Vive la evolution
People are still evolving, heart study numbers say
By Carolyn Y. Johnson
October 26, 2009
Charles Darwin famously studied evolution in the Galapagos Islands. Now a team of scientists has chosen a decidedly less exotic locale to study the subject - Framingham.
Residents of the Boston suburb have long participated in a landmark study of their cardiovascular health, which has shown that smoking and high cholesterol increase risk of heart disease. Now data compiled for the heart study are providing evidence of human evolution in action - and have led researchers from Yale University, Boston University School of Medicine, and the University of Pennsylvania to predict that the community’s next generation of women will be slightly chubbier and shorter and have lower cholesterol.
Evolution occurs because organisms with advantageous traits are more likely to survive and pass on those traits to their offspring - a process called natural selection. It is widely believed that modern medicine and technology have brought human evolution to a screeching halt, since most people - and not only the “fittest’’ - can now survive and pass on their genes. But the authors of the new research say their work shows natural selection is still occurring.
“As an evolutionary biologist, I’ve been aware for some time that people in the medical community have the misapprehension that evolution is not occurring in humans,’’ said Stephen C. Stearns, a professor of ecology and evolutionary biology at Yale and the senior author of the paper published last week in the journal Proceedings of the National Academy of Sciences.
The researchers found that a number of traits were associated with having more children - being shorter and chubbier, having a first child at a younger age, experiencing menopause later, and having lower blood pressure or cholesterol - meaning that those traits would likely become more prevalent in the next generation.
“The way we went at it is to use methods in evolutionary genetics that have been applied to plant and animal populations for about 30 years and simply then treat humans as though they were just another population of organisms,’’ Stearns said.
The researchers took data from the Framingham Heart Study, the long-running research study that started in 1948 and has now followed over 14,000 subjects. They looked at two generations of women and chose a handful of traits to analyze: cholesterol level, blood pressure, height, weight, age at first childbearing, and age at menopause.
They controlled for the baby boom and bust and factors such as smoking and used a statistical method to account for the fact that some of the traits they were measuring varied over a person’s lifetime, such as cholesterol and weight. Then, they used the Framingham study’s extensive family history data to calculate the extent to which any particular trait is inherited. They also looked at those traits’ influence on the number of children women had.
They found that a number of partly inherited traits were correlated with women having more children, and that enabled the scientists to make predictions about the future. The next generation of women would have slightly lower cholesterol, be nearly half a pound heavier, and less than a 10th of an inch shorter. They also predicted that a woman’s age at first childbearing would decrease by half a month and that age at menopause would be pushed back a month.
The bottom line, they said, was that evolution is occurring in people.
David Haig, a biology professor at Harvard University, said that he frequently hears people say that evolution has ceased in people.
“I think that’s clearly wrong - I hear that all the time, and it’s clearly the case that we’re not having the same number of children, so there’s some ongoing selection,’’ Haig said.
But he pointed out it was possible that some external environmental factor was affecting both a trait and a woman’s propensity to have more children.
“Weight has changed dramatically over that time span, so there are clearly environmental effects on weight. But the question is what is responsible for the correlation between weight and the number of children [a woman gives birth to], and it could be that being slightly heavier is increasing your fertility or it could be that some unknown factor is both affecting weight and affecting fertility,’’ Haig said.
The study corroborates a 2001 study in the journal Evolution, in which an analysis of 2,710 pairs of twins found that women who bear children at a younger age have more children, and that the trait is passed from one generation to the next. That suggested natural selection was having an effect.
Even though researchers found evidence that humans are evolving, it is occurring at a slow pace - slower than evolution in Galapagos finches and Trinidadian guppies, and more on par with evolution measured in New Zealand chinook salmon and Hawaiian mosquitofish.
What the researchers cannot definitively say is what is causing certain traits to be advantageous, or what set of genetic variations is causing any of the traits - which are also influenced by environmental factors. The traits they studied seem to account for only about 5 percent of the variation in reproductive success.
“The other 95 percent gives all the people who worry about their free will and decisions plenty of room to play in,’’ Stearns said.
Carolyn Y. Johnson can be reached at email@example.com.