Final answer:
For sympatric speciation to occur, disruptive selection and positive assortative mating are necessary. Disruptive selection favors extreme phenotypes and against intermediates, while positive assortative mating ensures similar phenotypes mate more often, contributing to speciation without geographical barriers.
Step-by-step explanation:
The conditions thought to be necessary for sympatric speciation to occur include disruptive selection and positive assortative mating. Disruptive selection, also known as diversifying selection, occurs when two or more extreme phenotypes are favored by natural selection, while intermediate phenotypes are selected against. It is one of the mechanisms that can lead to sympatric speciation, as it can result in a population splitting into two distinct groups without geographical separation. Positive assortative mating is another essential condition, which means that individuals with similar phenotypes are more likely to mate with each other, further driving the process of speciation.
Therefore, the correct answer to the question of which conditions are thought to be necessary for sympatric speciation to occur would be 4) a and c are correct, indicating that both disruptive selection (3) and positive assortative mating (3) are indeed necessary.