Ruby-throated hummingbirds are migratory birds that spend the summer in Canada and the northern United States and the winter in Mexico and Central America. Since all ruby-throated hummingbirds migrate at around the same time each year, many populations that were possibly isolated during the winter intermingle during the summer breeding season. This annual migratory behavior results in a transfer of genetic information from one population to another, which is known as _______. A. natural selection B. genetic drift C. gene flow D. speciation