If you had been in northern Canada 45 million years ago, you might have encountered the distant ancestor of all of the oaks in the Americas. That single species gave rise to 220 more and two distinct lineages—red oaks and white oaks—that moved south through the boreal zone to populate large swaths of the continent all the way into Mexico. These two findings—simultaneous evolutionary diversification in the red and white oaks, each following the same geographic routes; and two relatively recent origins of the Mexican oaks—are a surprise conclusion to a scientific mystery that went unresolved until now. Research published this week in the journal New Phytologist tells this story of the evolutionary history of American oaks for the first time.
Using a combination of next-generation DNA sequencing and statistical ecological methods, the researchers inferred the most detailed and comprehensive evolutionary history to date for the oak genus Quercus. Their work demonstrates that the two major groups of oaks in North America—the red oaks and the white oaks—independently and simultaneously radiated over the past 45 million years from a common ancestor. The red oaks and the white oaks simultaneously filled ecological space in California while also filling available habitats of eastern North America. Then, between 10 and 20 million years ago, both groups made their move down to the mountains of Mexico, where the two groups began to diversify at an increased rate, rapidly moving around to fill ecological (niche) space and producing species more rapidly than they had done in the north.