BUTTERFLYWEED RE‐REVISITED: SPATIAL AND TEMPORAL PATTERNS OF LEAF SHAPE VARIATION IN ASCLEPIAS TUBEROSA

Long-term monitoring of genetic changes in natural populations have been surprisingly rare, especially in view of the importance of documentation of such changes in assessing the role of natural selection in evolution. Among the more noteworthy of such studies have been those on morph frequency in the moth Panaxia dominula (Fisher and Ford, 1947), on changes in spotting pattern in the butterfly Maniola jurtina (Dowdeswell et al., 1960), and on inversion frequencies in Drosophila pseudoobscura in the southwestern United States (Dobzhansky and Levene, 1948). In plants there have been only two long-term studies. One is that of Stebbins and Daly (1961), who monitored changes in the characteristics of a hybrid population of H elianthus over an eight-year period. The other, and the subject of this paper, is the work on Asclepias tuberosa by Woodson (1947, 1953, 1962), who discovered evidence of a change over time in a clinalleaf shape pattern in populations from eastern North America. This change was originally documented on the basis of comparisons with herbarium samples (Woodson, 1947) but was continued by sampling of field populations in 1946 and 1960 (Woodson, 1962). The study has become a classic example cited in textbooks and reviews of evolutionary biology (e.g., Ehrlich and Holm, 1963; Heiser, 1973). Asclepias tuberosa L. (Asclepiadaceae) or "butterflyweed" (other common names are "coralweed, " "chiggerweed," and "pleurisy root") is a long-lived herbaceous perennial of roadsides, old fields, and other disturbed habitats. It is excellent ma-

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