Evolving on two fronts: Oak species and syngameons

Andrew L Hipp

  William ‘Bill’ 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, ecological 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 movement of alleles between species is called introgression. 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 natural selection working on a single population does so by grabbing hold of innovations (alleles) that evolved in many species—the suite of interbreeding species that constitute an oak syngameon. In this essay, I discuss Bill Burger’s species concept and ask how it aligns with what we know about oak species today.