I was looking for sweet cherries. Around here, one of the most popular sweet cherries is the Bing Cherry which requires a pollinator.
I will likely only get a single tree of any variety, so all it means is that I need to buy at least two trees (or one tree grafted with multiple varieties). But, a commercial orchard could potentially have hundreds of clonally propagated trees so having additional pollinator trees would be important.
In the wild, one would not have the clonal orchards so it would not be as big of an issue except that a single solitary tree would not be viable.
I will try to look at lists of trees and try to determine the characteristics of the ones requiring a pollinator. Perhaps the dark colored cherries are more likely to have the requirement.
Anyway, I find it interesting that they have evolved a mechanism to reject "self pollination". Perhaps with antigens?
It apparently is a different mechanism than is used with male sterile corn, as two non-self--pollinating trees of different types can cross pollinate.