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The Theme of Evolution in Plant Biology

A plant's "goal" in life is to survive and pass on its genes. Here we will look at different ways in which plants have evolved adaptations to their environment to enhance their survival. We’ll look at cacti and their arsenal of spines as well as the story behind the toxins in poisonous plants so you understand just why Poison Ivy’s kiss is so deadly for Batman.

Biology teachers like to quote the famous but funny-named biologist Theodosius Dobzhansky, who said, "Nothing in biology makes sense except in light of evolution." What he meant was that even though sometimes living things exhibit crazy behavior, like birds performing ridiculous mating dances, cacti growing tons of prickly thorns, or poison dart frogs producing absurdly toxic chemicals, all have an evolutionary explanation. In some way, each of these things help their organisms survive and reproduce. What does this mean for plants? We know plants can take on many different shapes and sizes, and some are spiky and others are poisonous or make delicious fruit. These characteristics always have some evolutionary context.

If you ever venture into a desert, you will notice that the plants have certain characteristics in common: their stems are fat and succulent, there aren’t a lot of broad green leaves, and they are prickly and hurt if you bump into them. If you’re thinking that deserts only have sand and palm trees, think again! Deserts are full of plant life and some, such as the Sonoran Desert, can be quite green and lush. So what’s the deal? Why do desert plants look so different and why do they want to hurt us?

The plants don’t really want to hurt us. They are just defending themselves, just as you would if someone came up and tried to take a bite out of you. Imagine a desert with a bunch of plants with leaves that look like this: (insert leaf photo here) and a bunch of plants that look like this: (insert spine photo here). You’re hungry, you’re thirsty, and like a good little herbivore you’re going to eat some leaves to satisfy these needs. Which leaves are you going to eat first? Right. You’re going to devour the tame, non-pointy leaves with such devotion that those plants have little chance of surviving and reproducing. 

Replay this situation thousands of times and you’ll see how we ended up with deserts full of spiny plants. It’s an example of survival of the fittest (thank you, Mr. Darwin). In this case the plants that are "fitter" are the ones that are spiny because they don’t get eaten. Pointy spines also lose water more slowly than flat leaves, giving their plants another advantage over the typical leaf shape found in other habitats. The succulent nature of plants in arid environments has also been selected for, evolutionarily speaking: plants that can retain water by storing it in their stems, such as a barrel cactus, survive dry spells much better than plants that don’t store water this way. In the photos below, the cactus is happy because it stored water in its stems, unlike the rose, which doesn’t store water in its stem. Never mind that the rose in question was probably cut off of the main plant, and now has no roots.

A similar problem has played out in other habitats, too. Leaves are green and juicy, and as Kermit the Frog told us, it isn’t easy being green. Or juicy. Plants from many habitats have evolved a way of dealing with herbivores: poison! Certain plants make toxic chemicals to discourage animals from eating them. These chemical defenses are called secondary compounds because they serve a purpose that is secondary to growth and reproduction. 

It should be no surprise that poison hemlock is poisonous, but did you know that apple seeds can also poison you if you eat too many? That’s right, they have cyanide in them. One or two won’t hurt, but don’t eat a whole cup of them. Tomato leaves, raw shells of cashews, and rhubarb leaves, and wild potatoes all contain serious amounts of toxins that can harm or kill humans.

So what’s the deal with all these crazy poisonous plants? How humans domesticated them and managed to overcome all the toxins is another question, but we can understand why they are poisonous by thinking about evolution. Plants that make toxic secondary compounds are trying to protect themselves from damage. If individual caterpillars of Species X die every time they eat the leaves of Plant 1, those caterpillars don’t get to reproduce and pass on their genes. Meanwhile, the individuals of Species X that eat non-toxic Plant 2 live, reproduce, and pass on the genes that make them find Plant 2 so tasty. In Plant 1’s ideal world, this is perfect. Plant 1 kills off the caterpillars that eat it and is left to live in peace. Plant 2 might also die out because it keeps getting eaten before it can reproduce.

In reality, Plant 1 doesn’t kill off all the caterpillars that try to eat it. Some caterpillars are resistant to the plant’s toxin and keep eating. Uh oh. This is bad news for Plant 1 because the caterpillars that can tolerate its toxin will probably pass this trait on to their offspring, and their offspring will be able to eat the plant too. As caterpillars wage war on Plant 1 and the rest of its species, the individual plants that have the strongest, most toxic poison have the best chance of surviving. Over many generations, the plants develop more potent toxic defenses and the caterpillars develop resistance to higher levels of the toxin. This process is called coevolution, and is why many plants (and many prey animals) make highly toxic substances. 

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