The Elements of Evolution (44-16) Toxic Adaptation

Toxic Adaptation

Numerous insects have independently evolved the ability to feed on plants that produce toxic secondary compounds and sequester these compounds for use in their defense. ~ Chinese evolutionary biologist Ying Zhen et al

Plants produce vast arrays of chemical protection for themselves that are toxic to would-be predators. In an evolutionary arms race, animals may overcome this barrier to consumption, at least to some tolerance. Some animals even adapt to employ plant toxins to their own advantage.

 Alkaloids

Some 3% of flowering plants produce pyrrolizidine alkaloids, which are a nasty trick. To an herbivore, the alkaloids go down well enough, but are converted into toxins as metabolized. Both the African grasshopper and European cinnabar moth caterpillar, by evolving the same specific enzyme, adapted to safely store the toxin themselves. This adaptation conferred protection from predation.

Separated by 300 million years of evolution, 14 species of insect – from aphids to beetles to butterflies – convergently evolved the same molecular mechanism to handle the cardenolides that are the natural defense for dogbane, milkweed, and other plants. This steroid cardiotoxin usually cripples the ability of cells to regulate sodium-to-potassium ratio. The different insects independently tweaked the gene that produces the protein which handles this chore in the same way, to nullify the effect that cardenolides normally have; identical convergent evolution.

 Glycosides

Cardiac glycosides are toxic molecules that perturb the cell membranes of animals, causing cardiotoxicity. Several plants and some toads produce these glycosides as a defense against predation.

Widely divergent insects, amphibians, reptiles, and mammals have overcome the toxicity to dine on these glycoside producers. All of them evolved resistance via 2 identical amino acid changes in a specific portion of a single gene. (There were possible alternate molecular routes to this common trait.) The gene produces a protein that is essential to the binding site that makes the glycosides toxic. The adaptation renders the glycosides harmless.

The predictability of this convergent evolution is underlined by its reversal in carnivorous lizards that migrate to toad-free areas. These lizards energetically economize by not producing unnecessary proteins.

Similar selection pressures have resulted in convergent evolution of the same molecular solution across the breadth of the animal kingdom, demonstrating how a scarcity of possible solutions to a selective challenge can lead to highly predictable evolutionary responses. ~ Australian biologist Beata Ujvari