Ecology and Evolution
Natural poisons, like morphological structures and behavior, allow organisms to manipulate their environments and enable them to better compete, defend, and exploit. The best competitors, defenders, and exploiters among a group of individual organisms are usually the best reproducers, and differential reproduction is the operational definition of natural selection, the principal force driving organic evolution. If an individual organism has a mutation that causes the production of a chemical substance that allows it to survive when other organisms die, then that organism will on average reproduce more than other individuals who do not possess the mutant gene. As a result, the mutant gene will be more frequently represented in the next generation. After many generations, this process may result in a population that contains only the mutant gene. Assuming that the mutant gene mutates again in such a way that it produces a more powerful chemical that improves its owner's ability to compete, defend, and exploit, this new mutant will be favored by natural selection and again increase in number in the population. So, by favoring small increments of improvement, natural selection results in the evolution of finely tuned, sometimes very potent poisons with elaborate delivery systems.
Many natural poisons are believed to have begun their evolution as metabolites, mildly toxic intermediate or waste products of essential metabolic processes. This has often been the point of departure in the development of most plant poisons. Enzymes, proteins essential to the control of the chemistry of life, were the likely starting compounds for the development of many animal venoms. For example, the digestive enzymes found in animal saliva have been modified to produce venoms in spiders, true bugs, flies, snakes, and shrews. Accessory glands associated with the female reproductive system of wasps, ants, and bees have evolved the secondary function of venom production, and the ovipositor has been modified through evolutionary time to produce the stinger. In the primitive wasps the stinging apparatus functions primarily to secure arthropod prey. Snake fangs have evolved from ordinary teeth that probably went through a grooved stage, as is found in the Gila monster, one of the poisonous lizards.
Most animals learn from unpleasant experiences to avoid negative circumstances. Therefore it is not surprising that many animals with particularly punishing chemical defenses have developed bright colors and distinctive behavioral displays that become recognizable to potential victims. Some species mimic others to avoid predation. For example, animals that prey on insects learn through experience that various brightly colored bees and wasps have painful stings. In response to this, a number of species of otherwise harmless flies have evolved to exhibit black and yellow markings strikingly similar to those of the stinging insects.
Natural Poisons and Humans
Although toxins from plants and animals kill few people in the United States each year, they kill many thousands worldwide. In North America, poisoning by rattlesnakes, Crotalus, coral snakes, Micrurus, widow spiders, Latrodectus, brown spiders, Loxosceles, and scorpions, Centruroides, may be life threatening if untreated. Poisoning from plants (excluding plant-derived narcotic drugs) amounts to less than 2,000 cases annually, and the number of human deaths attributable to plant poisons is insignificant. The most common plant poisonings of humans are by common ornamental and houseplantsÑsuch as holly, Ilex, pyracantha, Pyracantha, philodendron, Philodendron, and dieffenbachia, DieffenbachiaÑconsumed by children. Any plant or animal toxin can cause severe allergic reactions in persons who have a tendency to be allergic to various substances. Anaphylaxis (a severe reaction to specific substances such as wasp venom or penicillin) is the life-threatening manifestation of this conditon. It is more frequently caused by bee and wasp stings simply because of the frequency of human exposure. Anaphylactic shock kills far more people than the direct effects of all natural toxins combined. Most U.S. states have poison-control information networks that are operated 24 hours a day. These centers are staffed by well-informed personnel who can provide lifesaving information on all kinds of poisoning.
The benefits of natural toxins enormously outweigh their negative impact on humans. Natural toxins have evolved varied actions on virtually all species and all physiological processes, including nerve action, water retention, muscle contraction, and reproduction. These compounds, in regulated doses, have great utility as drugs for managing and treating human and domestic animal disease and as research tools to enhance the understanding of life processes. For example, digitalis, a substance obtained from the common foxglove, Digitalis purpurea, is used as a drug to regulate heartbeat rate; quinine, an alkaloid from the bark of the cinchona, Cinchona officinalis, was the first drug to be used successfully to treat malaria. In addition, plants supply commercial insecticides, such as rotenone and pyrethrum. The genes for thousands of compounds may be inserted into the major food and fiber crop plants using genetic engineering to protect them against insects and disease and contribute to global improvements in the standard of living.