Friday, January 7, 2011

Darwin and medicine

Abstract: Darwinian principles have been remarkably successful in explaining otherwise puzzling features of the living world. The human body surely qualifies as a part of the living world.

It would therefore be surprising if Darwinian principles were not helpful in explaining how our bodies work, and why they so often fail to work as we think they should. However, only recently have evolutionary biologists teamed up with physicians to try to understand the evolutionary causes of "why we get sick". The result is the new science of "Darwinian Medicine". This talk is a brief introduction to this exciting new field of research.

Introduction: What is "Darwinian Medicine"?

At present Darwinian Medicine is largely theoretical. It takes as its point of departure the oft-repeated saying by the geneticist Theodosius Dobzhansky: "Nothing in biology makes sense except in the light of evolution." Evolutionary biology is the theoretical foundation for all biology, and biology is the foundation for all medicine, so it is natural that there should be a connection. But it has taken a long time to appreciate this fact because of the very different time perspectives typically adopted by physicians and medical researchers, on the one hand, and evolutionary biologists, on the other.

The immediate and primary aim of medical research is to understand the human body's anatomical and physiological mechanisms as they currently exist, and to devise treatments to counter whatever malfunctions may arise. Medical research focuses on the proximate causes of illness, i.e., those causes operating on and in presently existing individuals. Evolutionary biologists, on the other hand, want to know why any organism has the characteristics it does in the first place.

They seek the ultimate causes of an organism's characteristics in its evolutionary history. Present organisms have the basic characteristics they do because their ancestors acquired these characteristics. Darwinian evolutionary biology does this by asking how a small set of processes leads organisms to have the characteristics they do.

At least some of the traits of living things can be explained by noting that such characteristics conferred a survival or reproductive advantage on their ancestors. In other words, such characteristics are a result of natural selection. A key strategy in framing Darwinian explanations has come to be called "The Adaptationist Program".

Essentially this means that when trying to explain some widespread biological characteristic, ask what adaptive function this characteristic has, or might have had in the past, and then seek a plausible selectionist explanation of this function. Of course, there is no guarantee beforehand that every characteristic is adaptive, or that a plausible selectionist explanation will be forthcoming. But as a research strategy the Adaptationist Program has enjoyed a great deal of success.

Despite its success, however, no Darwinian believes that all features of all organisms are adaptations. Selection is not the only evolutionary force at work shaping organisms. Chance, historicity, and constraints combine with natural selection to shape organisms.

The result is that while all organisms are incredibly well-adapted in some respects, in others they represent "trade-offs" and compromises, jury-rigged contraptions that succeed despite, rather than because of, the way they are constructed. Taking into account the various ways in which evolution designs organisms helps to explain why the human body is simultaneously a glorious work of art and a Rube Goldberg device that seems patched together by craftsmen lacking both wisdom and foresight.

I will follow Ness and Williams in distinguishing evolutionary explanations for human ailments into the following categories: evolved defenses, conflicts with other organisms, novel environments, trade-offs, and design flaws. I will conclude with some remarks on the future of Darwinian Medicine.

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