Macroevolutionary and geographical intensification of chemical defense in plants driven by insect herbivore selection pressure

• Plants produce defensive chemicals against insect herbivores and other enemies.
• Herbivores can often adapt to those defenses in time.
• Over the course of evolution, lineages of plants have increased the number and variety of defensive chemicals.
• Escalation in chemical defense can hinder herbivore counteradaptation.

Plants produce an extensive array of secondary chemical compounds that often function as defenses against insect herbivores. In theory, because of steadily herbivore adaptation, lineages of plants have reacted by escalating their chemical arsenals over time. Following this assumption, over the last three decades researchers have searched for potential signs of chemical intensification in plants. Although modern methodologies now allow the inference of macroevolutionary chemical trends with substantial confidence there are still only a handful of studies on this subject. These examples suggest that intensification of plant chemical defenses is the result of lineages progressively incrementing their compounds as well as recruiting an increasing number of biosynthetic pathways to produce more complex chemical mixtures.

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