Hybridization between species often results in offspring that are less fit than pure parental forms and may result in selection for traits that enhance prezygotic barriers to gene flow (a process known as reinforcement). Evolutionary biologists also recognize a role for interspecific hybridization in promoting the evolution of novel forms (Stebbins 1950; Anderson & Stebbins 1954; Harrison 1993a,b; Rhymer & Simberloff 1996; Arnold 1997). Natural hybridization occurs relatively frequently among divergent populations or species of plants and animals (Stace 1975; Barton & Bengtsson 1986; Grant & Grant 1992; Mallet 2005), and, despite the relative rarity of hybrids in any single population, only a few are needed to allow the exchange of advantageous alleles between species (i.e., introgression). The historical admixture of genomes has also contributed to speciation, especially in plants but also in some animal taxa (Arnold 1997; Dowling & Secor 1997; Mallet 2005; Gompert et al. 2006; Mavárez et al. 2006; Grant & Grant 2008).

In conservation biology, the predominant view is that introgressive hybridization is a cause of extinction (Rhymer & Simberloff 1996; Wolf et al. 2001), especially when hybridization events are anthropogenically mediated. Previously allopatric species are now commonly brought into contact due to anthropogenic introductions (Pysek et al. 1995). Likewise, previously parapatric or sympatric species now commonly hybridize due to human-induced changes in the environment that reduce elements of habitat heterogeneity that once contributed to diversification and maintained species independence (e.g., through ecological isolation; see Candolin et al. 2006; Hendry et al. 2006; Seehausen et al. 2008). Some have argued that this “smoothing” of landscapes is equivalent to a “speciation reversal,” a process that will become increasingly common in changing environments (Seehausen et al. 2008).