Genetics affects behavior at many levels. While we might be tempted to think of genes as guiding behavior ("my genes made me do it"), examples of direct genetic instructions for behavior are rare. Instead, the most prevalent effect of genes on behavior is to set limits, or restraints, on the range of behavior expressed by animals. Thus behavioral options are limited by genes, rather than genes directing behavior.
Humans figured out long ago that behavioral, as well as physical, traits run in families. When looking more broadly at animals, comparing behavior among family groups is a powerful tool for assessing the influence of genes on behavior. Natural processes, such as differentiation among subspecies, also allows comparisons which implicate genetic factors in behavioral differences among animals. In agriculture or in the laboratory, animals are bred to produce strains with differing characteristics, including behavior; comparisons among strains is also an important source of behavioral genetic information.
Simple genetic models of behavior
Behavioral genetics begins by looking for associations between genetic differences among animals and behavioral differences. Differences among families, subspecies, and species all help to indicate how much of a role genes have in behavior. Because an animal's behavioral phenotype is determined by a combination of environmental and genetic factors, scientists can estimate genetic influences by keeping animals in a shared environment. For example, testing to see if differences among subspecies from different environments persist when they are kept together allows comparison of genetic and environmental effects on behavior.
Single genes may have dramatic effects on complex behavioral systems. In surveys of Drosophila (fruit fly) mutants, scientists found many single-gene mutations which affect behavior. While the exact function of the genes are still unknown, in most cases, sophisticated techiques have begun to unravel the ways in which a simple mutation alters complex behavior. Other examples are extreme behavioral pathologies, such as Huntington's chorea in humans, in which mutations of a single gene cause myriad behavioral problems in humans.
Behavior as a quantitative trait
What is a quantitative trait? Many traits, or phenotypes, vary continuously, rather than having discrete forms. A good example of this is height in humans;"normal" adult humans range in height over a span of more than half a meter (more than two feet). Usually quantitative traits are normally distributed; a graph of the trait results in a bell-shaped curve.
Behavioral phenotypes often vary in this way. This is particularly true of phenotypes for activity levels. Honey bee colony pollen collection rates serve as a classic example of this.
A normally distributed trait, whether it is physical, such as height,
or behavioral, such as pollen collection, is usually the result
of the contribution of a number of genes. Recent advances in genetics
now allow scientists to map the genes having the greatest influence
on the trait. These genes are called quantitative trait loci, or
QTL's. QTL analysis is rapidly becoming the standard format for
studying genetic influences on behavior.
Artificial selection and behavior
The basis for artificial and natural selection on behavior is genetic variance. Animal breeders can only select for behavior found within the range of variation. The potential for artificial selection to modify a trait is assessed by measuring its heritability.
Artificial selection, in scientific laboratories and in animal husbandry, has dramatic effects on behavior. Perhaps the broadest range of artificially selected behavior is seen in domestic dogs, which display a wide variety of behavioral attributes. These behavioral patterns are the result of selection for dogs which assist humans in work (e.g. retrievers, shepherds) or as companion animals.
Most domestic livestock (such as chickens, horses, cattle, sheep, goats and swine) reflect the results of artificial selection for manageability in confinement, ease of training, and docility.
Natural selection and behavior
The behavior of animals in their natural setting results from many generations of natural selection; usually we observe the result of natural selection rather than natural selection in action.
The introduction of exotic plants and animals into ecosystems creates new opportunities for natural selection to act. Examples of striking behavioral differences between animals in new habitats and how the behavior of populations in their original habitats include:
copyright ©2001 Michael D. Breed, all rights reserved