Red hot peppers! Can evolution “design” anything, especially a chemical bomb a plant uses to be sure its seeds get spread properly?
There’s a desert plant in the Middle East that has an ingenious way of dispersing its seeds. Many plants rely on animals for help, but there’s a problem: the animal helper needs to spread the seed without destroying it. For instance, many plants surround their seeds by fleshy, delicious fruits, but if the animal munches the seeds, there they go, into oblivion instead of into the soil.
Current Biology tells the story of Ochradenus baccatus (“Taily Weed”; see photo in Flowers of Israel), a homely desert shrub that has a “mustard oil bomb” method of attracting animals but protecting its seeds from getting eaten. It attracts rodents with the delicious fruit, but if they bite into the seeds, a chemical reaction occurs between the fruit juice and the seed juice, and pow! a distasteful, toxic mustard oil bomb goes off in the mouth. The rodents quickly learn to spit out the seeds rather than eat them. Fortunately for the plant, the rodents (to avoid getting eaten by their own predators), take the fruits to their rocky habitats, the best places for the seeds to grow. This provides an especially tight example of commensal mutualism, where both parties benefit equally from their interaction.
In the Current Biology review article, K. C. Burns (U. of Wellington) did his best to evolutionize the story while admiring the designs of the plant world. First, he plagiarized the title of a well known book by Darwin champion Richard Dawkins, headlining his article, “Seed Dispersal: The Blind Bomb-Maker.” In the attempt, though, he personified evolution too often, starting right in the first paragraph:
Seed dispersal sets the stage for everything that happens to a plant during its lifetime — after germination, plants will never again be able to travel across the landscape. Seeds can’t move very far on their own, though, so they rely on wind, water or animals to get the job done. For example, coconuts float on water to reach their destination. Maple seeds fly through the air using auto-rotating wings that operate similarly to helicopter blades. Dandelion seeds use feathery plumes that function like parachutes. Another common mechanism of seed dispersal is to enlist the help of animals. Many plants surround their seeds with fleshy pulp to strike up mutualistic partnerships with fruit-consuming animals, who swallow seeds whole and defecate them intact in new locations. Animal-assisted seed dispersal can be a highly effective means of seed transportation, but it is often fraught with difficulty. In a new study in this issue of Current Biology, Samuni-Blank et al. demonstrate that a desert plant has taken an ingenious step towards solving the problems associated with animal-mediated seed dispersal.
Presumably, evolution taught problem-solving skills to the plant. The plant’s difficulty is simultaneously attracting partners and resisting predators. Well, red hot peppers! Chilis found a way, he said: they sneak capsaicin into the fleshy fruit. In mammals (except for some masochistic humans), the capsaicin sets the mouth on fire and sends the eater running for the cold water faucet. Birds, which are not affected by capsaicin, eat the fruit with the seeds and defecate them unharmed elsewhere via air mail, but rodents learn to leave the chilis alone.
Did you know your summer picnics are part of this symbiotic interplay?
We can relate to the rodents’ plight. Watermelons (Citrullus lanatus) are filled with large seeds, and most of us spit them out before swallowing the juicy pulp, largely because the seeds have a sour taste. This sour taste is our bodies’ way of telling us that the seeds are defended chemically. By listening to our taste buds and spitting the seeds out, we avoid investing the energy to metabolise these defensive chemicals and avoid any harmful effect they might have after ingestion. However, in the case of O. baccatus, it is the combination of chemicals stored separately in the fruit pulp and in the seeds that creates the chemical deterrent, not just the seeds themselves.
Here’s where Burns mixed his metaphors. He reminded us that human engineers have designed bombs that don’t detonate till two components mix, but then attributed a similar “design” to the unguided processes of evolution. He first presented the “evolutionary conundrum” for plants needing seed dispersers without attracting seed predators. He claimed that the Taily Weed and rodent “co-evolved” their mutualistic dance of seed dispersal and feeding. And then in the case of the chili pepper, he said, “capsaicin triggers receptors located in mammalian mouths that have been designed by evolution to respond to excessive heat.” Burns never quite bothered to explain how the complex secondary metabolites in the fruit (glucosinolates) and the enzyme (myrosinase) in the seeds that detonates the “mustard oil bomb” evolved by mutation and natural selection in the first place, let alone the complex heat receptors in the mammalian mouth.
It’s noteworthy that the main paper Burns was summarizing said nothing about evolution (Samuni-Blank et al., “Intraspecific Directed Deterrence by the Mustard Oil Bomb in a Desert Plant,” Current Biology, Volume 22, Issue 13, 1218–1220, 14 June 2012, 10.1016/j.cub.2012.04.051). In fact, it begins with a 4-minute video narrated cheerfully by lead author Michal Samuni-Blank (Israel Institute of Technology), who describes, without mentioning evolution once, how her team discovered and tested the “directed deterrence hypothesis” with chemical analysis and good old field work.
Let’s have some fun with the phrase, “evolutionary conundrum” (pretending, for the moment, that it is not redundant). So: Wonders of design happen whenever Evolution, the fairy godmother (identified as Tinker Bell), waves her mutation wand with no goal or purpose in mind. Our mouth receptors were Designed by Evolution to respond to excessive heat, we just learned. The capsaicin, on the other hand, was Designed by Evolution to turn these receptors on and signal, “Fire in the hole!” But then, the brains of weird people were Designed by Evolution to fan the flames and make chilis part of their fine cuisine. The plant was therefore Designed by Evolution to get these weird people to cultivate even more chilis so that they would spread their selfish genes even further.
Watermelon seeds, by contrast, were Designed by Evolution to make humans spit them out. Humans, in response, were Designed by Evolution to create watermelon seed spitting contests (or was that Evolution designing the watermelon to make the humans do this?). Evolution designed humans to retaliate by designing seedless watermelons. (This is known as an evolutionary arms race.) But if Evolution is such a good Designer, why didn’t Tinker Bell find the mutation to design watermelons with delicious seeds that pass through the human digestive tract? Oh, we get it; it’s because Evolution designed the human to design toilets and sewer systems, so the seeds would never make it to the soil. But the watermelon has the last laugh, because Evolution designed the human to realize that without propagation by other means than seeds, their favored watermelons would go extinct.
It must be fun to be an evolutionist. All you need is imagination, and imagination has no limits. To them, evolutionary imagination is like capsaicin. Most of us run for the cold water of observable science, but to them, imagination is delicious. They have lost all feeling. The fiery heat of imagination is normal; the more the better!
For a great 100% Darwin-free documentary on seed dispersal, see the Moody Video Journey of Life (also incorporated as Volume 1 of Wonders of God’s Creation). You’ll see the examples Burns mentioned and many others: coconut, dandelion, maple seed, and many more – illustrations of little living miracles all around us that can enrich our lives just to learn about.