Biological Altruism: Theories which Explain the Presence of Altruistic Behavior in Animals.
What is Biological Altruism?
Why would a person or animal choose to commit a seemingly selfless act that is disadvantageous to their own survival? Biological altruism is defined as the behavioral tendency of organisms to promote the survival of another organism (usually of the same species) at the expense of their own survival. The cost and benefits are measured as reproductive fitness (the average contribution to the gene pool of the next generation or the number of offspring produced by the average individual). By behaving altruistically an organism reduces the number of offspring it will possibly produce (if it produces any at all, as may be the case) to increase the number of offspring produced by another organism. In other words, it reduces its own reproductive fitness to improve the reproductive fitness of another organism.
So why behave altruistically you ask? Well, according to evolutionary scientists altruism or altruistic behavior promotes the survival of a group. There are many competing theories that explain altruism in nature, namely, group selection, multilevel selection, reciprocal altruism, and kin selection all of which will be discussed here.
The Traditional Theory of Group Selection.
The traditional theory of group selection states that the genes being selected to be passed on to the next generation are determined at the level of the group. This contradicts Darwin’s theory of natural selection which states that selection occurs at the level of the individual, not the group. However, the notion of group selection was first mentioned by Darwin himself in his book titled The Descent of Man, and Selection in Relation To Sex, 1871. In it, he attempts to explain the evolution of altruism in humans. He states:
When two tribes of primeval man, living in the same country, came into competition, if (other things being equal) the one tribe included a great number of courageous, sympathetic and faithful members, who were always ready to warn each other of danger, to aid and defend each other, this tribe would succeed better and conquer the other (Darwin, 1871, p. 113).
Here he states that altruistic behavior promotes the survival of the group, however, such altruistic behavior creates problems for the theory of natural selection. He also states:
It is extremely doubtful whether the offspring of the more sympathetic and benevolent parents, or of those who were the most faithful to their comrades, would be reared in greater numbers than the children of selfish and treacherous parents belonging to the same tribe. He who was ready to sacrifice his life, as many a savage has been, rather than betray his comrades, would often leave no offspring to inherit his noble nature (Darwin, p. 114).
The fundamental tenet of evolution is the passage of genes from an individual to its offspring and gran-offspring. As stated by Darwin, the possession of altruistic traits is not evolutionary advantageous for an individual because it does not guarantee that individuals who possess those altruistic traits will produce more offspring, and thus would have a greater contribution to the gene pool of the next generation than their non-altruistic counterparts. In fact, it is believed that in a population where such altruistic traits are prevalent the emergence of a cheat trait which is hereditary would be evolutionarily advantageous to those that possess it.
For instance, hunting in a pack or cooperative hunting is a form of group selection. This is disadvantageous for the individual because injuries may be acquired during the hunt that may reduce the capacity of the individual to produce offspring, and -- in some cases, injuries obtained from hunting in a pack may even lead to death. However, if a trait (a hereditary one) did arise that promotes the behaviour of sitting back during a hunt while still being able to enjoy the spoils of the hunt, then it is reasonable to believe that such individuals would have a higher chance of producing offspring (would, on average, leave behind more offspring) than their pack members that carry the altruistic trait for cooperative hunting, because they are less likely to acquire injuries during the hunt.
Logic would then dictate that eventually, with time, the frequency of individuals with the selfish trait within the population would increase until eventually the population consisted of only individuals with the selfish trait. As a consequence, the selfish trait will become disadvantageous and individuals within the population, and eventually the population itself, will starve off and die because there is no one to cheat off of.
The Theory of Multi-level Selection.
To make up for the shortcomings of the traditional theory of group selection a new theory called multi-level selection theory emerged. This new model is less restrictive than the more traditional concept of group selection because it doesn’t limit the definition of a group to that of a population. Multi-level selection examines selection at all levels of organization rather than at the population level: That is at the level of distinct subgroups within a population, subgroups that are distinguished from each other based on the level of interaction between the individuals within that subgroup and that of another subgroup (individuals that interact with each other more intensely than with individuals arbitrarily chosen from the population), such as, the interaction between siblings versus that of extended family members. If said subgroups within the population vary in the number of altruists each contain, then the most altruistic subgroup would contribute more to the total gene pool because between group selection favors altruism. In theory
If said subgroups within the population vary in the number of altruists each contain, then the most altruistic subgroup would contribute more to the total gene pool because between group selection favors altruism. In theory, group-level adaptations can evolve in the natural world, however, the conditions that must be satisfied for such an evolution to occur is so limited that it is unlikely to occur. The flexibility of what is defined as a group is also a weak point of multi-level selection theory. By stretching the meaning of the word “group” the new models fails at being instructive, crystal clear, simple and convenient.
The new definition, however, allows the components to be partitioned and weighed against each other:
Once a “group” is defined as a subset of interacting individuals, the variance in the fitness of individuals can be partitioned into two statistical components: how fit the individual is with respect to his group mates, and how fit his group is with respect to other groups. … Examples include huddling for warmth, mobbing a predator, and Tooby’s example of pooling resources to get higher expected returns in a risky investment. In such cases, one can separate the benefits that accrue to the entire group (including me) and whatever benefits or costs are assumed by me but no one else in the group.
This model also has another critique, that individuals are still maximising their fitness (natural selection within the group is still taking place): The frequency of a gene within a breeding population is determined by the increase in fitness it gives the individual that possesses it in comparison to those that don’t possess the gene and not by its interaction with other genes. Natural selection would thus select for genes with an accompanying trait that increases the likelihood that the frequency of that gene would increase in the population and not for social partners.
An Evolutionary Stable Form of Altruism: Reciprocal Altruism.
In 1871 Robert Trivers developed the theory of reciprocal altruism, which describes a process whereby the actions of an organism temporarily reduces its fitness to increase the fitness of another organism with the expectation that the action will be reciprocated immediately or at a later date. Some examples of reciprocal altruism include the warning cries of some bird species, the social behavior of grooming group mates in primates, and favor systems seen in human populations.
Observation has shown that primates groom each other to alleviate tension and to rid themselves of parasites. This behavior is potentially detrimental to the survival of the individual engaging in such behavior because it distracts them from potential predators and mates. It was once believed that such behavior arose in primates to increase the reproductive success of their family much like kin altruism. However, when further studies were done it was found that primates were more likely to groom others that had groomed them, regardless of their relatedness.
The evolution of reciprocal altruism is promoted by several factors:
- Long lifetime. The long lifetime of the individuals increases the probability that individuals with altruistic traits will encounter each other and many altruistic situations.
- Dispersal rate. A low dispersal rate increases the probability that individuals will intermingle with the same neighbors on multiple occasions.
- The degree of mutual dependence. The dependence of members of a species on other members for protection from predation reduces the dispersal rate of the members and so increases the likelihood that members of that species will encounter altruistic situations. If the benefits of mutual dependence are greatest when the group is small, this increases the probability that individuals will constantly interact with the same small set of individuals.
Whereas, for any instance of cooperation to be considered reciprocal altruism the following conditions must be satisfied:
- The act of performing the altruistic behavior reduces the fitness of the organism performing the behavior in comparison to a selfish alternative.
- Recipients of the altruistic behavior must have their fitness increased when compared to non-recipients.
- The performance of the altruistic behavior is not determined by the receipt of immediate benefits.
- All the conditions mentioned above must apply to individuals engaging in reciprocal helping.
- There must be a way to detect and punish “cheaters”. Organisms that display reciprocal altruistic behavior would cease such behaviors with individuals who do not reciprocate the behavior. This condition distinguishes reciprocal altruism from cooperative behaviors such as simple mutualism. It prevents “cheaters” from exploiting altruistic behavior without consequences, and allows reciprocal altruism to evolve.
- An unlimited number of opportunities to exchange benefits must be present. With only one instance of contact between individuals the evolutionary stable behavior (that which confers the greatest amount of fitness to the individual performing it) would be to “cheat”. However, if the increase in fitness given by cooperation is greater than the act of “cheating”, with multiple interactions cooperation will yield the greatest additions to fitness and would thus be evolutionarily stable.
Although some species may meet the requirements for reciprocal altruism to occur, genuine cases of reciprocal altruistic behavior outside of human populations is actually rare. This may be due to the mental incapacity of individuals to keep track of their previous interactions with other individuals: That is to say their ability to distinguish between individuals who reciprocated altruistic behaviour from those who didn’t. Also, traits that create a bias towards immediate benefits over long term benefits may prevent the evolution of reciprocal altruism.
A Family Level Type of Selection: Kin Selection.
Kin selection is a type of natural selection in which individuals increase the reproductive success of their close relatives at the cost of their own survival and reproductive success. Relatives, particularly close relatives such as siblings, share a number of genes in common. An individual’s fitness is determined by the number of offspring it produces (its genetic contribution to the gene pool of subsequent generations). Since relatives have a number of genes in common, an individual can effectively pass its gene (those genes it shares with its relatives) to the next generation by assisting a relative in the rearing of its offspring: This is an example of natural selection working at the level of the family, not the individual. Kin altruism is altruistic behavior whose evolution is driven by kin selection.
An example of kin altruism in nature is the cooperative breeding behavior observed in Florida Scrub Jays. When the birds reach maturity some birds choose to leave their parents, while other choose to stay for a period of time (1-5 years) and help with the upbringing of subsequent broods sired by their parents. Birds that help in the rearing of the offspring of relatives are called “helpers”. Helpers not only assist in the gathering of food but also in the feeding of the young and protecting the nest from the invasion of other jays and predators. They do not, however, assist in the building of the nest or incubation.
Helpers stay with the breeding pair throughout the entirety of the breeding season and in so doing give up their chance at reproducing that year. Both breeding pair and helpers take turns in watching the nest for hawks while the others search for food. If a hawk is spotted, the lookout signals its presence with an alarm call, which in turn stimulates the response of diving for cover. A different alarm call signals the presence of snakes and other terrestrial predators, which stimulates the response of mobbing said predator. Studies done on Florida Scrub Jays have shown that on average the number of young birds that survive to leave the nest is greater in the nests that had helpers than nests that didn’t have helpers.
Kin altruism can evolve in a population because:
- The benefit of altruism to the recipient is greater than the cost to the individual performing the altruistic behavior.
- The recipient of the altruistic behavior are relatives; it is likely that these relatives share the same gene for altruism, and so helping to rear their offspring allows the gene to be passed on to the next generation and subsequent generation.
The presence of the above conditions makes kin altruism evolutionary stable and allows the behavior to persist in a population.