Are the western and eastern populations genetically isolated, or do they sometimes mix during migration?

The eastern and western monarch populations are not genetically different. The eastern population refers to the monarchs that overwinter in Mexico, and the western population refers to the monarchs that overwinter along the Pacific Coast. There is evidence of interchange between the eastern and western populations, perhaps when individuals cross the Rocky Mountains, when butterflies fly from the western U.S. to the Mexican wintering sites, or butterflies from the Mexican sites fly into the western U.S. Learn more about monarch migration on the biology page of our website.