The mechanisms of reproductive isolation prevent the reproduction of two individuals from different species. The prezygotic mechanisms disrupt the stages of reproduction before the forming of the zygote or prevent the mating at the start, and the postzygotic mechanisms affect the stages of reproduction after the forming of the zygote.
So, the first and the third example (the urchins and the grasshoppers) show the prezygotic mechanism, as the two individuals are not able to mate or form a zygote.
The second and the fourth example ( zonkey and the death of a zygote) show the working of the postzygotic mechanisms, as the zygote is formed, but it seems to be inviable, and the zonkeys are sterile, preventing the individual to reproduce.
The child has a 100% chance of having Sickle Cell Disease
The human digestive system can fail if you eat something harmful, and it can break down nutrients, techinically the digestive system can stop working if you get something like Gastoparesis, which stops the movement of food through the digestive system.
<span>Epigenesis is the term which says that an organism develops from the fertilized egg by a succession of developmental events that leads to an adult. This theory was first published by Aristotle. It became a well established and accepted theory in the late 18th century.</span>
