Throughout this book, I occasionally bring up the cheetah's running speed as an example of an adaptation. I've always thought cheetahs were fascinating creatures and looked forward to seeing them on the Nature Channel. The nice story that I tell you is that there was variation for running speed, that cheetahs that ran faster left more descendents, and that, as a result, the cheetah population got really good at running. It's a good story, but science is more than just a good story.
When they're talking about adaptations, scientists have to be careful not to identify as an adaptation to current phenomena a trait that's really an adaptation to past phenomena that are no longer active.
Until fairly recently (not much more than 10,000 years ago), for example, North America and South America were home to a diverse group of large mammals such as gomphotheres (four-tusked, elephant-like creatures) that would have eaten a lot of the local vegetation and may have been potential seed-dispersers. As a result, these large herbivores would have been a key factor in the evolution of the local vegetation — that is, the plants would have evolved in concert with the herbivores that ate them and dispersed their seeds. Probably, the plants evolved defenses against herbivory (the eating of plants) and also evolved traits that led to increased seed dispersal. (This idea that two organisms evolve in response to each other is called co-evolution. You can read about it in Chapter 13.)
One of the main reasons plants produce fruit is to get their seeds dispersed. Animals eat the fruit; the seeds pass out of the animals' guts and land in new environments where they germinate and produce new plants (if the environment is suitable). One obvious benefit of such a system is that the seed ends up in a pile of fertilizer. A disadvantage is that the animal may eat the seed as well. So seeds need adaptations to prevent them from being digested on their journey through the digestive tract — hence, the hard coatings that many seeds have. Yet a coating hard enough to pass through an animal unscathed may actually make it more difficult for the seed to break through the shell and germinate. A seed with a coating that allows it to survive passage through an animal's gut may now need to go through the gut before it can germinate. And indeed, the Americas have plants that seem to require passage through a large herbivore for optimum germination.
Then humans arrived in the New World, spotted the giant herbivores, thought, "Dinner!" and gobbled them all up. (Well, not necessarily. Some controversy exists about the exact role that humans played in the extinction of these large animals. But the current thinking is that our species did play a significant role — and spent the next few centuries trying to come up with a suitable antacid.) Regardless of how the gomphotheres disappeared, disappear they did, leaving plants without the herbivores needed to facilitate germination of their seeds. In fact, some species of plants in the tropics may have been undergoing a gradual decline in population as a result of an absence of these seed-dispersal agents.
Then along came horses, which aren't native to the Americas. As it turned out, horses also serve as excellent seed dispersers for some American flora, and in some cases, they are the only animals able to facilitate the germination of native plants. But the plants didn't evolve in the presence of horses, which only just got to the New World a few hundred years ago.
Scientists can examine selection for increased running in the laboratory. Of course, it's tough to do experiments with big cats in the laboratory: You need a lot of room for them to run, feeding them is expensive, and if you're not careful, you could end up on the dinner menu. But cats are not the only things that run. Mice run, too. And they're a lot cheaper to keep and a lot less likely to turn on you. And the best part? They're happy to run in those little wheels, so with a minimum of analytical equipment, you can keep track of how far they're running.
^SjOJOK Theodore Garland, Jr., and coworkers set up a long-term evolution study ff'/^^VN looking at the consequences of artificially selecting for mice that like to run.
They got a bunch of mice, gave them all a chance to run, and then founded the next generation by selected the ones that just plain liked to run more. They've continued this selective regime for almost 15 years, continuing to selectively favor the mice than most like to run.
The results: Mice lineages selected for increased voluntary running now run everyday several times farther than the original mice, and they do it by running for just as long but several times faster. These fast running mice have undergone other changes as well as a result of this artificial selection:
1 They have larger hearts.
1 Their muscle fibers are different from the original unselected mice. 1 Their hind limbs or more symmetrical.
There was plenty of variation for characters associated with running on which artificial selection could act. It didn't take long to produce evolved mice that like nothing more than to go to the gym for a workout.
Oh — one last thing: The hearts of the mice that ran farther aged better, so when you're done with this book, go take a walk or jog. It'll be good for you.
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