title: Postzygotic isolation over multiple generations of hybrid descendents in a natural hybrid zone: how well do single-generation estimates reflect reproductive isolation?
abstract: Understanding speciation depends on an accurate assessment of the reproductive barriers separating newly diverged populations. In several taxonomic groups, prezygotic barriers, especially preferences for conspecific mates, are thought to play the dominant role in speciation. However, the importance of postzygotic barriers (i.e., low fitness of hybrid offspring) may be widely underestimated. In this study, we examined how well the widely used proxy of postzygotic isolation (reproductive output of F(1) hybrids) reflects the long-term fitness consequences of hybridization between two closely related species of birds. Using 40 species-specific single nucleotide polymorphism (SNP) markers, we genotyped a mixed population of collared and pied flycatchers (Ficedula albicollis and F. hypoleuca) to identify grand- and great grand-offspring from interspecific crosses to derive an accurate, multigeneration estimate of postzygotic isolation. Two independent estimates of fitness show that hybridization results in 2.4% and 2.7% of the number of descendents typical of conspecific pairing. This postzygotic isolation was considerably stronger than estimates based on F(1) hybrids. Our results demonstrate that, in nature, combined selection against hybrids and backcrossed individuals may result in almost complete postzygotic isolation between two comparatively young species. If these findings are general, postzygotic barriers separating hybridizing populations may be much stronger than previously thought.
authors.affiliation: Department of Animal Ecology, Evolutionary Biology Center, Uppsala University, Norbyvägen 18D, Uppsala 752 36, Sweden. cjw95@cornell.edu
authors.forename: Chris
authors.initials: C
authors.lastname: Wiley
authors.forename: Anna
authors.initials: A
authors.lastname: Qvarnström
authors.forename: Gunilla
authors.initials: G
authors.lastname: Andersson
authors.forename: Thomas
authors.initials: T
authors.lastname: Borge
authors.forename: Glenn-Peter
authors.initials: GP
authors.lastname: Saetre
mesh_terms: D000818:Animals; D049810:Genetic Speciation; D005838:Genotype; D006823:Hybrid Vigor; D006824:Hybridization, Genetic; D012726:Sexual Behavior, Animal; D012934:Social Isolation; D020308:Songbirds / Q000235:genetics* / Q000502:physiology