THE EVOLUTION OF SELF‐POLLINATION IN GRANITE OUTCROP SPECIES OF ARENARIA (CARYOPHYLLACEAE). I. MORPHOLOGICAL CORRELATES

Darwin (1876, 1877) demonstrated that outcrossed progeny of plants are usually more vigorous than those produced by self-fertilization. This observation led him to interpret many features of flowering plants as adaptations for outcrossing. Nevertheless, it is now well-known that many plant species have evolved mechanisms that promote selfing. Stebbins (1970) observed that "the evolutionary pathway from obligate outcrossing based upon self-incompatibility to predominant self-fertilization has probably been followed by more different lines of evolution in flowering plants than has any other. " There is indeed a long list of such taxa (see reviews by East, 1940; Fryxell, 1957; Stebbins, 1970; Wyatt, 1983). In other taxa, self-compatible but predominantly outcrossing plants may change such that self-pollination is assured. This usually involves a breakdown in dichogamy or other devices that promote outcrossing. In Clarkia xantiana, for example, stigma maturation regulates outcrossing rates and "appears to have a simple genetic basis" (Moore and Lewis, 1965). Determination of outcrossing rates in Lycopersicon also is largely through morphological characters with a simple genetic basis (Rick, 1950), although polygenic control of certain features is postulated for some species (Rick et al., 1978). In some taxa, there appears to be a large environmental component to breeding system variation (Jain, 1976). In some cases only minor morphological changes are associated with a shift from predominant outcrossing to predominant selfing (Stebbins, 1970). The selfing Funatnrim micrnshtmonn is nearly indistinguishable from its outcrossing relative, E. sinclairii (Baker, 1967). Autogamous populations of Lycopersicon esculentum and L. pimpinellifolium differ from their outcrossing ancestors by very slight rearrangements in structure and position of anthers and stigmas (Rick, 1950; Rick et al., 1978). In other taxa, major morphological reorganization may occur in association with a shift in the breeding system. Ornduff (1969) has listed a number of features characterizing a general trend from predominant outcrossing to predominant selfing. Obvious adaptations of outcrossing plants for pollination by animal vectors frequently are reduced or lost entirely in their autogamous derivatives. A particularly dramatic example is Rollins's (1963) and Lloyd's (1965) studies of Leavenworthia. Selfing has arisen many times in independent evolutionary lines to produce a number of new autogamous species. The morphological changes accompanying the evolution of autogamy in each case are strikingly similar. The state of the art with respect to the evolution of autogamy was succinctly summarized by Jain (1976), who stated that "we have more speculation than rigorous thought, and more theory than real data" regarding the questions of when, how, and why it evolved. A general problem with most studies of the evolution of autogamy is that comparisons are made between "closely related" congeneric species that differ with respect to their rates of outcrossing. This approach implicitly assumes that all morphological, genetic, and ecological differences can be attributed directly to differences in outcrossing rates.