The Evolutionary Psychology FAQ

Edward H. Hagen, Institute for Theoretical Biology, Berlin

What is domain specificity and why is it necessary?

Domain specificity is an important property of physiological adaptations, and is presumed to be an important property of psychological adaptations as well. Domain specificity means that adaptations evolve to solve problems in particular domains, and therefore are less well suited to solve problems in other domains. A domain is a selection pressure or (equivalently) a reproductive problem. It is a physical transformation that, completed successfully, would facilitate the reproduction of the organism (or, more properly, genes coding for phenotypic traits that effect the transformation). Examples of domains include oxygenating blood, killing harmful bacteria, focusing light, extracting nutrients from food, filtering or neutralizing toxins, regenerating damaged tissue, etc. In other words, any of the myriad physical processes necessary for reproduction. In order to be successful, reproduction requires that a vast number of physical transformations complete efficiently and effectively. Adaptations are the structures that effect or evoke the necessary transformations. Lungs oxygenate blood, the immune system kills bacteria, the lens focuses light, the intestines extract nutrients from food, etc.

Reliably and efficiently evoking a specific transformation requires a highly specialized structure. For example, extracting large quantities of oxygen from the atmosphere is a difficult task. Compared to solids and liquids, gases are quite dilute. Further, oxygen comprises only 20% of our atmosphere, and the earth's atmospheric pressure is relatively low. In order to transfer substantial quantities of oxygen into our (liquid) blood stream, it is necessary to efficiently expose blood to large quantities of air. In addition, it is important to keep blood inside the body while keeping air on the outside of the body. One solution would be to have a thin, very high surface area membrane that is continually 'washed' with air on one side, while a thin layer of blood circulates on the other side. This is exactly the type of structure that has evolved to solve this problem. The surface area of our lungs is about equal to that of a tennis court, and blood flows across one side in a single-celled layer--i.e., in the thinnest layer that is theoretically possible. Although lungs are ideal for oxygenating blood, they would be completely ineffective for circulating blood. Delicate lung tissue possesses none of the properties necessary to pump large quantities of liquids. The dense muscles of the heart are far more effective for this task. Thus, lung tissue has evolved to be highly specific to the domain of gas transfer, whereas heart tissue has evolved to be highly specific to the domain of pumping liquids. Specializing to solve a reproductive problem in one domain generally precludes an ability to solve reproductive problems in other domains. This appears to be a general property of mechanisms: mechanisms that do one thing well will not do other things well. Because reproduction involves a large number of distinct transformations of the world, organisms will be comprised of a large number of domain specific adaptations to effect these transformations.

Evolutionary psychology is betting that psychological adaptations have to be just as domain specific as physiological adaptations. Information processing appears to be an excellent model for the general class of problems solved by psychological mechanisms. In the field of information processing, no one has invented a computer program that solves all problems. Each information processing problem requires specialized software to solve that problem. Spreadsheets are different from word processors are different from video games. Similarly, vision is different from hearing is different from pain is different from smell is different from sexual desire is different from navigation. In short, transformations of information are just as specialized as any other physical transformation and require equally specialized mechanisms to complete the task. Thus, psychological adaptations are as likely to be as domain specific as any other adaptation.

Copyright 1999-2002 Edward H. Hagen