restricted access 6. Introgression and Parapatric Speciation in a Hybrid Zone
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Hybridization has been recognized by some as a potent evolutionary force that rapidly can generate new (novel) gene combinations for adaptive evolution and speciation (Arnold 1997; Burke and Arnold 2001; Schluter 2001; McKinnon et al. 2004). However, others have historically viewed it as a minor evolutionary force (barring allopolyploids in plants) or simply as a local or transient type of evolutionary noise or dead end (Rhymer and Simberloff 1996; Schemske 2000; Barton 2001). While definitive proof is generally lacking, especially for animals , diploid hybrid recombinant speciation may represent a mechanism of evolution of new species (Dowling and Secor 1997), especially if it occurs rapidly (Coyne and Orr 2004; Schwarz et al. 2005). The rarity of animal hybrid speciation may be partly due to the difficulty in detection of hybrids (until the use of recent technological tools). Hybrid Zones, “Evolutionary Novelties,” and Isolation Populations at the species borders that are under stress often show increased recombination (Hoffmann and Hercus 2000) and may exhibit increased dispersal tendencies or become more polyphagous (Thomas et al. 2001). When the species edge is a hybrid zone, some species-related traits, including X-linked traits, may move into different populations independently of others due to extensive interspecific introgression and recombination (Martinson et al. 2001; Scriber 2002a, 2002b; Scriber and Ording 2005)—all of which can result in genetically novel populations. Unfortunately , for most organisms there remains little information on geographic variation in specific traits known to be associated with range margins (Hoffmann and Blows 1994; Endler 1995). Homoploid hybrid speciation requires hybrids to be fertile , “fit,” and reproductively isolated from the parental species types (Buerkle et al. 2000; Coyne and Orr 2004). Hybrids may be more fit than parental species inside a hybrid zone (bounded superiority model), or less fit (tension zone model), or they may display variable fitness in a “mosaic zone” model (see review by Arnold 1997). Rarely have more than one or two fitness traits been measured in assessing hybrid fitness relative to parental types (Arnold and Hodges 1995; Barton 2001). However, hybrid fitness may in some cases give rise to “evolutionary novelties” that are not explained by the tension zone model or mosaic models (since hybrid genotypes may be very fit). The “novelty ” model also differs from the bounded superiority model since hybrids are not necessarily restricted to the ecotone in which they arose and may spread outside the historical hybrid zone (Arnold 1997). In addition, although they may arise infrequently, these hybrid populations with genetic novelties may give rise to new, long-lived evolutionary lineages (Arnold and Emms 1998), adaptive radiations (Seehausen 2004; Bell and Travis 2005), or new species via recombinant hybrid speciation (Rieseberg 2001; Coyne and Orr 2004; Schwarz et al. 2005; Gompert et al. 2006). Rapid chromosomal repatterning, ecological divergence, and/or spatial separation have been invoked to explain the reproductive isolation between hybrid lineages and parental gene flow (Rieseberg 2001). Necessarily sympatric or parapatric in origin, homoploid hybrid speciation is assumed to require a unique ecological niche for recombinant hybrids to avoid being outcompeted or gene-swamped by introgression from parental types before the establishment of an independent lineage from hybrid origins (Coyne and Orr 2004; Seehausen 2004; Schwarz et al. 2005). New host-plant-associated races could provide hybrids with such a competitionfree habitat (Emalianov et al. 2003; Schwarz et al. 2005). However, temporal reproductive isolation can provide a different but equally effective mechanism to avoid introgression or competition from parental types (Scriber and Ording 2005). Changes in voltinism patterns or other phenological 69 S I X Introgression and Parapatric Speciation in a Hybrid Zone J. MARK SCRIBER, GABE J. ORDING, AND RODRIGO J. MERCADER constraints in insect populations may affect their temporal isolation from other populations. Climate change is one mechanism that might cause such changes. Climate Warming, Thermal Constraints, and Voltinism The impact of recent regional climate warming on geographic distribution limits, abundance, and population dynamics has been significant for a variety of herbivorous insects. While many species may extend or expand their ranges poleward (Hellmann 2002; Parmesan and Yohe 2003), some contractions or extinctions of some local populations might also result from warming (Thomas et al. 2004). Phenological coordination between parasites and herbivores or specialized pollinators and plants (Thomas and Blanford 2003), altered susceptibility of different insect herbivores to insect pathogens (Altizer et al. 2003), sexual synchrony, size variation and mating coordination (e.g., with protandry) (Nylin et al. 1993; Zonneveld 1996; Nylin and Gotthard 1998) may prove...