6. which of the following examples describes a population where genetic drift is most likely to have the largest effects on allele frequencies? monarch butterfly populations decrease because overwintering grounds in mexico are deforested. a population of songbirds is reduced by 50% due to low food availability during a very cold, harsh winter. a storm causes a few mockingbirds to be transported from mainland south america to distant islands. the islands had no mockingbirds prior to the storm. the mockingbirds which colonize the islands and have a different set of allele frequencies than the mainland population. cackling geese used to be considered a subspecies of canada geese, but now cackling geese are considered a separate species. cackling geese are smaller with a shorter bill than canada geese. when cackling geese and canada geese mate, no offspring survive. black bears and grizzly bears interbreed (rarely) where their geographic ranges overlap.