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Evolving heart

By Elie Dolgin Evolving heart In 1948, 5,209 residents of a medium-sized New England town signed up for what would become the longest-running, systematic medical study in the world. The Framingham Heart Study, as it was called, was the first to show that smoking, obesity, and high cholesterol all increased people’s chances of developing heart disease. Six decades on, it’s also the first multigenerational human study to reveal that

By | August 1, 2009

Evolving heart

In 1948, 5,209 residents of a medium-sized New England town signed up for what would become the longest-running, systematic medical study in the world. The Framingham Heart Study, as it was called, was the first to show that smoking, obesity, and high cholesterol all increased people’s chances of developing heart disease. Six decades on, it’s also the first multigenerational human study to reveal that some of these same traits are actively undergoing natural selection.

The news should come as a surprise to many physicians. Ever since Charles Darwin, a prevailing attitude among medical practitioners has been that evolution does not operate in humans because modern medicine and culture have greatly leveled the playing field by homogenizing survival rates. The same sentiment has also been echoed by some leading evolutionary biologists, most famously the late Stephen Jay Gould.

Not so, says Yale University’s Stephen Stearns, who specializes in life history evolution. Survival rates have indeed evened out, particularly among children, yet human birth rates remain highly variable. Some people simply have more children than others. And if there’s variation in lifetime reproductive success, and if some heritable trait is associated with that variation, then natural selection must be acting. To demonstrate natural selection in humans, however, requires a multitude of data collected over successive generations. Enter Framingham.

In 2005, Raju Govindaraju, a medical geneticist at Boston University and the former director of the Framingham Heart Study Genetics Laboratory, approached Stearns with an idea: analyze the Framingham data from an evolutionary perspective. Govindaraju had read many of Stearns’s seminal evolution papers as a plant biology graduate student in the 1970s, and was sitting on a goldmine of Framingham data, so he gave Stearns a call. “That was one of the best phone calls I ever had,” Govindaraju says.

Are modern humans actively undergoing natural selection?

“The thing that immediately struck me,” recalls Stearns, “was that, gosh, we can actually study selection operating on a contemporary human population and thereby make clear to everybody that natural selection is operating on humans.”

Stearns hired a postdoc, Sean Byars, and together with Govindaraju and Douglas Ewbank, a University of Pennsylvania demographer, set to work analyzing a handful of medically relevant traits for their effects on women’s lifetime reproductive rates. They measured the statistical associations between the traits and family size in the first two generations of Framingham women to estimate the strength of natural selection and the potential genetic response to selection. Early results show that women with lower cholesterol, lower blood pressure, lower blood glucose, and women who conceive earlier in life and reached menopause at a later age, all had more offspring. As a result, a model based on the data (that also controlled for social factors that influence fertility) predicts that levels of all these genetically based traits will change over the next generation. “People, myself included, may have written off evolution in humans,” says Ewbank. “But it’s still there. It’s still happening.” But the same response won’t continue indefinitely. Obviously, says Stearns, “if selection continued to reduce cholesterol, we couldn’t build a brain.”

The researchers also plan to study several other traits of medical interest, including high-density lipoproteins, triglycerides, and bilirubin levels. The researchers have not yet looked at the genetics behind the trend, but neither did Darwin when he was studying natural selection, which can be demonstrated solely by showing statistically that a trait can be heritable and lead to larger family sizes.

“The findings are startling,” says Govindaraju. “This will set the stage for really a new way of thinking about modern populations,” and allow researchers to make short-term predictions about humanity’s future evolution. Peter Ellison, a Harvard evolutionary biologist who was not involved in the study, agrees. “We can now think about human phenotypes much more dynamically than we normally do,” he says.

Stearns and his colleagues, who plan to publish their findings later this year in a special supplement of the Proceedings of the National Academy of Sciences, are now looking at male reproduction and combining their phenotypic analyses with genome-wide single-nucleotide polymorphism data from Framingham participants to search for genetic signatures of the tradeoffs between survival and reproduction. “We’ll try to push Framingham as far as we can,” says Stearns.

Comments

Avatar of: Emma Leah

Emma Leah

Posts: 1

August 5, 2009

Is it not the social factors, filtered out in this study, that influence human reproductivity the most and will therefore determine the direction of human evolution? I find it hard to believe that cholesterol level would have a stronger influence on reproductive success than occupation or social standing, and both of these must be heavily influenced by genes. Would the cholesterol effect ever be noticeable?
Avatar of: anonymous poster

anonymous poster

Posts: 4

August 5, 2009

While survival of children may have leveled off where food is plentiful and since the emergence of antibiotics, we have no assurance that these conditions will pervail. Indeed, faced with massive disruptions that will likely occur in the years to come as the result of climate change, pressures will continue to hone the survial phenotype in our species and in others.
Avatar of: Jake Scott

Jake Scott

Posts: 2

August 8, 2009

Definetely some truth to this report.
Avatar of: Joe Carter

Joe Carter

Posts: 1

August 9, 2009

Despite China's attempt to curb population growth, the countries of Asia (including India) are still expanding in population at a significant rate. On the other hand, the populations of Europe and to a large extent, North America, are on a decline. Eventually, the dominant genes, in terms of the number of people, will be from parts of Asia. The population growth is by choice, not necessarily by evolution. This presents a case where the caucasian race may knock itself out of existence because of its choice to produce less offsprings from generation to generation... and cholesterol levels or triglyceride levels don't mean a darn thing!!
Avatar of: daniel miller

daniel miller

Posts: 40

August 10, 2009

What is being reported is self-evident from the Hardy-Weinberg equations. If there is any genetic factor that leads to more surviving, reproducing off-spring, that factor will be selected for. Some of these, such as later menopause, are self evident, but remember that there are limits so that selection does not continue indefinitely.\n\n As for cultural factors overwhelming these genetic factors, it doesn't work unless the genetic factors are not evenly distributed through the various social levels. If they are, then there is no reason to assume that the same things aren't being selected for in different cultural groups. I.e., if high cholesterol is over represented in a group that actively controls cholesterol, then possibly it won't be selected against. But if high cholesterol is evenly distributed through all socioeconomic levels, then it will be selected against in all socioeconomic levels, some more than others.
Avatar of: Jarrod Cusens

Jarrod Cusens

Posts: 2

August 12, 2009

Emma, are you suggesting that: 1) occupation or social standing have an influence on the number of offspring you have, 2) the occupation you choose or social standing you have is a result of you genetic makeup and not social and environmental? \n
Avatar of: quinn o

quinn o'neill

Posts: 1

August 16, 2009

Blood pressure, blood sugar and cholesterol levels aren?t strictly inherited traits; diet and lifestyle are important factors. Reproductive age may also have non-genetic determinants. Early-life factors, like low birth-weight and lack of paternal involvement during childhood, have been linked to early reproduction. \n\nIt?s probable that normalizing high blood pressure, high cholesterol and high blood sugar levels, either medically or through lifestyle modification, would reduce or eliminate negative effects on reproduction.\n \nObesity and polycystic ovary syndrome, which increase risk for high blood pressure, high cholesterol, and high blood sugar, remain very prevalent despite being important causes of female infertility.
Avatar of: Phil Terry

Phil Terry

Posts: 5

August 16, 2009

The article and comments have been helpful and interesting.\n\nI am intrigued by the fact that we *may* have found evidence showing how evolution is a dynamic active force in human populations.\n\nWhen speaking to general audiences, I have used other examples to demonstrate how evolution is active today. Two have been very powerful. First is the work of the Grants at Princeton - and the book about that work, "Beak of the Finch", by Jonathan Weiner. The Grants multi-decade research shows conclusively that evolution is very active - creating almost a constant vibration - with Galapagos finches.\n\nSecond, of course, and quite important at the moment, is the evolution of the flu virus. Mainstream adults who don't understand evolution are at a loss when trying to understand the flu. \n\nBut, now it seems we have a potential growing body of evidence that *may* demonstrate how evolution is at work in current human populations. \n\nThat evolution is active and with us humans here in the present may not be a profound insight for some evolutionary scientists. But, it sure is important for the general public to understand - and this genetic review of the Framingham data has the potential to be a powerful example for explaining the basics of evolutionary theory to a mainstream audience.\n\nI look forward to following this story. \n\nThanks,\n\nPhil \n\nVolunteer, Darwin Facebook Campaign\nOn our way to 1 million, help us get there\nhttp://www.facebook.com/group.php?gid=53320310123

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