Adaptations shaped by natural selection are so interesting because fascination with function gave our ancestors selective advantages. The mechanism works overtime, seeing functions where they don’t exist. And, not surprisingly given human nature, this has created critics who are skeptical about all adaptationism. This blog should help to connect skeptical adaptationists everywhere as we work together to figure out how to study adaptation and maladaptation.
This is important as well as interesting. At the very center of my work with George Williams to develop the field of Darwinian medicine is a question that needs to be answered about every disease. The question is:
Why has natural selection left the body vulnerable to this disease?
Notice that the question is not about how a disease is adaptive. Diseases are not adaptations and most were not shaped directly by selection. But every disease needs an explanation of its evolutionary etiology. Medicine has studied proximate etiology in detail, that is, what differences between individuals explain why some get a disease and others do not. The evolutionary etiology explains why all members of a species have bodies that are vulnerable to a disease.
We have proposed that there are six main kinds of answers: mismatch with the environment, co-evolution with pathogens, constraints, trade-offs, reproductive success at the expense of health, and defenses. Deciding which ones apply for a particular disease is hard work. Generalizations about adapations won't do, every proposal needs to be assessed independently. The contribution of Darwinian medicine depends entirely on how well we do this. I have published a cook-book style guide to testing evolutionary hypotheses about dieaase. But this is only a very modest beginning. Work to reach agreement on how to test evolutionary hypotheses will be well worth it.