded on the island. after 500 years, this small isolated group of sparrows had beaks 25% longer than the mainland population. this is an example of: