Adaptations And Spandrels: Internal Combustion Engine Analogy
[The] internal combustion engine. [...] [W]e could readily infer that the engine's purpose is to produce torque in order to drive wheels. Along the way, we will also notice that the engine produces excess heat. We are entitled to regard this excess heat as a pure by-produce, a side effect: if the engine functioned to produce torque without excess heat, it would make little difference to us -- or might even be considered advantageous, since the excess heat of internal combustion engines normally requires a cooling mechanism, in most cases a water pump and radiator. So is the cooling system, then, a by-product of the engine? Not exactly. The cooling system is a feature intrinsic to the design of internal combustion engines: it functions to disperse excess heat, and our reverse-engineering understanding of it would see it as part of the whole functionally interconnected mechanism. It is as much an intrinsic part of the design of the engine as, for instance, the heating and cooling systems of the human body. These involve life-protecting homeostatic reactions such as perspiration, shivering, and fevers. They are intrinsic elements of the body's physiological system and not by-products of adaptations -- spandrels or epiphenomena. Like the body, internal combustion engines could not function at all without specialized cooling systems: on the engine analogy, cooling systems are adaptations.
Let us push the analogy a step farther. Suppose water used to cool a car engine is diverted into a second, smaller radiator with a fan in order to heat the driver/passenger compartment. Are we now justified at least in calling the car heater a by-product of the system? The answer again is no: rather than being an extraneous epiphenomenon, the heater is an entirely calculated way of using what really is a by-product (excess engine heat) for the benefit of the driver, satisfying his desire to stay warm. The mobility human beings want from cars and the desire for warmth are in any event neither parts nor design features no by-products of the engine: they explain rather very existence of the car, with its engine and heater. The car heater, like any designed artifact that makes use of and depends on a by-product, does not itself necessarily become for that reason a by-product. That engines give of heat is an inconvenient contingent fact, which in this case human ingenuity turns into an asset. (If engines gave off light instead of heat, engineers might well figure out how to make productive use of the excess light, and devise alternative methods to heat cars in cold weather.) It is true that people do not design and manufacture cars in order to create car heaters, but that is like saying that evolution did not produce eyes in order to produce eyelids. Eyelids are an adaptation too (a further man-made enhancement of the eye would be spectacles). When the parts of a whole mechanism -- artifact of evolved system -- are functionally interconnected, understanding the machine, or the organ, becomes a matter of seeing how and why the parts are interrelated and what they accomplish.
-- Denis Dutton
from "The Art Instinct: Beauty, Pleasure, and Human Evolution"
Quoted on Wed Sep 28th, 2011