In 1884, Fritz Muller wrote a letter to the Journal Nature with the title, “Butterflies as Botanists.” The letter concerned the classification of a plant genus, Brunfelsia which for many years had been assigned to the snapdragon family. Muller noted that botanists had reclassified Brunfelsia, moving it from the snapdragon family to the potato family (Solanaceae).
Fritz Muller was interested in a group of related butterflies in the Tribe Ithomiini. This group all fed on plants in the potato family with the exception of a single genus of caterpillars, the (Thyridia) that fed on Brunfelsia. Moving Brunfelsia from the snapdragon family to the potato family meant that all of the caterpillars of the group fed on plants in the potato family. Muller remarked, “Thus it appears that butterflies had recognised the true affinity of Brunfelsia long before botanists did so” (i.e. it belongs in the potato family).
The specialization of related caterpillars on closely related plant species is a pattern that is frequently repeated. In the 1960s, ecologists proposed a model for how this pattern arises.
The model for the process is as follows.
1. A plant species evolves a defense (often chemical) that protects it from herbivores.
2. The defense allows the plant species to expand its range and successfully compete with other plants in its new range.
3. The plant species in its new range evolves adaptations to local environmental conditions and eventually radiates (over many generations) new species.
The chemical defenses of the ancestral plant species are retained in its descendants. This explains the patter of closely related plants containing similar toxins (chemotaxonomy) that is used by Botanists to aid the classification of plants.
Insects are excluded from feeding on plants in this group until one or more insect species evolves an adaptation that breaks through the plant defense. The caterpillar part of the process proceeds as follows:
4. An herbivore species develops a mechanism to break the plant defense.
5. The herbivore species expands its range to include many of the species in the plant group.
6. The herbivores in the new range evolve adaptations to local environmental conditions and eventually radiate as new species.
In this manner the pattern we observe (closely related species of caterpillar feeding on closely related species of plants) is produced. Caterpillars, it turns out, are good botanists because they are good chemotaxonomists.