An adaptation is a trait that has resulted from evolution by natural selection. In the example of the cheetahs earlier in this chapter, the ability to run faster is an example of an adaptation. Antibiotic resistance (see Chapter 17) is another example of an adaptation. This trait appears in a bacterial population because of random mutations that made some bacteria better able to survive and reproduce in the presence of antibiotics.
Some folks quibble over whether the evolution of antibiotic resistance is truly the result of natural selection since people are hosing down the world with tons of antibiotics and thus exerting selective pressure of their own. Others say that it is natural rather than artificial selection because even though people altered the environment in this instance, the bacteria responded (naturally!) to that change. (Also, antibiotics occur naturally — lots of microbes make them — and bacteria evolve in response to those. Many microorganisms produce antibiotics to inhibit the growth of other microorganisms. Penicillin, for example, is a natural antibiotic that gets its name from the mold that produces it, Pénicillium chrysogenum.)
If the whole concept of adaptation seems to be ridiculously straightforward, that's because overall it is. The following sections explain why recognizing an adaptation isn't not always easy though.
Is it an adaptation—or not?
Distinguishing adaptive from non-adaptive traits isn't easy. Identifying an adaptation and the selective force that caused it is pretty clear cut when it comes to something like antibiotic resistance because we've been around to observe the whole process. The situation gets a bit dicier when you talk about adaptations we haven't observed, because in nature, it's not always crystal clear whether a particular trait is an adaptation.
Adaptation, the result of evolution by natural selection, works at the species level, not at the individual level. Acclimatization, on the other hand, occurs within individuals.
Suppose that you decide to visit Denver, which is a mile above sea level and has less oxygen than lower-lying areas of the country. On your first day in the city, you decide to go jogging and find the activity much more difficult than usual. After a few days, however, you begin to feel like your old self and can jog just as you used to. Why? Because your body reacted to the lower levels of oxygen by producing additional red blood cells. Although most people in this situation would say that they've adapted to the altitude, they'd be more accurate if they said that they've acclimatized. This kind of distinction is exactly the idiosyncratic type of parsing that scientists (and English teachers) love.
While you won't be doing any adapting on your trip to Denver, there is evidence to suggest that human populations that have lived for long periods at high altitude show genetic changes that could be adaptations to the lower oxygen concentrations at altitude. When you acclimate in Denver, you make more red blood cells which are the same as the red blood cells you had before. The Sherpas living at high altitude in the Himalayas, however, have differences in blood chemistry that result in their red blood cells having higher affinity for oxygen than others.
Some limits exist to what natural selection can accomplish. If natural selection always favors more-fit genes, all species would be on the road to super-organism — forever getting faster, glowier, taller, whatever. And there'd be no limit in numbers of limbs, eyes, hearts, tails, and fins, if having more of these things means being more fit. But that's not what happens.
1 Some things may just not be physically possible: Mammals will never run at the speed of sound.
1 Others may not be biologically possible: The mammal lineage seems limited to four limbs — variation in limb number is absent.
1 Some adaptations may preclude others: A bird can have wings that function like a penguin or a hummingbird, but not both. This idea is explained in more detail in the later section "You can't get there from here: Constraints and trade-offs."
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