Some traits are obviously adaptive, such as antibiotic resistance in bacteria. Because scientists understand the selective force — humans added the antibiotics — they know that the evolutionary change they observe is a response to that selection. The increase in frequency in the population of antibiotic-resistant bacteria is an adaptation to the presence of antibiotics.
Other traits are obviously not adaptations. An example is a change at the level of the DNA that doesn't result in a change in the way a gene works or is expressed. Traits of this type are selectively neutral: They don't change the fitness of the organisms that carry them, so their frequency increases or decreases based solely on random factors (see Chapter 6 for details). And any change caused by random factors rather than selection isn't an adaptation.
Some traits that may seem to be adaptive aren't necessarily adaptations. Maybe the traits exist for reasons that we don't understand — developmental constraints, past events, whatever. The point is that although the traits appear to serve some purposeful function, that function had nothing to do with their evolution. In a paper written in 1979, Stephen Jay Gould and Richard C. Lewontin warn evolutionary biologists not to confuse current purpose with past adaptations. To make this point, they wrote about the spandrels of St. Mark's Cathedral in Venice.
A spandrel is a structure connecting the dome to the rest of the roof in a particular type of architecture. And in St. Mark's, the spandrels have been painted so amazingly, you'd think that they exist solely for the purpose of bearing the remarkable images. Wrong! The spandrels are just necessary consequences of how the rest of the structure is put together, meaning their purpose is solely architectural, not artistic. But that doesn't mean they didn't make a decent canvas for the artists who added the pictures later.
Unfortunately, the distinction between adaptive and non-adaptive traits isn't always obvious. Some traits appear to be adaptations but aren't. Why is knowing the difference important? Because you don't want to be fooled. Stating that things are adaptations that actually aren't can lead to errors in the way you interpret data.
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