“How extremely stupid not to have thought of that!” was Thomas Henry Huxley’s reflection on reading Darwin’s Origin of Species. What might elicit such a reaction from a contemporary biologist? Today the question is answered by Randolph M. Nesse, Foundation Professor of Life Sciences and Founding Director of the Center for Evolution & Medicine at Arizona State University. With George C. Williams, Nesse coauthored the classic Why We Get Sick: The New Science of Darwinian Medicine (1996).
I was the junior doctor on call, sitting in the nursing station on a warm summer evening in 1978 browsing the book I bought earlier in the day at the original Ann Arbor Borders bookstore. It was a huge volume by E. O. Wilson called Sociobiology. I was hoping that my wife would forgive me for spending our month’s entertainment budget at one go.
Within minutes of starting to read, I found a paragraph that made me stop, drop the book, and look out across the river valley for many minutes, shaken by the realization that my understanding of biology had been fundamentally mistaken.
Love joins hate; aggression, fear; expansiveness, withdrawal, and so on; in blends designed not to promote the happiness and survival of the individual, but to favor the maximum transmission of the controlling genes. (E. O. Wilson, Sociobiology, 1975, p. 4)
I had been taught that selection shaped bodies for health. I thought that happiness was normal, and that negative emotions were abnormal. But a moment’s reflection made it clear that an allele that increases an individual’s inclusive fitness will tend to become more common over the generations, even if it harms health, shortens life, or decreases happiness. In a flash, my view of the living world was transformed.
Why didn’t I think of that?! The complete absence of evolution in my otherwise superb medical education was a sufficient explanation. But I nonetheless felt extremely stupid not to think of it. As an undergraduate, I was preoccupied with a question that got me close. Rates of aging are influenced strongly by genetic variations, so why didn’t selection eliminate alleles that speed aging? I followed Peter Medawar’s conclusion that some alleles with effects at older ages were outside the reach of selection, but I wondered if other alleles might benefit a species by eliminating some individuals so the species could evolve more quickly in response to changing environments.
Ten subsequent years of intense medical education did nothing to correct my elementary misconception. But finally, frustrated with the lack of a scientific foundation for understanding behavior, and intrigued by reading Sociobiology, I started spending time at the museum. The biologists there were remarkably welcoming. However, when I shared my naïve notion about senescence, they hooted and pointed me to the 1957 article in which George Williams observed that alleles that caused senescence would nonetheless be selected for if they also gave benefits earlier in life when selection was stronger.
Reading that article changed my career, setting off a year of work to create a method for calculating the strength of selection on senescence. Applied to survival tables for populations in the wild, it showed selection coefficients vastly larger than could be explained by mutation accumulation. Williams’s notion of antagonistic pleiotropy offered an explanation for aging. What about cancer? What about schizophrenia? I started looking for an evolutionary biologist who wanted to work on medicine. George Williams was looking for a physician who wanted to apply evolutionary biology to disease.
In a series of papers and a book about how evolutionary biology could provide a deeper understanding of disease, we argued that evolutionary explanations were needed not only for why parts of the body work so well, but also for why some traits are so vulnerable to failure. It seems obvious now, but then it was a revelation. Mismatch with the modern environment became a theme, as well as co-evolution, inevitable tradeoffs, the limits of natural selection, and the utility of defensive responses. But at the core is the idea that selection maximizes inclusive fitness, not health.
The exigencies of sex offer the best example. In a collaboration with Dan Kruger, I looked at male vs. female mortality rates in WHO data and discovered that rates for young men are three times higher than those for young women in most modern societies. For many species, males that compete in ways that increase mating success get advantages—for their genes—at a severe costs to their health, to say nothing of the “hate; aggression, fear; expansiveness, withdrawal; and so on” aroused by sexual competition.
In a nicer vein, parents everywhere sacrifice their health and even their lives for their offspring, thereby benefiting alleles they share with those offspring. Usually the interests are aligned. But, as David Haig has argued, alleles can spread if they benefit paternal genomes at the expense of the maternal genomes, or vice versa. Then there are alleles that enhance their own transmission despite causing poor health or early death. Usually they are kept in check by recombination, but not always.
The idea that selection maximizes gene transmission even at the expense of health, is deeply disturbing. Maybe that helps to explain why the paragraph that so inspired me was edited out of later editions of Sociobiology. It is a pity. The idea, especially in sophisticated frameworks like Michael Wade’s, has huge potential to explain vulnerability to diseases. It can also give physicians, and everyone else, a more accurate view of life.
It can be frustrating to try to explain evolution to people who have had a different schema for explaining life. What works best, in my experience, is to give examples of how evolutionary biology is useful to understand why our bodies are vulnerable to disease, and how this knowledge can improve human health. Many people quickly realize that evolution is, like genetics and physiology, a basic science essential for medicine and public health. Once they do, abstract debates about evolution are replaced by fascination about why our bodies are not better, and recognition that evolutionary understanding can help relieve sickness and suffering.
If you find these ideas as exciting as I do, you may want to attend the first meeting of the International Society for Evolution, Medicine, & Public Health, to be held March 19–21, 2015, at Arizona State University (details).