ISOZYME AND MORPHOLOGICAL DIVERGENCE WITHIN EUPHORBIA SECTION TITHYMALOPSIS (EUPHORBIACEAE)

Euphorbia section Tithymalopsis is a group of seven species endemic to the southeastern United States. To assess patterns of genetic and mor phological differentiation and to test hypotheses of evolutionary rela tionships among species, we examined 14 isozyme loci from 45 populatio ns and 29 morphological characters from 29 populations representing se ven species of section Tithymalopsis and two related species. Although each species can be delineated using morphological characters, there is little quantitative or qualitative allelic differentiation among sp ecies. Morphological and isozyme data suggest that species in section Tithymalopsis have diverged recently. Isozyme-based trees support hy h ypotheses of monophyly for sect. Tithymalopsis, on the basis of morpho logical data. The isozyme data do not support a close relationship amo ng species constituting the E. corollata complex but suggest that E. d iscoidalis, E. polyphylla, and E. pubentissima form a closely related group.