This is a good read, provides a useful summary of the mechanisms that limit introgression, and offers new insights as to how hybrid speciation might be facilitated:
Stabilizing selection generates selection against introgressed DNA
www.journals.uchicago.edu/doi/10.1086/...
journals.uchicago.edu
Stabilizing selection generates selection against introgressed DNA | The American Naturalist: Vol 0, No ja