Pyrrolizidine alkaloids (PAs) are produced by angiosperms as a defense against insect herbivores. More than 660 PAs have been identified in over 6,000 plants. PA production capability independently evolved many times.
Milkweeds are one plant genus which adopted PAs as a toxin of choice to ward off predation. (Milkweeds also produce cardenolides to thwart insects. Cardenolides are steroids which stop the hearts of the little pests that dare prey upon milkweeds.) There are over 140 distinct milkweeds.
The problem that certain milkweeds encountered with PAs is that some predators adapted to the toxin. Danaine (milkweed) butterflies lay their eggs on milkweeds. When the eggs hatch, the larvae feed on the milkweeds, concentrating PA in their bodies to make themselves inedible to birds that would otherwise pick them off.
Pyrrolizidine alkaloids are an ineffective defense against milkweed butterflies. Furthermore, they are actually beneficial to them since they take in these chemicals for their own defense against their predators. ~ American botanist TaTYAna Livshultz
Recognizing that there is no good reason to waste energy producing poisons that don’t work, at least 4 different milkweeds gave up making PAs.