Proximate and Evolutionary Explanations
A complete explanation of anything in biology requires two kinds of explanation. The first is a description of what it's like and how it works. Such descriptions are "proximate explanations.” The second is a description of how it got to be the way it is. These are "evolutionary explanations." Both are necessary for a full explanation.
Most medical research tries to find proximate explanations. That is sensible because identifying what is malfunctioning is often the key to finding ways to prevent or treat diseases. Genetics, physiology and anatomy all provide proximate explanations for how the body works. There are two kinds of proximate explanations. One describes what a trait is like and how it works. The other describes how it developed from a genetic code to an adult form. Both are about mechanisms and how things work. Together they provide a complete proximate explanation.
Evolutionary explanations are also of two kinds. One describes the history of a species and how traits change across many generations. Such studies of phylogeny and the relationships among organisms are profoundly useful in evolutionary medicine. They reveal the origins of pathogens, how pathogens evolve, and our own history as a species.
The other kind of evolutionary explanation describes how variations in a trait influenced how many offspring individuals had; this process of natural selection explains why most parts of a body work so well. A classic example is sweating. Individuals who sweated in hot environments survived better in hot environments and had more children than those who could not sweat, so sweating became universal. This is an evolutionary explanation. In contrast, a proximate explanation would describe the mechanisms that detect heat, and that turn on sweat glands.
Most evolutionary explanations describe why most traits work so well. Tiny variations in the eyes and ears and fingers of our ancestors influenced how many children they had. That process of natural selection explains our extraordinarily abilities to see, hear and feel things. It also shaped bones that are resistant to breaking, heart valves that usually last a lifetime, and minds that can recall details from events decades in the past. Natural selection is, however, only one aspect of evolution; genetic mutations, population migrations, and random changes in gene frequencies called genetic drift are also relevant. Together with natural selection, they offer explanations not only for why most things work so well, but also for traits that make us vulnerable to disease such as the narrow birth canals and our limited abilities to prevent cancer and infection. Explaining such apparently maladaptive traits is a major focus for evolutionary medicine.