Complex sentence: This is shown in the study of hybrid zones, as well as differences in levels of prezygotic isolation (by factors that prevent formation of a viable zygote) in both sympatric and allopatric populations.
Split into simpler sentences: This is shown in the study of hybrid zones. It is also apparent in the differences in levels of prezygotic isolation (by factors that prevent formation of a viable zygote) in both sympatric and allopatric populations.