American scientists Paul Ehrlich and Peter Raven observed that closely related species of plants were fed upon by closely related species of butterflies. In 1964, they cited this observation as evidence that plants and their herbivores are caught up in an evolutionary arms race: Plants evolve new forms of defense, and herbivores evolve mechanisms to overcome these defenses. In particular, Ehrlich and Raven hypothesized that the evolution of novel defenses has been an important force in the diversification of both plants and herbivorous insects.

A novel form of defense may allow a plant population to expand into new areas. If the population in a new area remains separated from the original population, it may evolve into a new species. Similarly, a novel means of overcoming plant defenses may allow an insect or pathogen population access to new plant resources. If these insects or pathogens became separated from their original population, then they might also evolve into new species. Ehrlich and Raven’s “escape and radiate” hypothesis thus predicts a burst of diversification following the evolution of novel defenses.

This hypothesis makes intuitive sense, but it could only be tested with the advent of phylogenetic trees based on DNA sequence comparisons (Chapter 23). Biologists knew that latex or resin canals evolved independently in more than 40 different groups of plants. Reasoning that these features represent a novel defense, they asked whether the groups that evolved latex or resin canals were more diverse than closely related groups that lacked these forms of defense. In 14 of the 16 groups examined, the groups with the protective canals were significantly more diverse than their closest relatives that lacked latex or resin canals. This pattern supports the hypothesis that the evolution of latex and resin canals provided plants with the freedom to expand into new habitats.

Escalation of defenses, however, is not the only possible evolutionary outcome. Phylogenetic research shows that, as milkweeds diversified, many of the newly evolved species produced fewer chemical defenses such as latex canals and cardenolides. Instead, these species are found in resource-rich habitats and replace tissues by growing quickly following damage. The presence of specialized herbivores such as monarch caterpillars that can disarm milkweeds’ defenses may help explain why these more recent plant species reduce investments in defense in favor of more rapid growth.