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Does the same process work with naturally occurring selection? Does natural selection
explain our human tendencies? Nature has indeed selected advantageous variations from
the new gene combinations produced at each human conception plus occasional mutations
(random errors in gene replication that become nature’s preliminary tests of alternative pos-
sibilities). But the tight genetic leash that predisposes a dog’s retrieving, a cat’s pouncing, or
a bird’s nesting is looser on humans. The genes selected during our ancestral history provide
more than a long leash; they give us a great capacity to learn and therefore to adapt to life
in varied environments, from the tundra to the jungle. Genes and experience together wire
the brain. Our adaptive flexibility in responding to different environments contributes to our
fitness — our ability to survive and reproduce.

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Evolutionary Success Helps Explain Similarities
Human differences grab our attention. The Guinness World Records, for example, entertain
us by highlighting the tallest, oldest, longest-haired, and most-tattooed humans. But our
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deep similarities also demand explanation. At the Amsterdam Airport’s international arriv-
als area, one sees the same delighted joy on the faces of Indonesian grandmothers, Chinese ®
children, and homecoming Dutch. In our genes and our behaviors, we humans are more AP Science Practice
alike than different. “Your DNA and mine are 99.9 percent the same,” observed Francis Research
Collins (2007), who led the human genome’s decoding. “At the DNA level, we are clearly In research terminology, a popu-
all part of one big worldwide family.” lation refers to all those in a group
being studied, say in a survey or
experiment, from which a sample
Our Genetic Legacy can be drawn. You can review
this and other terms related to
Our similarities reflect our shared human genome — our common set of genes. No more than research methods in Unit 0.
5 percent of the genetic differences among humans arise from population group differences.
Some 95 percent of genetic variation exists within populations (Rosenberg et al., 2002). Thus,
the typical genetic difference between two South Africans or between two Singaporeans is
much greater than the average difference between the two groups (Lewontin, 1982).
®
And how did we develop this shared human genome? At the dawn of human history, AP Science Practice
our ancestors faced certain questions: Who is my ally, who is my foe? With whom should Research
I mate? What food should I eat? Some individuals answered those questions more suc-
cessfully than others. For example, women who experienced nausea in the critical first The text refers to average differ-
ences here. Recall from Unit 0
3 months of pregnancy were genetically predisposed to avoid certain bitter, strongly fla- that the average is determined by
vored, and novel foods. Avoiding such foods had survival value, since they are the very calculating the mean, which is a
foods most often toxic to prenatal development (Profet, 1992; Schmitt & Pilcher, 2004). Early measure of central tendency.
humans disposed to eat nourishing rather than poisonous foods survived to contribute their
genes to later generations. Those who deemed leopards “nice to pet” often did not.
Similarly successful were those whose mating helped them produce and nurture
offspring. Over generations, the genes of individuals not disposed to mate or nurture
tended to be lost from the human gene pool. As success-enhancing genes continued to be
selected, behavioral tendencies and learning capacities emerged that prepared our Stone
Age ancestors to survive, reproduce, and send their genes into the future, and into you.
For all such universal human tendencies, from our intense need to give parental care
to our shared fears and lusts, evolutionary theory proposes a single, all-encompassing
explanation (Schloss, 2009).
As heirs to this prehistoric legacy, we were not born as unprogrammed “blank slates.”
Instead, we are genetically predisposed to think and act in ways that promoted our biologi-
cal ancestors’ survival and reproduction. But in some ways, we are biologically prepared for
a world that no longer exists. We love the taste of sweets and fats, nutrients that prepared mutation a random error in
our physically active ancestors to survive food shortages. Few of us now hunt and gather gene replication that leads to a
for our food; instead, we too readily find sweets and fats in fast-food outlets and vending change.
machines. Our deeply rooted natural dispositions are mismatched with today’s proliferation

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