By cross-breeding a tiger and a lion, a hybrid animal was created, but was infertile. Which evolutionary barrier caused the infertile offspring?

The evolutionary barrier that caused the infertile offspring of the cross between a tiger and a lion is postzygotic reproductive isolation. The reproductive isolation is a mechanism to prevent closely related species, such as lions and tigers, to mate each other. The reproductive barriers can be either prezygotic (before fertilisation) and postzygotic (after fertilisation) barrier. Since the lion and the tiger have the offspring, the barrier that caused the infertility of the offspring occurred after fertilisation. Thus, this is the example of postzygotic isolation.