Population Differentiation and the Effects of Herbivory and Sand Compaction on the Subterranean Growth of a Desert Lily

Differences in level of herbivory can select for local adaptation and genetic differentiation of plant populations in different environments. Mean bulb depth of the desert lily Pancratium sickenbergeri, differs considerably among populations differing in the level of herbivory by the dorcas gazelle. The gazelle digs in the sand to remove most of the bulb of the lily. Deeper bulbs have less material removed by herbivory than shallow bulbs and have higher fitness. A possible confounding factor is the degree of sand compaction, which may retard the downward growth of the bulb. We conducted a common garden experiment with 2 sand types with seeds from source populations with different levels of herbivory. There was a large genetic difference among populations. Two of 3 analyses indicated that there was an interaction between population and sand type, indicating that there is a heritable component of plasticity. Studies of the evolution of plant resistance to herbivores re-quire an understanding of the origin and dynamics of genetic variation for resistance traits in natural populations (Simms and Rausher 1992). Life-history traits, that is, growth and resistant patterns, are quantitative traits under continual selec-tion, and the genetic variation of such traits should be lo