Lecture 37—The Evolution of Aggression

In order for biological evolution to occur there must be variability in the population and that variability must be under genetic control—meaning that traits must be inherited. Is animal behavior under genetic control and is it subject to the principles of evolution? The answer is obviously “yes” in the case of animal instincts: the bee dancing behavior, the migratory drive of salmon, birds and butterflies. But what about human behavior, what if any of our emotions and behaviors are genetically determined and subject to evolutionary forces?

Sociobiology is the study of the biological basis of social behavior. In this lecture we examine the evidence for aggression among all species, including humans.

Aggression= abridgement of the rights of others forcing him to surrender something he owns or might have attained. It involves fighting, hostility, intimidation, and attack. It occurs in virtually all organisms and this alone suggests it has a strong genetic basis and since is varies among individuals of a species, this suggests that it can evolve. Moreover, aggression clearly has survival value under many circumstances:

Interspecific aggression (i.e. between species) is common in predator/prey, parasite/hostand competition interactions.

Intraspecific aggression occurs among individuals of the same species and is common in competition for resources, mates, between parents and offspring, moral authority, territory, and social dominance. We see it among battling individuals leading to intimidation and possible murder or in group conflict such as war.

Is there a selective advantage for aggression? Yes, for individuals are competing for vital resources (food, shelter, territory, breeding rights), protection of kin, and life itself. All of this argues that there must be a strong genetic component driving aggression. But is any of this learned?

How much of behavior is genetically determined?Behaviors seem to fall along a spectrum of being totally genetic (a stimulus occurs and the behavior automatically occurs—eye blink behavior) or totally learned (the learning of a particular language). Most anatomical structures and physiological reactions have both a genetic and environmental component so it is reasonable to expect that behavior will fall along a similar trend.

Evidence for learning in human aggression: When we look at human behavior we can see that cultural forces are at work: homicide rates and judicial torture have steadily fallen over the centuries in most countries; the frequency of war has dropped. People can be trained to control their temper. But it is not all a learned behavior.

Evidence for genetic involvement in human aggression:

Chimpanzees and humans have similar aggressive traits

Different human populations have similar aggressive traits.

Breeding experiments in other mammals show we can select for aggressive or docile individuals.

We can identify regions of the brain that when stimulated evoke rage or anger.

The physiological reactions of anger or fear follow a predictable pattern of nervous and hormonal response (flight of flight).

Since there is a genetic component in aggression, and aggression has survival value, why isn’t there a continual escalation of aggression?

Answer: there are disadvantages to too much aggression: possible injury, increased vulnerability to predators and disease, waste of time and energy. And in the case of humans and other social organisms, individuals need the cooperation of others in their group.

Terms/Concepts to Define:

Sociobiology

Aggression

Territoriality

Social Dominance Hierarchy

Fight or flight response

Can you answer these questions?

  1. Scientists argue that aggressive behavior evolve like physical traits. What is the logic and basis for that claim?
  2. Give three specific examples of interspecific aggression.
  3. Give three specific examples of intraspecific aggression.
  4. Since aggression is apparently a universal behavior, can you list three examples of behavior that we would find in plants or bacteria?
  5. What are animals competing for when they employ aggression and why can we claim this is consistent with evolutionary principles?
  6. In many social dominance systems, individuals at the bottom of the peeking order do not fight to the death for their rights even though it means they will not leave any offspring behind. Why would such a defeatist behavior evolve?
  7. Most social animals show an immediate distrust of strangers, why is this behavior a selective advantage?
  8. What is the most convincing argument that humans have a strong genetic tendency to be aggressive?
  9. Why is extreme aggression counterproductive in humans?
  10. Why do you think that primates seem to be the most lethally aggressive mammals?