Seminar Summary - Plant biologists have debated the nature of oak species for more than 200 years. Opinions range from the view that oak species hybridize relatively rarely to the view that related oaks form syngameons, near-freely interbreeding complexes of species. Understanding species boundaries and gene flow in oaks is essential to conserving the ca. 425 global oak species on which humans and hundreds to thousands of arthropod, fungus, vertebrate, and plant species depend. In this talk, I provide an overview of how our understanding of oak species boundaries and hybrids has grown from the early 19th Century to today. Molecular data from the past two decades show that individual oaks exhibit a wide range of mixed-species ancestry, with as many as 20% of individuals averaged across studies admixed at a level of 10% or higher. This means that hybridization is quite common in many oak species, and some of the resulting gene flow may play a role in population adaptation and species migration. Nonetheless, oaks form genetically distinct species, and that species diversity is crucial to the function of forests, savannas, and other oak-dominated forests across much of the northern hemisphere. The lecture will include both historic and recent research.