The Benefits of Gene Flow: Hybridization and Introgression in the Genus Quercus

Janet Rizner Backs

Published May 2025 in International Oaks No. 36: 79–94

Abstract

The genus Quercus is made up of about 450 identified species plus some 180 named hybrids. The ability to hybridize is an evolutionary advantage for oaks that has allowed them to adapt to ecological niches and changing climate since they first appeared in the fossil record approximately 56 million years ago. Their propensity to hybridize has made them stand out as outliers in attempts to define the concept of “species”. Recent research makes trying to define a “species” even more complex, as awareness of a holobiome made up of a host species and its related organisms becomes more prevalent. Advances in molecular genetics have clarified the evolution and population structures of oaks. Entire genomes have now been sequenced, nuclear and chloroplast DNA is analyzed in greater detail, genes and their expression have been identified, and the role of epigenetic processes in altering gene expression is better understood.

This review presents some of the latest study results and ideas about what defines a “species”, as well as those pointing to hybridization and introgression as part of the explanation of how Quercus has come to be such a successful genus.

Keywords

epigenetics, holobiome, holobiont, gene flow, oak lineages, named hybrids

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