THE GENETICS
AND EVOLUTION OF BEHAVIOR
Darwin firmly believed that all
of his claims about natural selection applied both to organisms’ structural
traits (like a finch’s bill width, or a fish’s coloration) and to its behavior
traits (like being a protective parent or a skilled problem solver). Indeed,
the evolutionary logic is the same whether we’re considering an animal’s
anatomy or its behaviors, capacities, and preferences. For behaviors just as
for physical traits, evolu-tion requires the same three conditions: variation
among individuals; a higher rate of reproductive success for individuals with
some of the variations; and some means of passing the successful variation from
one generation to the next. If this selection process continues generation
after generation, eventually the advantageous traits or behaviors will
characterize the entire species.
But how does evolution shape behaviors, or capacities, or preferences? We’ve suggested the answer in some of our earlier examples: Psychological traits, just like physical features, are part of an animal’s phenotype. If this phenotype makes it more likely that the animal will survive and reproduce, then the animal’s genes will be well represented in the next genera-tion. And to the extent that an animal’s behaviors, capacities, and preferences are shaped by those genes, then these traits also are likely to be well represented in the next generation.
Of course, the genotype does not
produce these traits directly. Instead, the genes (as always) guide the
production of proteins, which in turn, lead to the construction of a nervous
system with a specific design. It’s then the nervous system, modulated by other
signaling systems (e.g., hormones) and various environmental influences, that
makes the behaviors (or capacities or preferences) more likely.
In fact, many behaviors in many
organisms have been shaped in exactly this way— and this point leads us to
another common misunderstanding about evolution: Some people have the idea that
natural selection would lead to rigidly defined behaviors— favoring organisms
that always produce the same (presumably adaptive) responses to a predator, or
to a mate, or to their young. If we observe flexibility in a response,
there-fore, or if we see that a behavior is shaped by learning, we conclude
that the behavior depends on the organism’s experience, not evolution.
Once again, though, this view is
mistaken. To be sure, evolution has in many cases guided an organism (human or
otherwise) toward relatively well-defined behaviors, like a particular
courtship dance or a specific style of nest building repeated season after
season (Figure 2.15). Even in these cases, though, evolution has favored
mechanisms that produce flexibility
in how an animal acts.
To understand this point, bear in
mind that virtually all species evolved in changing environments. In some
cases, the organism encountered changes from one day to the next, so that the
berry bush that was filled with fruit yesterday is empty today; the pred-ator
that lurked by the water hole just a day ago has now moved to other hunting
grounds. In other cases, the changes were slower paced—climatic conditions
might vary from one year to the next, and food supplies that were available in
some seasons might disappear in others. In any of these settings, animals that
always behaved in the same manner would fare poorly. As a result, natural
selection would favor individuals that could shift their behavior in response to new circumstances and that
could rapidly deploy new skills appropriate for an altered setting.
This need for flexibility in an
animal’s behavior is amplified by the process of nicheconstruction—a process in which organisms, through their
behaviors, alter their environ-ments and thus create their own circumstances.
In biology, a niche refers to all of
the factors in an organism’s environment that have the potential to affect its
life. Animals can and do alter these factors; and by doing so, they alter the
opportunities and challenges they face.
These considerations are
especially important for humans, because our ancestors were quite skilled in
niche construction—building new shelters, finding (or develop-ing) new sources
of food, and creating social alliances that favored individuals with
communication skills. Because of changes like these, ancient humans would have
had a substantial survival advantage if they were flexible in their behavior,
responsive to new cues in the environment, and able to share information with
others. Thus, natural selection would, in many cases, have favored innovators, learners, and probably teachers (Csibra & Gergely, 2009;
Tomasello, 2000).
Evolution therefore favors
flexibility and learning—in part because environments inevitably vary, and in
part because organisms end up changing their environment and need to cope with
those changes. But can we be more specific about the origins of particular
human behaviors? As we’ll see throughout this text, we can often draw important
lessons from considerations of the proximate causes of a behavior, with an
emphasis on genetics; just as often, we can draw insights by asking about
ultimate causes, with an emphasis on evolu-tion. Let’s develop these points by
considering three extended examples.
The first case we’ll consider
involves the expression of emotions—and
smiles in par-ticular. Here we’ll see what can be learned by comparisons across
cultures and species. The second case involves intelligence and illustrates how research can illumi-nate the
proximate causes of a complex capacity. Our third case involves a topic that’s
central to natural selection—sexual reproduction. How do we choose our
part-ners? Why do we choose to be loyal to our partner (or not)? Research in
the last few decades has made it clear that an evolutionary perspective on
these questions can be enormously useful.
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