There are at least two plausible introduction scenarios. One scenario posits a northward migration of the common morning glory along with maize culture over 1,000 years ago. A second scenario posits an introduction of the common morning glory into the southeastern U.S. by European settlers of this region as a horticultural plant. The genetic evidence points to a strong founder effect associated with the introduction of the common morning glory into the southeastern U.S. Maize does not appear to have experienced so extreme a founder effect, and it is not obvious why the two species would experience different population restrictions during a common migration process, so we regard the first scenario as less likely. Under either scenario, we may regard the southeastern U.S. populations as a crude series of experiments in the microevolution of flower color determining genes.
Epperson and Clegg (1986) conducted geographic surveys of flower color variation within the southeastern U.S. at three different spatial scales. The smallest spatial scale (the intrapopulation scale) was analyzed via spatial autocorrelation statistics (Sokal and Oden, 1978); second, the sub regional scale (defined as local populations that range from 0.8 to 32 km apart) was analyzed via gene frequency distances between populations (Nei, 1972); and third, the regional scale ranging from 80 to 560 km between populations, and including much of the southeastern U.S., was also analyzed by using genetic distance statistics. A strong result of these analyses is that there is no correlation between genetic distance and geographic distance at subregional or regional scales for either the W/w or the P/p loci. Populations separated by a few kilometers are as differentiated from one another with respect to the P/p and W/w flower color determining loci as those separated by hundreds of kilometers. Such a pattern is consistent with the hypothesis that the flower color variants were randomly introduced into multiple locations in the southeastern U.S. Analyses of spatial distributions within local populations led to two major conclusions: first, spatial autocorrelation statistics were heterogeneous between the W/w and P/p loci within the same local populations; and, second, analyses of spatial correlograms for the P/p locus revealed genetic neighborhood sizes consistent with an isolation-by-distance model of population structure (Epperson and Clegg, 1986). We begin by elaborating on the second conclusion; we then turn to the importance of the first conclusion in establishing that the W/w locus is subject to selection within local populations.
For clarity, it is useful to review a few elementary definitions in spatial statistics. A spatial autocorrelation measures the correlation in state of a system at two points that are separated by x distance units. For example, in the common morning glory case, the state may be the flower color determined by the P/p locus x distance units apart. The autocorrelation is