In biology, there exists the concept of "ring species."
For example, in Western Europe, there is a type of bird called the Little Black-backed Gull.
Many years ago, a group of Little Black-backed Gulls set out from Western Europe and migrated eastward. Due to the high latitude, they wouldn't continue north into Arctic regions. Eventually, they settled in Siberia and became the Siberian subspecies of the Little Black-backed Gull.
Then, a group of Siberian subspecies Little Black-backed Gulls continued to set out, migrating eastward all the way, eventually evolving into Herring Gulls.
A group of Herring Gulls continued migrating eastward, evolving into the Vega Gull subspecies, Vega Gulls, Federation Herring Gulls.
Finally, a group of Federation Herring Gulls re-crossed the Atlantic eastward and returned to Western Europe, evolving again into Silver Gulls.
