Many public-policy decisions are based on implicit assumptions about “human nature,” and it is currently popular to speculate about how evolution might have shaped human behavior and psychology. But this raises some important questions: are humans continuing to evolve – and, if so, is our basic biological nature changing – or has modern culture stopped evolution?
For some traits, we do not have to speculate – we can measure and compare on the basis of studies covering thousands of individuals over several generations. Such studies have not yet been done on most of the traits where speculation is popular, but they have been done on some traits of medical interest. What have we learned?
Two approaches of scientists
Scientists are taking two approaches. In one, they sequence the DNA of particular genes in many individuals with or without some determined health condition and look for differences between the two groups. This genetic approach measures effects that have accumulated over hundreds to thousands of generations, and the message is clear: humans have evolved in these respects fairly recently, some in one direction, some in another, depending on their environment and other conditions encountered.
From this approach, we have learned, for example, that the ability to digest milk as adults evolved within the last 10,000 years – and several times – in cultures that domesticated sheep, goats, or cattle. Similar studies have taught us that sensitivity to alcohol consumption and resistance to diseases like malaria and leprosy also evolved within the last several thousand years.
Some scientists, myself included, have taken a different approach. Instead of looking for changes in genes that take many generations to accumulate until they can be detected, we have measured natural selection directly. This method can reveal selection in action, working over periods of time as short as one generation – so that it can answer the question of whether modern culture has stopped evolution.
About women in Massachusetts
The message of this approach is also clear: natural selection continues to operate in modern cultures. Whether it will operate consistently enough for a long enough time to produce significant genetic change can be answered only by future generations. Nevertheless, it is interesting to see in what direction natural selection is starting to shape us. Some of the answers are surprising.
We measured natural selection operating on women in Framingham, Massachusetts, where a long-term medical study on heart disease produced the data that we used. The women were born between 1892 and 1956. We found significant selection and projected that if it continued for ten generations, the women would evolve to be about two centimeters shorter and have their first child about five months earlier.
This is a surprising result, because women in developed countries have become taller thanks to better nutrition, and are having children later in life for many reasons, some of them cultural. So what is going on here? Three things:
Why women become smaller
First, we know that giving birth for the first time when younger carries a cost in increased infant mortality, but modern medicine and hygiene have strikingly reduced infant mortality, reducing the cost of younger age at first birth. We therefore expect a shift toward earlier reproduction, because the costs previously associated with doing so have disappeared – exactly what we found in Framingham.
And we expect those younger women to be shorter simply because they have had less time to grow. In five other cases, scientists found women in developed countries maturing earlier, in two of them at smaller size (in the other three cases, size was not measured). It is too early to say that this is a general trend, but right now, all signs point that way.
Second, we measured the effects of natural selection by noting that women who were shorter and first gave birth when younger had more children. But genes are only one of many factors that influence height and age at first birth. Personal decisions, nutrition, income level, education, and religious affiliation all enter the mix.
Five percent biology, 95 percent culture
When we estimated how much of the variation among individuals could be attributed to biology, the answer was less than 5%. That left 95% of the variation to be explained by the effects of culture and individual decisions. But, while the effects of biology on the traits that we measured are relatively small for humans living in complex modern cultures, even small effects, when repeated consistently, will accumulate.
Third, traits like these are always the result of interactions between genes and environment. A woman could have genes that would tend to make her taller than average but end up shorter than average if she had been poorly nourished as a child.
If evolution took its steady course and changed the genetic basis of height and age at first birth, we might not see women ten generations later who were shorter and matured earlier, for the effects of nutrition and culture could more than compensate for the genetic change. As a colleague of mine likes to put it, one good school-lunch program could be enough to obscure the biological effects.
Even when we focus on a simple physical trait like height, natural selection in humans turns out to be a multifaceted and nuanced process. Similar studies on human behavior and psychology, where causation is more complex, remain beyond our grasp. In such cases, silence may be wiser than speculation.
Stephen C. Stearns is Professor of Ecology and Evolutionary Biology at Yale University.