GENETIC VARIATION IN ANNUAL PHLOX: SELF‐COMPATIBLE VERSUS SELF‐INCOMPATIBLE SPECIES

The effects of the breeding system on the amount and organization of genetic variation within and between populations, and on the evolutionary kinetics of species has been treated extensively within mathematical and conceptual contexts by Wright (1931, 1932, 1946, 1964), Mather (1943), Baker (1953, 1959), Stebbins (1950, 1957, 1958), Grant (1958, 1971), Allard et al. (1968) and others. The impression one receives is that predominantly selffertilizing species would be less variable than cross-fertilizing species, all other factors being equal. One also would expect single populations of self-fertilizing species to contain less of the total genetic diversity of the species than single populations of cross-fertilizing species but populations of the former to be more highly differentiated inter se than populations of the latter. However, we are cautioned by Allard (1970, 1975) and Jain (1976) not to place intuition before observation because any pattern of variation is not confined to one group of species or the other. Indeed some empirical observations are consistent with the aforementioned expectations, others are not. In Leavenworthia, self-incompatible taxa exhibit less interpopulation differentiation than self-compatible taxa (Lloyd, 1965) but have more allozymic variation (Solbrig, 1972). In Trifolium, outbreeders have more within-population heterogeneity for quantitative characters and less between-population heterogeneity than inbreeders (Katznelson, 1969). On the other hand, allogamous and autogamous Limnanthes have similar levels of allozymic variation within and among populations (Arroyo, 1975); inbreeding Triticum species have greater total as well as withinfamily variability than outbreeding species (Hillel et al., 1973). A lack of correlation between estimates of outcrossing rates and coefficients of genetic variation have been reported in Lupinus (Shroader and Harding, cited in Jain, 1976). The present study was undertaken to shed additional light on genetic correlates of self-incompatibility and self-compatibility in congeneric annual species. Three species of Phlox will be considered, two (Phlox drummondii Hook. and P. roemariana Scheele) are self-incompatible, and one (P. cuspidata Scheele) is self-compatible. These species are largely allopatric, but all are restricted to Texas (Fig. 1). Phlox roemariana grows principally on shallow calcareous soil covering limestone shelves on the Edwards Plateau, and is the most xerophytic annual Phlox. P. drummondii grows principally on sandy soils in the prairies east, south and north of the Plateau, but also is found on the Plateau in deep sandy soils. This species has the broadest range of those being considered, and is composed of six subspecies (Erbe and Turner, 1962). The morphological diversity and the range of habitats occupied by this species is considerably greater than those of the others. P. cuspidata replaces P. drummondii to the east where the soils are less well drained and have higher clay content. Occasionally these species occur in mixed populations, with P. cuspidata inhabiting the wetter sites. Hybridization has been described in a number of such populations (Erbe and Turner, 1962; Levin, 1967, 1975). P. drummondii and P. cuspidata are closely related whereas both are distantly related to P. roemariana