The Selective Origin of Incompatibility Barriers in the Plant Genus Gilia

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Abstract
In 2 sections of the genus Gilia there is an inverse correlation between interspecific geographical relationships and crossing relationships. In the leafy-stemmed Gilia 5 sympatric foothill species are isolated from 1 another by very stron incompatibility barriers, forming an average of 0. 2 hybrid seeds/ flower pollinated in interspecific crosses; whereas 4 allopatric maritime species can be crossed inter se with the greatest of ease, forming an average of 18. 1 hybrid seeds/flower. In the cobwebby Gilia the sympatric species from the center of the distribution area yield an average of 3. 4 hybrid seeds/ flower in interspecific crosses; while crosses between central and peripheral species yield an average of 7. 6 hybrid seeds/ flower. The higher cross-incompatibility of the sympatric species is interpreted as evidence for the selective origin of hybridization-preventing barriers, a process designated as the Wallace effect.