CONCLUSIONS: Based on first- and second- generation cross types, our results suggest that the initial barrier to gene flow between these two species was relatively weak, and subsequent evolution has been generally slow. The persistence of low-viability recombinant hybrid genotypes in some contemporary populations illustrates that while hybridization can provide a potent source of genetic variation upon which natural selection can act, the sorting of fit from unfit gene combinations might be inefficient in highly admixed populations. Spatio-temporal fluctuation in selection and/or complex genetics has perhaps stalled adaptive evolution in this system despite selection for admixed genotypes within generations.
Hybridization and natural selection
Retention of low-fitness genotypes over six decades of admixture between native and introduced tiger salamanders: