In a previous post Chris introduced the concept of the evolutionary arms race. Predators get better at catching prey, yet prey get better at avoiding predators. Organisms develop immunity to diseases, but diseases develop new ways to infect organisms. No matter how good cheetahs are at catching gazelles, there are still plenty of gazelles on the Serengeti. Conversely, no matter how good rabbits get at dodging wolf attacks, some of them still get eaten. It seems that without some outside influence (like a natural disaster, or human activity), most antagonistic biological relationships (meaning one organism is being harmed by the interaction) will settle out in something of a stalemate. There will be gains and losses for both parties. This led Leigh van Valen to make the comparison to what the Red Queen said to Alice in Lewis Carroll's Through the Looking Glass.
Organisms must continue to adapt, lest they be overcome by predators or left behind by prey, but all of that adapting leaves them with no more advantage than they had before. This idea has come to be called the Red Queen Hypothesis, and it made quite an impact on the field of evolutionary biology. The hypothesis is now an essential part of every evolution curriculum.
Now, you may notice that it still bears the label of a hypothesis, meaning that it hasn't been tested thoroughly enough to be called a theory. One of the main reasons for this is that evolution takes a long time. Even small changes take generations to become established in an entire population. It's also difficult to compare different generations in order to establish that they really are adapting to each other. By the time a predator or parasite has adapted, so has the prey or host.
Enter Ellen Decaestecker et al.
This group found an elegant way around both of those problems by looking at Daphnia and bacterial parasites.
Here's the necessary background information: Daphnia magna (also called the water flea) is a freshwater crustacean that is parasitized by bacterium called Pasteuria ramosa. Both of these organisms can go through many generations in a short amount of time (that solves Problem One), and both can produce dormant offspring (resting eggs in D. magna, and spores in P. ramosa). The dormant offspring will accumulate in the lake sediment, where they can remain for decades before becoming active and healthy individuals. Eggs and spores aren't the only thing that settles to the bottom of a lake; there's also silt, mud, dirt, and all sorts of other sediment. That means that as you dig down in the bottom of a lake, you are looking at older layers (almost like tree rings, but a little more wibbly-wobbly).
Here's where Dr. Decaestecker's work comes in. They took core samples of the lake, divided them into layers (corresponding to about two to four years) and isolated the bacteria and daphnia from each layer. Now they were able circumvent Problem Two by exposing different generations of daphnia to bacteria from their past (lower sediment layers), present (same sediment layer), and future (higher sediment layers).
The results were fantastic. As one might expect the bacteria from the past were less effective at infecting the daphnia than the bacteria from the present. That makes sense, the daphnia have already adapted to resist those parasites. Now, what about the bacteria from the future? Well it turns out that they were also less infective than the present bacteria.
Hang on. How can the Daphnia be adapted to bacteria from their future?
While physicists haven't completely ruled out time travel as a theoretical possibility, I think that most would agree that it is beyond the capabilities of Daphnia magna. Something else must be at work here.
That something is the concept of evolutionary tradeoffs. One adaptation often has to replace another. Many terrestrial animals have long lost their gills in favor of lungs. Now they are well suited to living on land, but they can no longer survive in the water. Birds traded heavy scales for lightweight feathers. Now they are able to fly, but they have lost the protection that reptiles have. Pasteuria ramosa lost adaptations that allowed it to infect past generations of D. magna in favor of ones that let it infect the present population.
As is always the case with science, there's always more work to be done. We have a great demonstration of the Red Queen Hypothesis at work in a single host-parasite relationship, but that alone is not enough to say that it applies to every situation. What about predator-prey relationships? What about long-lived species? What about terrestrial animals? What about plants and herbivores? The list of follow-up questions is endless. Every answer creates even more questions. The data monster is always hungry.
Here are some other interesting articles about evolution
It's summer time, and that means temperatures are heating up. While humans are migrating to swimming pools or air-conditioned movie theaters, plants are still stuck outside without the luxury of beach umbrellas or some mediocre fiction to keep them entertained. While there's not much they can do about boredom, plants have evolved plenty of innovative ways to keep cool in harsh environments, and that's the topic of this new series.
"Now, here, you see, it takes all the running you can do, to keep in the same place."
Organisms must continue to adapt, lest they be overcome by predators or left behind by prey, but all of that adapting leaves them with no more advantage than they had before. This idea has come to be called the Red Queen Hypothesis, and it made quite an impact on the field of evolutionary biology.
There’s always been a natural appeal to understanding the battle between two enemies. As one adapts, so must the other.
Consider the sit-and-wait predator in his shiny patrol car waiting behind a freeway overpass to catch speeders on the highway. As drivers became more aware of police, they had to employ radar guns to catch their prey...
References and further information
Decaestecker, E., S. Gaba, J. A. Raeymaekers, R. Stoks, L. Van Kerckhoven, D. Ebert, and L. De Meester. 2007. Host–parasite ‘Red Queen’dynamics archived in pond sediment. Nature 450:870-873.