Hybridization and Introgression

Abstract

Spontaneous hybridization between taxa provides a source of genetic variation upon which selection may act. Though individual genetic variants are not new, the combinations of variants from diverse genomes found in hybrids are new (novel epistatic gene interactions), and hybrid populations may, and often do, contain more genetic diversity than their parental species (Stebbins 1959; Lewontin and Birch 1966; Grant 1971). While investigators have long debated the evolutionary significance of hybridization and the subsequent incorporation of genetic variation from one species into another, several studies reviewed in this chapter would suggest these phenomena play an important role in conifer evolution. This chapter seeks to summarize the scientific literature on hybridization and introgression in conifers with particular attention given to studies from the last quarter century. As in previous decades, much of this literature is descriptive. Increasingly, however, investigations have expanded to investigate such important principles as which speciation model is most appropriate, what are the mechanisms by which species boundaries are maintained, and how methods used to detect and measure introgression affect our interpretation of the results. Such important queries are reviewed by looking at specific case studies.