AMSTERDAM—Insecure
about your height? You may want to avoid this tiny country by the North
Sea, whose population has gained an impressive 20 centimeters in the
past 150 years and is now officially the tallest on the planet.
Scientists chalk up most of that increase to rising wealth, a rich diet,
and good health care, but a new study suggests something else is going
on as well: The Dutch growth spurt may be an example of human evolution
in action.
The study, published online today in the Proceedings of the Royal Society B, shows that tall Dutch men on average have more children than their shorter counterparts,
and that more of their children survive. That suggests genes that help
make people tall are becoming more frequent among the Dutch, says
behavioral biologist and lead author Gert Stulp of the London School of
Hygiene & Tropical Medicine.
“This study drives home the message that the human population is
still subject to natural selection,” says Stephen Stearns, an
evolutionary biologist at Yale University who wasn’t involved in the
study. “It strikes at the core of our understanding of human nature, and
how malleable it is.” It also confirms what Stearns knows from personal
experience about the population in the northern Netherlands, where the
study took place: “Boy, they are tall.”
For many years, the U.S. population was the tallest in the world. In
the 18th century, American men were 5 to 8 centimeters taller than those
in the Netherlands. Today, Americans are the fattest, but they lost the
race for height to northern Europeans—including Danes, Norwegians,
Swedes, and Estonians—sometime in the 20th century.
Just how these peoples became so tall isn’t clear, however. Genetics
has an important effect on body height: Scientists have found at least
180 genes that influence how tall you become. Each one has only a small
effect, but together, they may explain up to 80% of the variation in
height within a population. Yet environmental factors play a huge role
as well. The children of Japanese immigrants to Hawaii, for instance,
grew much taller than their parents. Scientists assume that a diet rich
in milk and meat played a major role.
The Dutch have become so much taller in such a short period that
scientists chalk most of it up to their changing environment. As the
Netherlands developed, it became one of the world’s largest producers
and consumers of cheese and milk. An increasingly egalitarian
distribution of wealth and universal access to health care may also have
helped.
Still, scientists wonder whether natural selection has played a role
as well. For men, being tall is associated with better health,
attractiveness to the opposite sex, a better education, and higher
income—all of which could lead to more reproductive success, Stulp says.
Yet studies in the United States don’t show this. Stulp’s own
research among Wisconsinites born between 1937 and 1940, for instance,
showed that average-sized men had more children than shorter and taller
men, and shorter women had more children than those of average height.
Taken together, Stulp says, this suggests natural selection in the
United States pulls in the opposite direction of environmental factors
like diet, making people shorter instead of taller. That may explain why
the growth in average American height has leveled off.
Stulp—who says his towering 2-meter frame did not influence his
research interest—wondered if the same was true in his native country.
To find out, he and his colleagues turned to a database tracking key
life data for almost 100,000 people in the country’s three northern
provinces. The researchers included only people over 45 who were born in
the Netherlands to Dutch-born parents. This way, they had a relatively
accurate number of total children per subject (most people stop having
children after 45) and they also avoided the effects of immigration.
In the remaining sample of 42,616 people, taller men had more
children on average, despite the fact that they had their first child at
a higher age. The effect was small—an extra 0.24 children at most for
taller men—but highly significant. (Taller men also had a smaller chance
of remaining childless, and a higher chance of having a partner.) The
same effect wasn’t seen in women, who had the highest reproductive
success when they were of average height. The study suggests this may
be because taller women had a smaller chance of finding a mate, while
shorter women were at higher risk of losing a child.
Because tall men are likely to pass on the genes that made them tall,
the outcome suggests that—in contrast to Americans—the Dutch population
is evolving to become taller, Stulp says. “This is not what we’ve seen
in other studies—that’s what makes it exciting,” says evolutionary
biologist Simon Verhulst of the University of Groningen in the
Netherlands, who was Stulp’s Ph.D. adviser but wasn’t involved in the
current study. Verhulst points out that the team can’t be certain that
genes involved in height are actually becoming more frequent, however,
as the authors acknowledge.
The study suggests that sexual selection is at work in the Dutch
population, Stearns says: Dutch women may prefer taller men because they
expect them to have more resources to invest in their children. But
there are also other possibilities. It could be that taller men are more
resistant to disease, Stearns says, or that they are more likely to
divorce and start a second family. “It will be a difficult question to
answer.”
Another question is why tall men in Holland are at a reproductive
advantage but those in the United States are not. Stulp says he can only
speculate. One reason may be that humans often choose a partner who’s
not much shorter or taller than they are themselves. Because shorter
women in the United States have more children, tall men may do worse
than those of average height because they’re less likely to partner with
a short woman.
In the end, Stearns says, the advantage of tall Dutchmen may be only
temporary. Often in evolution, natural selection will favor one trend
for a number of generations, followed by a stabilization or even a
return to the opposite trend. In the United States, selection for height
appears to have occurred several centuries ago, leading to taller men,
and then it stopped. “Perhaps the Dutch caught up and actually overshot
the American men,” he says.
Source Article from http://feedproxy.google.com/~r/AscensionEarth2012/~3/2d366QnCzNM/did-natural-selection-make-dutch.html
Did natural selection make the Dutch the tallest people on the planet?
No comments:
Post a Comment