Darwinian medicine, which is named for English naturalist Charles Darwin, whose theory of evolution by natural selectionbecame the foundation of modern evolutionary studies, is not a method of practice or a specialized area of research. Like embryology, evolution provides a basic science foundation for all research and clinical practice. Some applications are very practical, such as using evolutionary modeling to understand antibiotic resistance or the reasons why disease-causing genespersist. Other applications are more fundamental. For example, an evolutionary foundation deepens scientists’ understanding of what disease is, and it explains why the metaphor of the body as a designed machine is inadequate.
Evolutionary applications in medicine are diverse, ranging from established methods such as population genetics to newer attempts to understand why the body has traits, such as the narrow birth canal in females, that leave it vulnerable to disease. Evolutionary explanations can be based on the phylogeny (evolutionary history) of the trait or on its proposed adaptive significance. They can address five kinds of traits acted on by evolution (human traits, human genes, pathogen traits, pathogen genes, and cell lines). The intersection of these two kinds of explanations with five objects of explanation defines 10 areas of work in the field.
Much of Darwinian medicine consists of well-established applications of evolution to medicine. For instance, population genetics is intrinsically based on evolutionary biology, phylogenetic methods have long been useful in medicine, and antibiotic resistance is recognized as an example of natural selection. New methods and data have expanded these applications. In genetics, for example, methods have been developed to identify chromosome locations subjected to strong recent selection, such as locations near the lactase gene that influence whether adults can digest milk. Modern phylogenetic methods use genetic data for diverse tasks, from tracing the specific source of an infection to tracing the genetic heritage of an individual. Informal evolutionary thinking about antibiotic resistance has been replaced by rigorous mathematical models that have major implicationsfor public health.
Other applications of evolutionary biology to medicine are still developing. In particular, studies to test hypotheses about why natural selection has left the human body vulnerable to disease expanded after 1991, when an article titled “The Dawn of Darwinian medicine,” published in The Quarterly Review of Biology and written by American evolutionary biologist George Williams and physician Randolph Nesse, argued that evolutionary explanations are needed to explain not only why bodies usually work well but also why they have aspects that leave them vulnerable to disease. The major evolutionary reasons that explain why bodies remain vulnerable to disease can be organized into six categories. Mismatches between the environments that humans evolved in and that they now occupy account for the prevalence of substance abuse, obesity, high blood pressure, atherosclerosis, and breast cancer. A second reason for vulnerability is the speed with which infectious organisms evolve ways to deal with antibiotics and the protective defenses of the human body. This process of coevolution results not in benign coexistence but in levels of virulence (ability to damage tissues) shaped to maximize the rate of pathogen spread. Virulence often depends on the route of transmission. For instance, respiratory viruses severe enough to keep victims in bed are likely to be displaced by less-severe strains whose victims are mobile enough to infect others. In contrast, malaria parasites spread faster when they make the host too sick to defend against mosquitoes; thus, malaria tends to be quite virulent.
Vulnerability results also from constraints. For example, the eyes of vertebrates are poorly designed, with a blind spot, and nerves and vessels run between the point where light enters the eye and the retina. The octopus eye, by contrast, has no blind spot. Another constraint is the inevitability of DNAreplication errors. Bodies are also subject to engineering constraints and trade-offs. Bones could be thicker, but bodies would then be heavier and slower. Darwinian medicine emphasizes that nothing in the body can be perfect, since every trait is subject to constraints and trade-offs.
Selection shapes bodies for maximum reproduction rather than health. Usually optimal health and reproduction coincide, but mutations that increase reproduction tend to spread, even if they decrease health and longevity. Higher male than female mortality rates in polygynous species (species that have more than one mate) are an example. In such species an incremental investment in bodily protection and repair increases reproductive fitness more for females than for males.
Additionally, many symptoms are not diseases but protective responses shaped by natural selection. Pain, fever, cough, and anxiety are aversive and useful responses. Nonetheless, medications can often safely block their expression, because of the “smoke-detector principle.” Humans put up with sensitive smoke detectors set off by making toast because such false alarms are a minor nuisance compared with the huge cost of not being alerted to a fire. Likewise, the cost of many bodily defenses is low compared with the cost of not expressing a defense when it is needed, so the normal mechanisms shaped by natural selection give rise to many false alarms and apparently excessive responses.
Darwinian medicine has narrowed the gap between evolutionary biology and medicine and contributed to improvements in the understanding of health and disease. Some advances have been straightforward, such as new public health policies based on formal evolutionary models of antibiotic resistance and evolutionarily informed searches for genes that cause disease. Other advances have come from asking new evolutionary questions about why natural selection has left bodies vulnerable to disease. Applications of these advances are less direct, but they may be more fundamental. They encourage new studies of phenomena with enormous clinical importance, such as why males die younger than females and how selection shapes mechanisms that regulate protective responses such as pain and fever. They offer a more fully biological view of the body and disease