Some evolutionary changes are adaptive, meaning that a character has changed as a result of natural selection in a way that makes that character better suited to perform its function. Here's an example of the process of adaptation: Gazelles run away from cheetahs. The slow gazelles get eaten, leaving the faster gazelles to reproduce. In the next generation, the gazelles are faster on average than those in the past generation, because the run-away-from-cheetahs character has evolved. Being able to run really, really fast is an adaptation.
It's not always easy to tell whether a particular character is an adaptation because sometimes things that appear to be adaptive characters aren't. Suppose that you have a cat and decide to put its food outside. At some point, you notice that birds eat the cat food. Knowing a bit about evolution, you think that eating from the cat dish may well be good for the birds; they probably have more energy to sing songs, build nests, and raise baby birds. If you observe such successful foraging behavior in a different environment, you might conclude that the birds are foraging in your cat dish as the result of natural selection. But eating cat food isn't an adaptation (the birds haven't evolved to eat out of cat dishes); it's opportunistic. The food's available, and the cat . . . well, he's probably trapped behind a patio door. For more about adaptive characters, go to Chapter 5.
The Study of Evolution, Post-Darwin
Darwin had only a vague idea of what genes were and didn't know squat about DNA, but he hit the evolutionary nail on the head. Today, scientists know that the process of evolution by natural selection occurs pretty much the way Darwin first proposed it: Natural selection results in changes over time in any given population, and good genes (those that make the organism more fit — that is, more successful at surviving long enough to reproduce) become more frequent over time. Still, scientists' understanding of evolution has continued to evolve as they expand the theory of evolution to include some elements Darwin was unable to address:
¡^ Many DNA mutations are selectively neutral. The DNA code contains a certain amount of redundancy, which means that many changes in the DNA don't result in a fitness advantage or a fitness cost. The extent to which these genes increase or decrease in a population has entirely to do with chance.
¡ Chance can be an important factor contributing to the change in gene frequencies through time. Imagine that half the deer in the forest have blue eyes, and half have brown eyes. Now suppose that a couple of trees fall over and accidentally crush a couple of deer with blue eyes. All other things being equal, the next generation will have a higher proportion of the brown-eyed gene than the previous generation. Evolution has happened, but not as a result of natural selection. (Yes, I know that deer don't have blue eyes; it's just an example.) For more information on how chance factors into the evolutionary process, head to Chapter 6.
I can imagine what you must be thinking: Two deer more or less are hardly going to make much of a difference. In a large population, you'd be right, but in a small population, a few deer more or less can make a difference that would be noticed in the future. When the population is large, chance events aren't as important, but when the population is small, random events can have larger repercussions.
m m il Not all the characteristics of any particular organism are positively correlated with fitness. This idea stems from scientists' understanding that not all evolutionary change is the result of natural selection. Sometimes, it's the result of chance; sometimes, it's the result of bad genes hitching a ride into the future with the good genes that made the organism more fit.
i The environment affects fitness. Populations in different places experience different selective forces. A gene for being able to survive a long time without water, for example, may offer a fitness advantage in the desert, but it may have rather negative consequences in a rain forest. Interaction between the gene and its environment is important in determining whether a given gene increases or decreases fitness.
Sickle cell anemia is an example of how the environment determines whether a particular gene increases or decreases fitness. The gene that causes sickle cell anemia produces a slightly different form of hemoglobin. The most extreme case occurs when someone has two copies of the sickle cell gene: one from the mother and one from the father. But even having just one sickle cell gene causes illness. At first glance, it seems obvious that this gene wouldn't increase anyone's fitness, yet it's present in high frequencies in certain areas of Africa. By examining the system from an evolutionary perspective, scientists learned an interesting thing about the sickle cell gene: Having a copy of this gene helps protect against malaria, which is present in those areas of Africa where the gene occurs at high frequency. So yes, it's bad to have this gene in the current era in the United States. But in the days before antimalaria drugs, it was a good gene to have in parts of Africa.
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