The Selective Origin of Incompatibility Barriers in the Plant Genus Gilia

Nine species of annual Gilias in Pacific North America and South America, belonging to the same section of the genus-the Leafy-stemmed Gilias, fall into two classes with regard to geographical distribution. Five species in the California foothills and valleys are sympatric. Four maritime species on the coasts of North and South America are completely allopatric in relation to one another and either completely or mainly allopatric in relation to the foothill-and-valley species. The sympatric foothill species are isolated from one another by very strong incompatibility barriers, forming an average of 0.2 hybrid seeds per flower pollinated in interspecific crosses, whereas the allopatric maritime species can be crossed inter se with the greatest of ease, forming an average of 18.1 hybrid seeds per flower. A somewhat similar geographical distribution pattern of crossing barriers is found in another branch of the genus, the Cobwebby Gilias, which occur in western North America and are also annual herbs. The sympatric species from the central part of the distribution area of this group yield an average of 3.4 hybrid seeds per flower in interspecific crosses. By comparison, crosses between central and peripheral species yield an average of 7.6 hybrid seeds per flower. The species hybrids are highly sterile in all combinations in the two groups of annual Gilias. The formation of such sterile hybrids represents a loss of reproductive potential on the part of their seed parents in the parental species population, a loss which must be especially disadvantageous in short-lived annuals. The inverse correlation between interspecific geographical and crossing relationships provides critical evidence in the case of the Leafy-stemmed Gilias, and supporting evidence in the case of the Cobwebby Gilias, for the hypothesis of a selective origin of hybridization-preventing mechanisms in annual plants under conditions of sympatry. It is proposed to name the process of selection for reproductive isolation the Wallace effect.