EVOLUTIONARY ECOLOGY OF DATURA-STRAMONIUM L IN CENTRAL MEXICO - NATURAL-SELECTION FOR RESISTANCE TO HERBIVOROUS INSECTS

It has been assumed that herbivores constitute a selective agent for t he evolution of plant resistance. However, few studies have tested thi s hypothesis. In this study, we look at the annual weed Datura stramon ium for evidence of current natural selection for resistance to herbiv orous insects. Paternal half-sib families obtained through controlled crosses were exposed to herbivores under natural conditions. The plant s were damaged by two folivorous insects: the tobacco flea beetle Epit rix parvula and the grasshopper Sphenarium purpurascens. Selection was estimated using a multiple-regression analysis of plant size and of d amage by the two herbivores on plant fitness measured as fruit product ion for both individual phenotypes and family breeding values (genetic analysis). Directional phenotypic selection was detected for both lar ger plant size and lower resistance to the flea beetles, whereas stabi lizing phenotypic selection was revealed for resistance to S. purpuras cens. However, performing the same analyses on the breeding values of the characters revealed directional and stabilizing selection only for plant size. Thus, no agreement existed between the results of the two types of analyses, nor was there any detectable potential for genetic change in the studied population because of selection on herbivore re sistance. The narrow-sense heritability of every trait studied was sma ll (all < 0.1) and not different from zero. The potential for evolutio nary response to natural selection for higher resistance to herbivores in the studied population of D. stramonium is probably limited by lac k of genetic variation. Natural selection acts on phenotypes, and the detection of phenotypic selection on resistance to herbivores confirms their ecological importance in determining plant fitness. However, ev olutionary inferences based solely on phenotypic selection analyses mu st be interpreted with caution.