For the past 50 years, theories have been developing to explain the diversity, abundance, and distribution of plant defensive strategies. These range from physiological explanations and optimality models to coevolutionary predictions. Save a few exceptions, what has been strikingly lacking is a comparative phylogenetic approach to test these predictions. With the advent and availability of molecular phylogenies, the time is ripe to revisit some of the classic hypothesis to explain plant defense strategies. In this talk I review the hypotheses, methods to test them, and provide novel results from work with the new world milkweeds (Asclepias spp.).
Work with 55 species of Asclepias show many exciting patterns addressing classic defense theory. The key finding was a pattern of phyletic decline in the three most potent resistance traits (cardenolides, latex, and trichomes), and an escalation of regrowth ability (or tolerance). Several traits showed a pattern of early trait divergence, which is consistent with a role in adaptive radiation. Resistance traits did not appear to trade-off, and we did not find a signature of growth rate influencing defense allocation. Our neontological approach complements more common paleontological approaches to discover trends in the evolution of life, and points to the importance of natural enemies in the macroevolution of species. The finding of macroevolutionary escalating regowth ability and declining resistance provides a new window into the ongoing coevolutionary dynamics between plants and herbivores, and suggests a revision of classic plant defense theory. Where plants are primarily consumed by specialist herbivores, regrowth may be favored over resistance traits during the diversification process.