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- Evolution and medicine: the long reach of "Dr. Darwin"

It is clear that evolutionary biology has an enormous potential to enrich our understanding of biomedical phenomena. It is also clear that the study of biomedical phenomena can greatly enrich our understanding of evolutionary processes. These observations should be of relevance to biological and biomedical investigators and educators. Moreover, the examples drawn from immunology and oncology show that the human body itself is a laboratory for fast evolution. This fact has significant philosophical implications for the philosophy of science, especially as it relates to the nature of explanations in the biological sciences.

Much of this review has been devoted to ways in which evolutionary biology can enrich our understanding of biomedical phenomena. However, the study of biomedical phenomena shows the need to rethink some aspects of evolutionary biology. Traditional Darwinists draw a sharp distinction between mechanistic explanations on the one hand, and evolutionary explanations on the other. Thus, in dealing with the question, "What is biology?" the great evolutionary biologist Ernst Mayr observed:

When we try to answer this question, we find that biology actually consists of two rather different fields, mechanistic (functional) biology and historical biology. Functional biology deals with the physiology of all activities of living organisms, particularly with all cellular processes, including those of the genome. These functional processes ultimately can be explained purely mechanistically by chemistry and physics [[39]:24].

But the story does not end so simply; Mayr continues:

The other branch of biology is historical biology. A knowledge of history is not needed for the explanation of a purely functional process. However, it is indispensable for the explanation of all aspects of the living world that involve the dimension of historical time – in other words, as we now know, all aspects dealing with evolution. This field is evolutionary biology [[39]:24].

Mayr observes that the most frequently asked question is mechanistic (functional) biology is "how?" whereas the most frequently asked question in evolutionary biology is "why?" He adds, "To truly appreciate the nature of biology one must know the remarkable difference between these two branches of biology" [39].

Focusing their attention on the contours of the new science of Darwinian medicine, traditional Darwinists Nesse and Williams distinguish between two types of causes that are medically relevant (and thus require two different types of causal explanation):

Consider heart attacks. Eating fatty foods and having genes that predispose to atherosclerosis are major causes of heart attacks. These are what biologists call proximate ("near") causes. We are more interested here in the evolutionary causes that reach further back to why we are designed the way we are. In studying heart attacks, the evolutionist wants to know why natural selection hasn't eliminated the genes that promote fat craving and cholesterol deposition. Proximate explanations address how the body works and why some people get a disease and others don't. Evolutionary explanations show why humans, in general, are susceptible to some diseases and not to others [[12]:6].

The distinction is between mechanistic explanations that answer "how" questions, and evolutionary explanations that answer "why" questions. Evolutionary explanations are typically viewed as long-term, historical explanations (one might have to consider the entire course of human evolution, for example), whereas mechanistic explanations are immediate – and for many purposes, essentially ahistorical.

It is true that looking at medical phenomena from the standpoint of traditional Darwinism typically means taking a historical perspective – and as we have seen above, it certainly has a legitimate role to enrich our understanding of biomedical phenomena. It is also true that traditional Darwinists recognize that rapid evolution is possible for organisms with short generation times, such as viruses and bacteria – organisms where the relevant history may concern events occurring over the course of a few months. But we now see that traditional Darwinism is only a part of the Darwinian medical story. Consideration also needs to be given to the role of Darwinian explanations of biomedical phenomena occurring in the life-cycles of animals – including humans.

The examples we have presented concerning the role of Darwinian explanations in the realms of immunology and oncology show that it is not easy to draw a sharp distinction between mechanistic explanations and evolutionary explanations. For phenomena in the domain of humoral immunity and oncology, important aspects of the mechanistic explanation involve a rapid evolutionary explanation. In this way the study of biomedical phenomena shows the need for a critical reassessment of generalizations about the nature of biological explanation that have been forthcoming from traditional Darwinists.

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