Headlines

As reported by The Hartford Courant, February 11, 2011.

Considering Evolution and C-Sections

Have Procedures Led To Higher Birth Weight? Will They Lead To Higher IQ?

By William Weir

This being the occasion of Charles Darwin's 202nd birthday, it seems as good a time as any to consider the evolutionary role of cesarean sections.

Dr. Joseph Walsh, an obstetrician at the University of Connecticut Health Center, thinks that the increasing numbers of C-sections have played a part in natural selection, particularly in mothers' pelvic size and babies' birth weight.

Not only does it account in part for babies' higher birth weight, he says, but it possibly could lead to larger brains and higher IQ. He discusses the theory in this month's issue of the American Journal of Obstetrics & Gynecology.

Walsh points out that the first cesarean sections the surgical delivery of a baby by cutting through the mother's abdominal and uterine walls in the United States were performed around 150 years ago, about the time that Darwin's "Origin of the Species" was published.

Over time, medical advances made the operations safer and more common. In the developed world, mortality rates during childbirth have decreased dramatically because of C-sections. In the U.S. about 15 percent of deliveries are cesarean.

The C-section liberated humans from the natural selection against mothers with smaller pelvises carrying larger fetuses. Because both mothers and their children survive delivery, they pass down their traits to next generations. As a result, birth weight has increased. A study Walsh cites shows that birth weights between 1960 and 1997 have increased 1 to 2 percent.

Walsh first posited his theory in 2008 in the journal The American Biology Teacher. He said he was prompted to write it after the 2005 court case over whether intelligent design should be taught in the Dover, Pa., school district to balance the teaching of evolution. He wanted his article to fuel the argument for evolution.

"It seemed the biology teachers needed some practical example of evolution in action," he said. "People think of it as something that happened, and it's something that's still happening."

More speculatively, Walsh wrote in 2008, the rise in C-sections could lead to higher average IQ. The larger the baby's cranium, the more potential trouble there is in a natural birth. But because C-sections minimize such risks, more large-brained babies survive delivery. Studies have shown a correlation between IQ and newborn brain size.

He revisits the matter of C-sections and natural selection this month in a letter responding to an earlier study published in the American Journal of Obstetrics & Gynecology on changes in birth weight. Walsh faults that study for failing to count C-sections as a factor.

"The maternal pelvis can get smaller over time and fetal birth weight can get greater over time, because there is now nothing to limit these changes," he wrote.

McGill University's Michael Kramer, one of the authors of the earlier study, writes back in the same journal that he doesn't believe that there has been enough time for C-sections to affect natural selection. Increases in birth weight instead could be attributed to other changes, such as the average weight of mothers, diet, smoking habits and such.

David Haig, a professor of biology at Harvard, said he sees some truth to both arguments. He also doubts that enough time has passed for significant changes in natural selection. But that doesn't mean medical advances aren't affecting our evolution.

"I agree with Walsh that we are changing the nature of selection," Haig said, "and that could change our biology."