What is the difference between punctuated and gradual evolution? Why does gradual evolution not completely explain how the giraffe got its long neck?
You need first to define punctuated and gradual evolution. Most students understood the difference although some missed the fact that punctuated evolution was followed by long periods of stability after the initial rapid evolution. Many students developed the point that in puncuated evolution speciation and morphological change happen at the same time while in graudual evolution considerable morphological change can happen before speciation occurs.
Then you need to devlop the idea that vertebrate in the necks of giraffes elongate to lengthen the neck. The natural selection force which allowed individuals with longer vertebrate to survive was drought. Only the tallest trees with the deepest roots on them would have the young green shoots on which giraffes like to browse. Some students suggested that giraffes evolved from okapi which is probably not correct. Giraffes and okapis have a common ancestor which had a short neck. Modern okapi have a slightly elongated neck. Another misconception was that the giraffe was a grazing animal (eating grass) and not a browser (eating leaves and shoots of bushes and trees). Other students fell into the trap of talking about the neck getting long because the graffe was stretching to reach leaves on the tallest trees. This is considered to be 'Lamarkian evolution' and is not accepted as being a valid mechanism of evolution.
Then the blood sinus which allows the giraffe to drink without having a brain haemorrhage should be described. Once that was done then you could effectively argue that the neck had to be by punctuated evolution. The neck alone might elongate through gradual evolution but when the blood sinus has to be considered there is no force requiring a blood sinus to evolve gradually. The sinus has to be in place for it to work properly. A partial sinus won't do. There is no selection for a blood sinus until the neck becomes fairly long and then it will have to be in place or the animal won't survive. This must occur by rapid or punctuated evolution.