Although it was believed for years that Monarch butterflies in the eastern US and the western US were genetically different, DNA studies have proven that they are the same.
There are two groups simply because they do not naturally fly over the Rocky Mountain Range as they migrate and fly about to mate and lay eggs. The western Monarch butterflies usually stay west and the eastern Monarchs usually stay east. Although Monarch butterflies have been seen in the Rocky Mountains, in high altitudes, it is not their natural flight direction. Unless they are migrating, they seek out milkweed and nectar plants, males searching for female butterflies which are often around milkweed and nectar plants.
The practice of tagging and releasing Monarch butterflies has revealed a fact that was not expected. First, some western Monarchs will migrate to Mexico. They are tagged in the west, wild Monarchs, and the tags are found in Mexico.
Second, it was discovered that if a Monarch raised west of the Rocky Mountains is moved to the Eastern US and released, it will migrate to Mexico and vice versa.
Migration destination depends on where it is located when it begins migration, not where it was when it emerged as an adult butterfly. Its DNA signals it what to do depending on its location at the current time and which direction to fly, depending on where it is when it is captured, tagged, and re-released.
This map is a Monarch Watch map, the information based on recovery of tagged Monarch butterflies.
Dr. Tom Emmel, University of Florida, believed that some Monarchs fly down Florida to Cuba to stay in mountains there. Some, after arriving in Cuba, he believed would fly to the Yucatan Peninsula and over to the overwintering sites in Mexico.
Although the genetics are the same in both eastern and western Monarch butterflies, the USDA does not permit them to be shipped across the continental divide.
Mia Munson provided this link to a Monarch Watch webpage that mentions the DNA study.