THE EVOLUTION OF SELF-POLLINATION INCLARKIA XANTIANA

The evolutionary effects of outbreeding and inbreeding systems have caused considerable interest and speculation over the years and their influence in changing gene frequencies has been formalized mathematically (see Fisher, 1949; Wright, 1921). A large body of data is available on the genetical basis of the sex and incompatibility mechanisms promoting outcrossing in plants but similar information on the devices favoring self-fertilization is much more scanty. Self-pollinating races or species occur widely throughout the angiosperms and there seems little doubt that they have been derived from outcrossing relatives (Stebbins, 1957). In self-compatible plants, self-pollination is often limited either by temporal or physical separation of the mature anther and stigma or by some combination of these two types of mechanism. Automatic self-pollination is effected by the juxtaposition of the mature anther and stigma. It can be the result of regular environmental changes as in Viola (Evans, 1956; Bergstrom, 1939; Bergdolt, 1932) where open, cross-pollinated flowers occur early in the season and cleistogamous flowers are produced during the longer, warmer days of summer; a comparable seasonal sequence occurs in Bromus carinatus (Harlan, 1945). Self-fertilization may also result from temporary or local environmental changes as in Narthecium ossifragum, Stipa, and Danthonia (Stebbins, 1957). In the majority of self-pollinating plants, however, the mechanism is apparently less subject to environmental influences. Stebbins (1950, pp. 176-179) has suggested that self-pollination is most likely to arise