THE SIGNIFICANCE OF ALLOZYME VARIATION AND INTROGRESSION IN THE LIRIODENDRON-TULIPIFERA COMPLEX (MAGNOLIACEAE)

Fifty populations of Liriodendron tulipifera were sampled and scored f or electrophoretic variation at 23 loci. The level of genetic polymorp hism and population differentiation is greater in L. tulipifera than i s usual for an outbreeding species. Since this species exhibits a dine of morphological and ecological variation from north to south, the 50 populations were divided into seven geographically defined regional g roupings: three from the Appalachian uplands, three from the southeast ern coastal plains, and one from the Florida peninsula. Nei's genetic identity, I, was calculated for all within- and among-population and r egion comparisons. The populations from the upland regions clustered c losely together while the coastal plain populations were similar but m easurably separated from the upland ones. The populations from the Flo rida peninsula were markedly divergent. A principal components analysi s of the same data set revealed a nearly identical pattern of populati on clustering. Two hypotheses were explored to explain the pattern obs erved: 1) post-Pleistocene differentiation and migration from a single refugium and; 2) sympatry of two previously isolated taxa during glac ial maxima, followed by introgression and migration. The weight of evi dence best supports the introgression hypothesis which is explained in terms of plant migration events during Pleistocene time. The level an d distribution of electrophoretic variation in L. tulipifera is compar ed to that of other woody taxa in which historical events of the Pleis tocene may have contributed to modern levels and patterns of variation .