A new kind of evolution in action in Australia
Lesson summary
Hi there, I’m Jeff, and thanks for joining us for Plain English lesson number 340. JR is the producer and he has posted the full lesson at PlainEnglish.com/340. The full lesson, remember, includes the transcript, translations of key words into nine languages, the audio at two speeds, a video lesson, and one extra word or phrase from today’s lesson, all for you at PlainEnglish.com/340.
Coming up today: the cane toad is an invasive species in Australia. It has no natural predators there, and it kills the animals that try to eat it. But there is one fascinating thing about this toad: some of them are evolving far faster than normal evolution would suggest. We’ll explore what’s happening and how on today’s lesson. Our expression is “pick up” and we have a quote of the week.
By the way, before we start, a “toad” is a kind of frog, usually brown, with drier skin, and bumps all over its body.
Australia’s cane toads evolving to jump faster
Species change over time; it’s a fascinating part of nature. Unfortunately for curious humans, though, these changes happen so slowly that it’s impossible for any of us to observe them in our own lifetimes. Changes to animal species happen over dozens, hundreds, or thousands of generations, and hundreds, thousands, or millions of years.
However, a strange thing is happening in Australia. The cane toad—a despised species Down Under—is changing much more quickly than other species evolve. In fact, the evolution is playing out at a high speed.
I’ll tell you about that in a second. But first, let’s meet this species. In 1934, Australian sugar cane farmers were battling a pernicious species of beetle: the beetles were eating their sugar cane stalks. So they found a kind of toad in Hawaii that eats this exact kind of beetle—a match made in heaven, from the perspective of the farmers. They brought 102 toads from Hawaii to Australia, bred them in captivity, and eventually released 2,400 toads into the sugar cane farms.
They watched in horror as their experiment not only failed to control the sugar cane beetle but proceeded to destroy wildlife all over their country. You see, the beetles were munching on sugar cane high on the stalk, but the toads didn’t jump high enough to eat the beetles. Instead, the toads ate other things on the ground.
Very well, other animals must eat this toad, right? Indeed they do: the cane toad is ugly to us, but looks delicious to Australian animals. Unfortunately, the cane toad is also poisonous. It releases a deadly serum when it’s in trouble, so the animals that eat the cane toad also die. Crocodiles, turtles, other reptiles, some mammals, and even some birds all try eating the cane toad, and many die. Some native Australian species are in danger of extinction as a result.
The cane toad does have natural predators, just not in Australia. That’s a problem when an individual toad can lay 30,000 eggs a year. In fact, the cane toad population has grown to over 200 million since 1934.
The cane toads are on the move. They were introduced to a small area in northern Queensland in 1935. They moved slowly at first, but their spread picked up . They began moving up and down the coastline in the 1940s and 1950s and started pushing farther inland in the 1970s and 1980s. They now cover large parts of northern and eastern Australia.
In the early days, they were advancing slowly. By 1994, though, they were conquering new territory at a rate of 40 kilometers, or 25 miles, a year. Twenty years later, they were conquering new territory even faster: 60 kilometers, or 37 miles, per year. Think about that. In the space of twenty years—the blink of an eye in evolutionary terms—in just twenty years, the toads were jumping and spreading fifty percent faster. How is that possible?
Scientists call the toads near the edges the “frontier toads.” These are the ones that are pushing the frontier, or the border, of the species into new territory. And guess what? These toads along the frontier have stronger back legs and they jump straighter than the toads that are living in areas where the toad population is more well-established. It’s almost as if evolution is on fast-forward just for these toads on the frontier.
This is unusual. If you look at the population of a single species, there will be natural variation from animal to animal. But big differences in characteristics—the structure of bones and muscles and such—won’t be different from place to place. A whole population tends to evolve together, at least in the short run. Sure, over millions of years, species branch off and adapt; the population in one place will change differently than the population in another. But that’s over the long run.
The cane toad is changing quickly. Here’s what scientists think is happening.
Picture a population of cane toads. Some toads will have genes that make them jump farther and straighter; others won’t jump as far, or they’ll jump around in circles and stay in the same general area. The ones that jump straight and far will be the ones to push the frontier deeper into Australia. When it comes time to mate, those toads near the frontier will find partners nearby, partners that are just like them: long, straight jumpers.
So guess what happens to the offspring of the toads at the frontier? This new generation is the product of two parents that are long, straight jumpers. So this new generation will naturally be a little stronger, and jump a little straighter. Out of this new generation of strong, straight jumpers, some will be stronger than others. These strongest ones—the strongest of a strong set of parents—will push the frontier farther still, mate with one another, and pass along the strongest genes. The same thing happens with each successive generation at the frontier. That’s how this one species is evolving at a high speed to push deeper and deeper into Australia, and that’s why the toads at the edges of the population are different from the lazy ones that stay close to home.
This isn’t exactly Darwin’s natural selection, but it’s close. In natural selection, the individuals with the strongest characteristics reproduce more often so that the offspring have the most favorable characteristics to survive and continue mating. That’s evolution of a whole population over time. What’s happening with the cane toad is more like evolution over space rather than long periods of time.
The ironic thing for the cane toads at the frontier is that jumping farther and straighter doesn’t confer a survival advantage. If anything, it’s the opposite: more of the toads on the frontier suffer from arthritis from jumping so far. They may even be eaten more often.
Seeking a solution
A few years back, we talked about a new effort to genetically modify mosquitos so that they stop reproducing. The point would be to control the spread of malaria. Scientists are working on a way to do that with these cane toads, but it’s not easy.
Invasive species are scary. The cane toad is a big deal in Australia. Here in the US, we have the Asian carp, a kind of fish that is just demolishing rivers. It doesn’t help either that both these invasive species are really ugly!
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