Reproductive isolation between species appears, in certain cases, a long time after fertilization and the formation of the zygote, as happens – for example – in the twin species Drosophila pavani and D. gaucha.
A postzygotic reproductive barrier is a mechanism that reduces the viability or reproductive capacity of hybrid offspring. For example, a plant that's structured for bee pollination will not be compatible with flowers that rely on hummingbirds to spread their pollen. After fertilization, a zygote is formed that develops into a hybrid.

While this is still a result of differing shapes, it's not the shape of the actual gametes that matters, but rather, the incompatibility of the flower shape and the pollinator. In the example of the female horse and male donkey crossing, the offspring is a mule which has the inbred postzygotic barrier that is sterilization, it cannot reproduce. There are both prezygotic and postzygotic barriers. Reproductive barriers are present to reduce interbreeding between similar species that have overlapping habitats.

Reproductive barriers encourage speciation.

The purpose of postzygotic barriers is to reduce the fitness of the hybrid.

The hybrids between both species are not sterile, in the sense that they produce viable gametes, ovules and spermatozoa.