The nervous and endocrine systems that in part shape behaviors are the product of genetic instructions. If you take an extreme view, then, all behaviors have a genetic component. After all, it is the instructions in the genome that build the nervous system, muscles, and glands whose actions lead to a behavior. On the other hand, all behavior can be considered environmental since, without the appropriate environment, an organism would not develop normally and would therefore lose the ability to perform the behavior. On a very basic level, an organism in an environment without appropriate nutrients would lack the necessary energy to perform a given behavior. How, then, do we study the genetic basis of behavior? Modern approaches harness the power of molecular genetics, but more traditional analyses are also highly informative.

Artificial selection (Chapter 21), in which humans breed animals and plants for particular traits, provides strong evidence of the role of genetics in influencing behavior. Dogs were domesticated from wolves about 10,000 years ago. Today, there is great diversity among dogs, although they are all the same species. Obviously, there has been extensive selection on physical traits, with the result that a Dachshund, for example, looks quite different from a German Shepherd or Great Dane. Selection has also been applied to behavior: A Pointer has extraordinary “pointing” behavior that indicates the location of a hunter’s prey, and a Border Collie is an excellent herder (Fig. 45.6).

In the late 1950s, William Dilger studied the nest-building behavior of lovebirds in the genus Agapornis. Some species transport their nesting material in their beaks, whereas others tuck pieces into their tail feathers. Is there a genetic basis to the nesting behavior of these birds? To answer this question, Dilger set up crosses between species with different nest-building techniques to produce hybrids. He then observed how the hybrids built their nests. Interestingly, the hybrid offspring showed nest-building behavior intermediate between that of the parents: They tried to tuck material under their feathers, but were not successful. These experiments suggest that this behavior has a genetic basis.

It was not possible to do a true Mendelian analysis of the lovebirds’ behavior (Chapter 16). Such an analysis would require additional crosses, which were not possible because the hybrids were sterile. Like most behaviors, the lovebirds’ nest-building behavior is probably controlled by a large number of genes, each with a relatively small effect (Chapter 18). Therefore, it is difficult to identify which genes are responsible for the behavior. Even with molecular methods, the task of mapping and identifying the underlying genes when a large number of different genes govern a particular trait is daunting.

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Quick Check 1 Why is it typically difficult to identify individual genes that govern particular behaviors?