Self Fertilization and Population Variability in the Higher Plants

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Abstract
In the flowering plants, there is a series of types of breeding systems from obligate cross fertilizers with self incompatibility to facultative selfers with increasingly restricted percentages of crossing to species which are almost entirely self fertilized. In such genera as Bromus this entire series can be found. The reasons for the assumption that all predominantly self fertilized plants are derived from cross fertilizing species are: (1) They often possess many specialized features of morphology, particularly the annual habit of growth. (2) Many of them have specialized structures, particularly in their flower form, which could have a high selective value only in connection with insect pollination and other forms of cross fertilization. (3) Some self fertilizing types arose in historical times from cross fertilizers. (4) Cross fertilization through self incompatibility requires the occurrence of many rare mutations; self compatible types can be produced from self incompatible species relatively easily by spontaneous or induced mutation. The advantages of self fertilization are insurance of seed production under unfavorable climatic conditions, or in a single plant which has become established after long distance migration; and genetic constancy due to homozygosity giving the self fertilizer an advantage in colonization of newly available habitats. The variation pattern existing in self fertilizing species is best explained on the assumption that although each successful biotype maintains itself for many generations, occasional crossing between genetically different biotypes permits the origin and establishment of some new types. When such a variation pattern exists, the recognition of sympatric subspecies is permissible. Many self fertilizing groups contain a large number of genetically isolated microspecies. These can be produced by crossing between previously existing microspecies, and subsequent establishment of new combinations of the genetic factors (genes or chromosomal rearrangements) which produced the initial isolation. A genetic model for this process is presented. Polyploidy is in general less common in self fertilizers than in related cross fertilizing species, and consists entirely of allopolyploidy.