The mechanism of the transfer of pollen maybe wind, insects, birds, or even water. Cross-pollination brings about a greater diversity of hereditary traits from the two parents. Blueberries, for instance, need two different varieties planted in the field, or berry production will be severely curtailed. I have said that pollination is how nature selects genes for future generations. GM crops are those which are artificially manipulated by humans, by inserting genes into a plant by means other than pollination.
Many experiments involve manipulations such as putting a fish gene into a tomato.
The plants are then bred and multiplied in the normal manner. Some people will argue that this is no different than what nature does. Of course, in nature, an animal gene will never find its way into a plant. Even in nature, selection mistakes can occur, such as when one grower produced potatoes with an excess of a poisonous alkaloid. All of this discussion of definitions leads up to my point.