Hybrid Zones

by

Sean P. Mullen

• LAST REVIEWED: 13 January 2014
• LAST MODIFIED: 13 January 2014
• DOI: 10.1093/obo/9780199941728-0040

Introduction

Hybrid zones are regions where genetically distinct populations meet, mate, and produce at least some offspring of mixed ancestry. Historically, interest in hybrid zones has focused on understanding their origins, dynamics, and, ultimately, their fate. Hybrid zone formation occurs either via primary intergradation along an environmental gradient or as a result of secondary contact between two populations that have previously differentiated in allopatry. Unfortunately, both processes can produce similar patterns of phenotypic and genetic variation, making it difficult or impossible in many cases to distinguish between these alternative hypotheses. Established hybrid zones are traditionally classified as transient or stable. Transient hybrid zones are ephemeral, and outcomes include (a) fusion of parental populations, (b) extinction of one of the parental types, or (c) speciation via “reinforcement.” In contrast, stable hybrid zones may persist for hundreds or thousands of generations. The countervailing forces of gene flow and selection govern the apparent stability of such zones. Although gene flow is predicted to break down neutral divergence and lead to fusion of the parental types, ecologically divergent selection on adaptive phenotypic variation or intrinsic hybrid incompatibilities (e.g., hybrid sterility, inviability) are both expected to reduce gene flow across the zone for the genomic regions underlying these traits. Indeed, the antagonism between selection and recombination, which arises as a consequence of gene flow, can lead to a mosaic of genome regions experiencing either divergence or homogenization. This insight led to the characterization of hybrid zones as “windows” into the evolutionary process and has fueled interest in their study, given the promise that such zones provide to reveal the genomic architecture of speciation.

Textbooks

There are no traditional textbooks dealing solely with hybrid zones. The definitive reference on the topic is Harrison 1993b, published by Oxford Univ. Press. Consisting of twelve edited chapters, the book is divided into two parts; the first four chapters focus on pattern and process in hybrid zones, with Harrison 1993a providing a historical perspective on their study. Barton and Gale 1993 then review the theoretical expectations governing the dynamics of hybrid zone maintenance, Howard 1993 includes an in-depth discussion of the reinforcement hypothesis, which is still largely relevant, and Rieseberg and Wendel 1993 covers the causes and consequences of introgressive hybridization in plants. The second part of the book, in contrast, consists of detailed case studies on taxa, including examples drawn from plants, insects, and vertebrates. While hybridization was historically considered a rare process, it is now recognized to be a common, if not integral, part of the divergence process, a view that is explicated and expounded on in Arnold 1997. This book, published as part of the now-defunct Oxford Series in Ecology and Evolution, also provides a historical overview of research on hybridization, with a strong emphasis on the plant literature. In the second part of the book, Arnold discusses the relevance of hybridization for several distinct conceptualizations of the species boundary (BSC, RSC, PSC, etc.) and in Part 3 synthetically summarizes the literature to estimate the frequency of hybridization both in plants and animals. Part 4 of the book deals primarily with specific barriers to gene flow (e.g., post-insemination processes in animals) that can prevent introgression due to hybridization between closely related species or forms. The fifth part of the book reviews evidence for several models of hybrid zone formation (see Introduction) and articulates the view that hybridization provides a natural mechanism for the origin of evolutionary novelty. The sixth (and final major) portion of the book details potential outcomes of hybridization, and the book concludes with a brief summary of the main argument for novelty (Part 7).

• Arnold, M. L. 1997. Natural hybridization and evolution. Oxford Series in Ecology and Evolution. New York: Oxford Univ. Press.

  This book synthesizes an enormous amount of empirical data on hybridization both in plants and animals and argues convincingly that hybridization likely plays an important role in the origin of adaptive phenotypic novelty.

• Barton, N. H., and K. S. Gale. 1993. Genetic analysis of hybrid zones. Paper presented at a symposium at the Fourth International Congress of Systematic and Evolutionary Biology, held at College Park, MD, in July 1990. In Hybrid zones and the evolutionary process. Edited by R. G. Harrison, 13–45. New York: Oxford Univ. Press.

  The seminal description of hybrid zone dynamics, explaining how genotype frequencies found in hybrid zones can be used to infer the overall strength of selection, the number of genes involved in reduced hybrid fitness, the rate of individual dispersal, and the speed of introgression between hybridizing species.

• Harrison, R. G. 1993a. Hybrids and hybrid zones: Historical perspective. Paper presented at a symposium at the Fourth International Congress of Systematic and Evolutionary Biology, held at College Park, MD, in July 1990. In Hybrid zones and the evolutionary process. Edited by R. G. Harrison, 3–12. New York: Oxford Univ. Press.

  The historical overview of the field provided in the introduction by Harrison is a must-read for anyone interested specifically in hybrid zones but also in speciation more generally.

• Harrison, R. G., ed. 1993b. Hybrid zones and the evolutionary process. Papers presented at a symposium at the Fourth International Congress of Systematic and Evolutionary Biology, held at College Park, MD, in July 1990. New York: Oxford Univ. Press.

  The most important text on hybrid zone research published to date. Harrison provides an insightful and concise summary of the history of research on hybrid zones, and contributed chapters from additional authors deal with many core issues relating to hybrid zone origins, maintenance, and the potential outcomes of hybrid interactions.

• Howard, D. J. 1993. Reinforcement: Origin, dynamics, and fate of an evolutionary hypothesis. Paper presented at a symposium at the Fourth International Congress of Systematic and Evolutionary Biology, held at College Park, MD, in July 1990. In Hybrid zones and the evolutionary process. Edited by R. G. Harrison, 46–69. New York: Oxford Univ. Press.

  This chapter provides a synthetic review of the literature and articulates a series of criteria necessary to support the reinforcement hypothesis.

• Rieseberg, L. H., and J. F. Wendel. 1993. Introgression and its consequences in plants. Paper presented at a symposium at the Fourth International Congress of Systematic and Evolutionary Biology, held at College Park, MD, in July 1990. In Hybrid zones and the evolutionary process. Edited by R. G. Harrison, 70–109. New York: Oxford Univ. Press.

  A compendious review of the plant literature, focusing on introgressive hybridization and its consequences.