Evolving on two fronts: Bill Burger on the nature of oak species

William Burger wrote in 1975, “I believe that the classical species-concept in Quercus defines a very real population system and that it evolves on two fronts. One is that of continuing to adapt to a niche that differs slightly from its close relations. The second is in sharing the broader evolutionary advances of these same close relations that together comprise the genetically isolated biological species.” Burger’s view of oak species reflected morphological study going back at least to 1947, but since Burger’s time, molecular and genomic data have accrued to further support his hypothesis: oak species are distinctive ecologically, morphologically, and genomically, but interspecific gene flow moves alleles (gene copies) between species. This process of genetic introgression increases genetic variation within species and shuffles alleles into new ecological contexts, where they may shape the evolution of the species they enter. Thus interfertile oak species act as a kind of an extended workshop for evolution, where natural selection on a single population can take advantage of innovations that evolved in multiple species. In this essay, I discuss Bill Burger’s species concept and ask how it aligns with what we know about oak species today.