Genetic introGression: an inteGral but neGlecteD component of speciation in birDs

The Auk, Vol. 128, Number 4, pages 620−632. ISSN 0004-8038, electronic ISSN 1938-4254.  2011 by The American Ornithologists’ Union. All rights reserved. Please direct all requests for permission to photocopy or reproduce article content through the University of California Press’s Rights and Permissions website, http://www.ucpressjournals. com/reprintInfo.asp. DOI: 10.1525/auk.2011.128.4.620 3E-mail: [email protected] Hybridization, or the act of reproduction between different species, is common in birds (Grant and Grant 1992) and can be an agent of adaptive evolution (Veen et al. 2001). When hybridization occurs, a species’ genetic material may enter the gene pool of another and thereby introduce genetic novelty. This process is called “genetic introgression” (hereafter “introgression”; Anderson 1949). Although known for decades, the incidence of introgression in nature has long been underappreciated (e.g., Currat et al. 2008). Only recently have evolutionists come to regard it as an important and pervasive mechanism in speciation, in the maintenance of genetic diversity and in the introduction of advantageous novelty into the gene pool (Arnold et al. 1999, Noor et al. 2000, Seehausen 2004). In fact, genetic data now suggest that our own species may have undergone a complicated history of introgression with at least two extinct hominine lineages (Green et al. 2010, Reich et al. 2010). Another reason why introgression is an important topic is the impact of human-induced climate change, which is rapidly shifting vegetation boundaries and creating new hybrid zones, with little-known consequences for the genetic integrity of species (Mank et al. 2004, Brumfield 2010). There has been a tremendous increase in introgressionrelated avian research since a comprehensive overview of the introgression complex between the Blue-winged Warbler (Vermivora cyanoptera) and the Golden-winged Warbler (V. chrysoptera) appeared in The Auk five years ago (Confer 2006). Here, we seek to fill a gap by summarizing the most important recent advances in avian introgression research and our current understanding of locus-specific differences in introgression dynamics, the detection of introgression, its directionality, and its relevance to conservation, phylogenetics, and speciation research.