Open pollination increases biodiversity but results in some plants less suitable for their environment or intended human use. This is in contrast with hand pollination, which is controlled so that all seeds of a crop carry the same traits. Another type, hybrid pollination, increases suitability, especially through heterosis, but decreases biodiversity. Some hybrid strains are inbred and selected for desired traits until a strain that breeds true by open pollination can be developed.
One of the bigger challenges in maintaining a strain by open pollination is avoiding introduction of pollination by other strains. Based on how broadly the pollen for the plant tends to disperse, it can be controlled to varying degrees by greenhouses, tall wall enclosures and stuff, or field isolation.
Popular examples of plants produced by open pollination include the heirloom tomato.