the island is a small specie of lizards so if the mainland specie was a large population it would have been the one to be affected by an increase instead of a decrease. The genetic drift was caused by not having enough of the same kind to keep the heredity going, most likely the largest population is the one that had enough of the same heredity to keep the population of lizards going. In a way the hurricane with the force on both species caused a genetic drift but increased a larger population for a year later.