Escalation of Plant Defense: Do Latex and Resin Canals Spur Plant Diversification?

TLDR

Ehrlich and Raven's postulate links rapid diversification to innovation in plant defense, and secretory canals—repeatedly evolved protective features—are hypothesized to enable plant radiation by reducing herbivory and disease. The study aims to test the postulate by defining a novel defense class based on chemical/anatomical criteria and examining whether lineages that evolved this defense are consistently more diverse than their sister groups. The authors quantified evidence by comparing diversities of lineages with independently evolved canal systems to their sister groups across all available taxonomic lineages. A sign test revealed that canal-bearing lineages consistently have higher diversities than sister groups (P = .0021), and alternative explanations were examined and provisionally rejected.

Abstract

Ehrlich and Raven's postulate that rapid diversification follows innovation in plant defense has often been invoked a posteriori for plant lineages of unusual diversity and chemical distinctiveness. The postulate can be more rigorously tested by defining a novel class of defense using chemical and/or anatomical criteria, independent of taxonomic lineage. If multiple plant lineages have evolved the new defense type, then according to the postulate they should be consistently more diverse than their sister groups (of equal age, by definition) when the latter retain the primitive defensive repertoire. Secretory canals are an independently defined, repeatedly evolved feature that functions to protect plants from herbivores and pathogens. The canals might therefore be expected to allow plant radiation in an adaptive zone of reduced herbivory and disease. We have quantified the evidence for this hypothesis by comparing the diversities of lineages that have independently evolved canal systems with their sister groups for as many plant lineages as current taxonomic evidence allows. A sign test showed that canal-bearing lineages have consistently higher diversities than their sister groups (P = .0021). Explanations for this result, other than selective advantage conferred by secretory canals, are examined and provisionally rejected.