THE EVOLUTIONARY STORY OF HOMO SAPIENS 565
Natural selection and evolution in real time
Many people think that humans living in the twenty-first century are largely free
of natural selection. Don’t modern medicine, hygiene, and diet flatten the fitness
landscape? No, not entirely.
The Framingham Heart Study is the longest-running longitudinal study in
medical history. It has collected a wealth of physical and demographic data on
three generations of Americans. We can ask how fitness changes with the values
of the traits the study measured [6]. FIGURE 21.21A shows the fitness function for
total cholesterol level in the blood, based on data on lifetime reproductive suc-
cess from 1948 to 2008. The implications are dramatic. Individuals with low total
cholesterol (15 mg/l) had an average of just over three children in their lifetime.
Those with high cholesterol levels (35 mg/l) had only two children. There is strong
directional selection favoring lower cholesterol levels, acting right now, on people
living in the United States.
An obvious next question is: Will selection on cholesterol levels cause evolution-
ary change? The answer is yes. The heritability of total cholesterol is h^2 = 0.61 in
the population studied by the Framingham Heart Study. Thus we have the two
ingredients needed for evolution of a quantitative trait: directional selection and
heritable variation (see Chapter 6).
Many other traits are also currently under selection, even in industrialized soci-
eties [68]. One is body height. FIGURE 21.21B shows the fitness function (again
based on lifetime reproductive success) for height in Finland between 1935 and- Stabilizing selection acted on females: women with average height had the
largest number of children over their lifetime, while short and tall individuals had
fewer. In males, however, directional selection was acting, favoring even taller
people in what is already one of the world’s tallest populations. Height is highly
heritable (h^2 averages about 0.8 across human populations), so we can expect that
it too will evolve. How height affects fitness is not clear.
Many people think that modern hygiene and medicine have alleviated or even
ended natural selection on humans. This is true for some genes and for some traits
in some populations. But in much of the world, many people do not have access
to sophisticated health care. And even in affluent and technologically advanced
societies, natural selection continues to act on modern Homo sapiens, and to shape
the evolution of our species.
Evolutionary mismatches
Environments are constantly changing, and adaption always lags behind. While this
is true for all species, it is a particularly conspicuous fact in our own species. Some
of the biggest challenges to human health are the results of bodies that have not yet
adapted to the agricultural revolution that began just a few hundred generations ago.
Obesity is a growing epidemic in many countries. It is strongly correlated with
major causes of mortality, including diabetes and heart disease. Americans with a
body mass index^4 over 40 have a 2.8 times greater risk of developing diabetes than
do those with average weight [49]. Diabetes brings with it greatly increased risks of
blindness, kidney failure, neuropathy, and other maladies. In 2015, more than 400
million people worldwide had diabetes. The tragedy of this situation is that most of
these cases are preventable. They result largely from low physical activity and bad
diet, such as consumption of the high-fructose corn syrup used in processed foods
and soft drinks.(^4) The body mass index (BMI) is one measure of obesity. It is defined as a person’s mass divided
by the square of height, and is expressed in units of kilograms per square meter (kg/m^2 ). In
many populations, a BMI in the range of 18–25 is considered normal. Futuyma Kirkpatrick Evolution, 4e
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Evolution4e_21.21.ai Date 02-02-2017
(A)
(B)
Relative height (s.d.)
Females
Males
–2 –1 0 1 2
2.4
2.2
2.3
2.1
Total cholesterol (mg/l)
15 25 35
8
Fitness
Fitness
2
4
1
0
FIGURE 21.21 Selection is still acting on
humans in modern industrial societies.
(A) The red curve shows the fitness func-
tion acting on total cholesterol for a U.S.
population between 1948 and 2008. Fit-
ness was measured as lifetime reproduc-
tive success; each circle represents an in-
dividual. (B) The fitness function acting on
body height in Finland between 1935 and
- Height is measured in standard de-
viations (s.d.) from the mean, with shorter
individuals to the left and taller ones to the
right. (The meaning of a standard devia-
tion is discussed in the Appendix.) (A after
[6]; B after [70].)
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