The Deadly Tomato

 

As I walked through the weeds between the early-successional trees in the field about which I earlier wrote, I noticed a Solanum dimidiatum weed (horsenettle). It was in the same genus as the potato, a close relative of the tomato. And it had what looked like little green tomatoes on it. This is the time of year when tomatoes are really good. But if you think these tomatoes might be delicious, think again. The entire plant is filled with toxins that are dangerous, even deadly, to most animals.

 

[Photo from Carolina Biological Supply Company] 

The horsenettle plant expends a lot of its energy creating these toxic compounds [what are they]. Expensive they may be, but they protect the plant from having its leaves eaten by most animals. But there is at least one group of animals that can eat the leaves as if they are not toxic at all. The sphinx moth (Sphingidae) caterpillars (hornworms) of the genus Manduca can eat them with little effect on their growth. I know this because I have measured the growth of hornworms when they ate ground-up horsenettle leaves and when they did not; their growth was the same. The toxins do not protect the horsenettles from hornworms, but in many cases, there are no hornworms about. The plant is taking a chance. It invests in imperfect, but usually good, protection.

You may wonder how to compare hornworms that eat horsenettle tissue and those that do not. To what do you compare the ones that do? This is an interesting story. You can purchase little hornworms, special vials to grow them in, and hornworm chow. They do not naturally eat this chow, but if you start them on it when very young, they will eat it and grow grossly big. It is sort of hornworm junk food. The control worms are the ones that eat only the chow; the experimental worms have horsenettle tissue mixed in with the chow.

Herbivores, and the defenses of plants against them, are a constantly moving evolutionary target. They hardly ever get it right, but they come close enough that there are lots of plants, and lots of herbivores, in the world. Any plant that invested too much in defense would grow slowly and die; but if it invested too little, it would disappear down the gullets of extinction. There is a balance of nature, but it is imperfect and shifting.

Oklahoma Evolution Workshop, Part Four

The evolution workshop for Oklahoma teachers continued after lunch. I had the first session. I began by advertising my own website, where you can find essays and photographs, as well as copies of scholarly articles  I have written. It has links to my YouTube channel and to this evolution blog.

Then I began with the story of the Ghosts of Evolution. (I have a recent YouTube video where Darwin discusses this subject.) I held up a big, sticky, green fruit of Maclura pomifera, known as bois d'arc, Osage orange, or horse-apple. The native range of this tree includes Oklahoma. I modeled the kind of questions that they can ask their students. Why do some plants produce juicy sweet fruits? Animals eat them, swallowing the seeds and transporting them to a new location. This is useful to the plant only if the animal swallows undamaged seeds, as we do with strawberry or kiwi seeds, or as raccoons do with persimmon seeds. With their finicky fingers, some raccoons could pick out the persimmon seeds, but this would waste valuable time, during which other raccoons would eat up all the fruits. The concept that students might grasp is coevolution. In this case, juicy fruits evolved in response to animals that eat them and disperse their seeds.

But what eats big, sticky bois d'arc fruits? In Oklahoma, someone will always say, horses. In other parts of the country, nobody may have any idea. But modern horses are not native to North America. There were native horses in North America at the end of the last ice age, but they have become extinct. Perhaps at the end of the last ice age, there were large animals (horses, mammoths, mastodons, gomphotheres, etc.) that ate bois d'arc fruits, but today, there are no native animals that eat them. Thousands of years ago, there was a coevolutionary waltz between bois d'arcs and gomphotheres; today, the bois d'arc is doing a waltz with no partner.

We went outside, into the cold wind. The temperature has dropped 50 degrees in two days. If you want to get kids outside and still teach them about evolution, just take them to an oak tree and ask them what they know about oak trees. The wood is strong. The trees live a long time. Acorns are large, as are the seedlings that emerge from them. Now, think about what kind of environment would benefit trees that have these characteristics. In an old, stable forest, large seeds and large seedlings would be better able to compete with the dense populations of plants that are already there. They invest for the long term in strong wood. Then take the kids to a cottonwood tree. Cottonwoods produce cheap wood and do not live very long. Their seeds are numerous and small. These characteristics make sense for trees that live near rivers, where they have a lot of water but also face a great risk of being destroyed in a flood. It would make no sense for a cottonwood to invest in wood that will last for centuries, since a cottonwood tree would probably get killed in a century or less. This may help students to understand that evolution does not always produce the same "superior" set of adaptations; the "superior" traits for a tree in an old, stable forest are different from those in a young, frequently-disturbed, floodplain forest.

Then Dr. Cecil Lewis had a second session, this time devoted to his specialty: the evolutionary and medical genetics of humans. He explained how population genetics works. A large, old population that has been large for a long time may have a great deal of genetic variation, including a lot of rare variants. Large populations can lose genetic variation if they become small--that is, they go through a population bottleneck. Small populations can lose genetic variation just by chance. If the population then grows rapidly, it retains the genetic makeup that it had while it was small. That is, a newly-large population will have no more genetic variability than it did when it was small. If you find a population that is large but has low genetic variability, this probably means that there was a recent genetic bottleneck. This appears to have happened in the human species. The entire human species, all seven billion of us, has the same amount of genetic variation that one would expect from a mammal population with only twenty thousand individuals. This suggests that (perhaps 70,000 years ago) our species had a brush with extinction--there may have been, at that time, only about twenty thousand of us.

You can make a closer examination of the genetics of human groups. The greatest genetic variation is in Africa. The genetic variation in the Middle East is a subset of African genes. The genetic variation in Europe is a subset of Middle Eastern genes; Asians are a separate subset. North American native genetic variation is a subset of Asian genes; South American native genetic variation is a subset of North American native genes. Each time a migration occurs, only a subset of people leave home, therefore some genes get left behind. Human genes, therefore, are an invisible record of the pattern of human migrations during the last 200,000 years.

Some human genetic patterns have resulted from natural selection. Lewis gave an example: the ALDH2-2 gene, the gene that codes for alcohol dehydrogenase. One variant of this gene is much more common in Asians than in anyone else. The Asian variant is the reason that Asians generally have a lower tolerance of alcohol than other people. However, since refiend alcohol is a recent invention, the Asian variant of the gene was probably selected by exposure to pathogens.

Let the Earth Bring Forth Plants

The first chapter of Genesis in the Old Testament is a hymn to an orderly and bountiful cosmos. Surprisingly enough, it begins by describing a universe that existed before God began to create. This universe had two problems: it was chaotic and empty (the old King James English says “formless and void”). During the six days of creation, according to this hymn, God solved these two problems. On the first three days, he brought order into the heavenly, the fluid, and the earthly realms: making light separate from darkness, air separate from water, and land separate from sea. On the last three days, he filled the heavenly, fluid, and earthly realms: filling the heavens with stars, the water and air with fish and birds, and the earth with “creeping things.” It is obvious to everyone except creationists that this chapter was never, even when it was first written, intended as an historical description of the history of the cosmos. It described the work of creation as categories, not as a timeline. Astronomy, geology, and biology now provide explanations for the processes of creation described poetically in Genesis.

As beautiful as it is, however, the first chapter of Genesis reflects the human-centered viewpoint of the time in which it was written (about 500 BCE), a viewpoint to which most modern people still cling, even if they are not particularly religious. This viewpoint puts humans at the pinnacle of creation, something that most people (including those who think they understand evolution) still do. The writer of Genesis did not praise humans as much as you might think, however: humans were created on the same “day” as all of the creeping things. Humans did not even get their own special day of creation. But Genesis clearly states that humans rule all of the creation, and uses terms that imply that humans have the right to conquer and rape the creation.

One example of this domination is the creation of plants. Plants appear on Day 3, along with the dry land. That is, in this hymn, plants are not considered to be living organisms; they are just a part of the landscape. In a sense this is true: any landscape without plants is quickly ravaged by erosion, as I have explained in my book Green Planet. But the God of Genesis does not give plants any role of their own other than to be eaten by animals. Behold, he says on Day 6, I have given you every green plant for food.

But plants are organisms. They’ve got to make a living like everybody else. Plants have populations that evolve. It is not in the interests of the plants to just sit there and allow themselves to be eaten by animals. The plants that natural selection favors are those that can avoid getting eaten by animals.

As a result, the world is not just a big salad bowl. Nearly every wild plant has several or many characteristics that make it unpalatable to animals. In some cases, plant defenses are clearly visible: spines on cactuses, stinging hairs on nettles. In other cases, the defenses are invisible but even more effective. Most plants have leaves that are at least mildly toxic. Plants produce thousands of different kinds of chemicals that are toxic to animals. Some of them accumulate poisonous mineral ions out of the soil and put them in little sacs (vacuoles) in the leaf cells, where the poisons will not harm the leaves but will harm any animal that chews the leaf, breaking open the sacs of poison.

Humans have bred crop and garden plants that are palatable, and this has created for most of us the illusion that plants are just waiting to be eaten. They are like Al Capp’s cartoon shmoos, which fall all over themselves in an attempt to get eaten, with stupid grins on their faces. Be warned: wild plants do not have brains, they are in fact stupid, but evolution has produced in them many defenses that appear to those who study them to be very clever. Wild relatives of crop plants, such as beans, tomatoes, and potatoes, are poisonous. Some tribal peoples subsist on wild cassava. But they have to boil and squeeze the cassava pulp to remove poisons first.

Even the parts of plants that are not poisonous are not simple gifts to feed animals. Many species of plants produce soft, colorful, sweet, fragrant fruits. These fruits make themselves very visible to animals, and reward the animals for eating them. Such fruits, abundant and hanging low, are in fact part of our image of paradise. (The word paradise itself comes from the Persian word pairidaeza, a walled garden.) But the plants have something to gain from this arrangement. The fruits have seeds in them. Animals usually eat the entire fruit, swallowing the seeds. It is just too much work for the animal to pick out the seeds, especially since the pulp clings closely to them. Plants cannot move around and carry their seeds to new locations, nor can the seeds move under their own power. Many plants therefore get animals to carry their seeds to new places by getting the animals to eat them. They pay the animals by feeding them the sweet and nourishing pulp of the fruit. The animals deposit the seeds, along with a pile of fertilizer, far away from the parent tree. Both the plants and the animals benefit. In other cases, plants take advantage of the animals. Seeds with spiky burs entangle the fur of mammals, which carry the seeds to a new location while they are trying to scrape the burs from their fur.

Plants and herbivores are involved in what has been called an arms race. Plants evolve defenses, and herbivores evolve ways of circumventing the defenses. Plants then evolve new defenses, and herbivores evolve ways of circumventing these as well. For both the plants and the animals, this interaction is expensive. The plants have to use their energy and molecules to make poisons rather than using them to grow new leaves and roots. Animals have to use energy and molecules to digest the plants despite their defenses. In human societies, spending more money on defense means spending less on growth (such as infrastructure and education). Plants and animals find themselves in the same predicament as humans do.

This is coevolution: natural selection favors plants that are prickly or poisonous, but not so much that they cannot grow, and animals that can tolerate or avoid the poisons, but not so much that they divert too much energy away from their own metabolism, movement, and growth.

One you have learned about the coevolutionary relationship between herbivores and plants, you can never again look at a serene natural landscape the same way as before. The Lord is my shepherd, said the psalmist, leading sheep into green pastures. The psalmist also said that God prepared a table for him in the presence of his enemies. But the psalmist could not have guessed that, to a certain extent, the green pastures themselves were one of the enemies. There never was a natural Garden of Eden in which all the plants were uncomplicatedly nutritious.

This entry was part of my book Life of Earth: Portrait of a Beautiful, Middle-Aged, Stressed-Out World, published earlier this year by Prometheus Books.