Life History Evolution in Seven Milkweeds of the Genus Asclepias

Abstract

The demographic characteristics of plants and animals have attracted the widespread attention of evolutionary ecologists only in the last decade, in spite of the early theoretical work of Cole (1954), the empirical work of Salisbury (1942) and the obvious close relationship between features of the life history and Darwinian fitness. Recent discussions of life history evolution, e.g. those of Gadgil & Solbrig (1972), Pianka (1972) and Southwood et al. (1974), have focused on competition as an agent of natural selection and have used the terminology of r-selection and K-selection introduced by MacArthur & Wilson (1967). Wilbur, Tinkle & Collins (1974) argue that a discussion of the selective effects of competition alone cannot explain the diversity of life history patterns in several wellstudied species of plants and animals and they advocate the consideration of predation and environmental uncertainty in addition to competition. The purpose of this paper is to illustrate the arguments of Wilbur et al. by an empirical study of the life histories of seven species of milkweeds (Asclepias spp.). First the species are compared with respect to growth form, seed number and seed size. These life history adaptations are then correlated with independently measured indices of the effects of competition, predation and environmental uncertainty on populations of each species in south-east Michigan. This study is based on the premise that reproductive adaptations represent evolutionary adjustments of life history characteristics such that the lifetime contribution of a plant to the next generation is maximal. Milkweeds provide excellent material for the study of evolutionary ecology. Asclepias is a widespread genus with 108 species in nine subgenera in North America and the Antilles (Woodson 1954). All species which have been studied are isoploid (Moore 1946) and have only a low level of self-fertility (R. Wyatt, personal communication). Pollination is effected by large insects, primarily hymenopterans and lepidopterans (F. C. Evans, personal communication), which transfer pollinia (Macior 1965; Willson & Rathcke 1974). Most species have narrow microhabitat tolerances, are perennial and do not propagate vegetatively. Seeds are large compared to other forbs of meadows and wood margins (Stevens 1932; Salisbury 1942); each seed has a tuft of comose hairs which assist in dispersal by wind and, in the case of A. incarnata L., by water. All of the species produce alkaloid compounds (Punyarajun 1965), which deter herbivorous insects, are distasteful to livestock (Whiting 1943), and include cardiac glycoscides which are poisonous to most vertebrates (Parsons 1965; Reichstein et al. 1968; Duffey 1970). Milkweeds are occasionally eaten by deer or by a small number of insect species (Table 1), most of which are aposematic, toxic and have a history of co-