Higher plants are characterized by bisexuality—a prerequisite to autogamy—and by species which are broken up into populations of remarkably small effective breeding size even if distributed continuously (wright, 1946; Ehrlich and Raven, 1969). Such genetic discontinuity is an outcome of the random, but leptokurtic form of gene dispersal by pollen and seed, and of disruptive selection encouraging polymorphism (Mather, 1955) in extremely localized environments. Plant population differentiation over distances of less than 30 m and in less than 40 years has been demonstrated experimentally (Snaydon, 1970). Local differentiation of outbreeding species is maintained by selection in spite of foreign pollen contamination as high as 50–60% at the site (Jain and Bradshaw, 1966). Therefore, the adoption of self-fertilization by many plants as an evolutionary strategy must be regarded mainly as a response to certain conditions of existence such as present in pioneer habitats, where initial reproduction must be assured and a rapid build-up of uniform population may be advantageous (Stebbins, 1957; Baker, 1959; Rollins, 1967; Levin, 1972). As a matter of fact, the majority of the most successful noncultivated colonizers are predominantly self-pollinated annual species (Allard, 1965). Cultivated plants have been selected for genetic constancy and reliable fertility since the beginning of plant domestication. Selection for genetic constancy and stable fertility mean restriction of genetic recombination and independence of the vaguaries of pollination vectors in the cultivated field; these are facilitated by self-pollination. Actually, the evolution of many self-pollinated cultivated plants can be traced to outbreeding ancestors. Such evolution most likely arose through a gradual adjustment of the mating system (loss of self-incompatibility, structural adjustment of flowers) as well as through restriction of the effective breeding population.
KeywordsPollen Tube Seed Parent Hybrid Seed Production Hermaphrodite Flower Pollen Collection
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