A huge question in evolutionary biology is the very basic one: How do species form? It turns out that the Dark-eyed Junco, one of the most common birds at winter feeders, is providing a clear picture of that process.
First, a quick review of what defines a species:
Biologically, it's a group able to interbreed and produce viable offspring. Back when a massive ice sheet covered most of North America, there were no Juncos here; they were forced south. As the glacier melted and retreated northward, one Junco species from Mexico headed in that direction as well, to colonize new, unexplored territory.
Encountering the broad North American continent, the Mexican Juncos radiated out in different directions. In time, they separated into five main population groups in five distinct regions.
Geographic isolation and breeding within regional groups led to changes in looks, behavior and genetic make-up as groups adapted to local conditions and challenges. That's how species form, slowly, given time and separation.
In the Juncos' case, genetic mutations that typically occur over millions of years, only took thousands of years, dating back to the retreat of the last glacier. The Mexican Junco pioneers led to our Dark-eyed Junco.
Given more time and separation, it's likely that today's Dark-eyed Junco will be split into several species. For now, there's evidence of interbreeding in the rare instances where one group overlaps another.
Junco populations continue to diverge--it's a work in progress!
Indiana University produced a film that highlights the common, but amazing bird, the Junco. Watch the trailer below and click here to see the full film.