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How Did Altruism Evolve?
Four Categories of Social Behavior
If natural selection works to increase the fitness of an individual, how could it be possible for acts that decrease an individuals fitness to have evolved? This has always been a tricky issue, but theories have developed to attempt to explain it. First however, let us consider the four types of social behavior that exist: spite, selfish acts, cooperation, and altruism. Here are some quick explanations:
- Spite - Spiteful behaviors decrease the fitness of both the actor (the individual doing the behavior) and the recipient (the individual affected by the behavior). An example would be an animal destroying the food of another animal. Neither would benefit from this.
- Selfishness - Selfish behaviors increase the fitness of the actor and decrease the fitness of the recipient. An example of selfishness would be stealing.
- Cooperation - Cooperative behaviors increase the fitness of the actor and the recipient. An example would be the teamwork of a pack of wolves while hunting. Together they can acquire more food than alone.
- Altruism - Altruistic behaviors decrease the fitness of the actor and increase the fitness of the recipient. An example would be a prairies dog crying out and alerting other members of a predator. The prairie dog that warns the others makes itself a greater target for predation and thus decreases its fitness to help the others escape.
As it appears, altruistic alleles should die out since they lower the fitness of their bearers! However, it is not that simple. The success of an allele is not tied to its effect on a single individual. If a "deleterious" altruistic allele improves the fitness of others who carry that allele to a greater extent than it decreases the fitness of the individual then it will spread! This is known as Hamilton's Rule.
Kin Selection and Hamilton's Rule
Kin Selection is a subsection of Natural Selection that is based on indirect fitness gains. Indirect fitness is the fitness gained by "aiding the survival and reproduction of nondescendant relatives" who carry the same genes. There is no distinction between allele copies that are produced by the individual itself and those of its relatives so through kin selection an altruistic allele can succeed! Knowing this, we can deduce the inclusive fitness of an individual by taking the sum of its direct fitness and indirect fitness.
- Inclusive fitness = direct fitness + indirect fitness
But how can we predict whether an altruistic allele will be successful? This can be done by using Hamilton's rule:
- rB > C
In this equation r stands for the relatedness between an altruistic individual and the recipient, B stands for the benefit to the recipient, and C represents the cost to the recipient. The rB taken together represent the indirect benefit.
If rB is greater than C then the allele will spread. For the individual to know its relatedness to the beneficiary, some form of kin recognition is necessary. This can be visual, scent-based, proximity-based, or something else entirely.
What is the Green-beard Effect?
A Green-beard Effect can occur when an altruistic gene is pleiotropic (or multiple linked-genes can do this as well) and has the following three effects:
- A perceptible trait (for example a green beard phenotype)
- Ability of others to recognize this trait
- Preferential treatment for others with this phenotype
The Green-beard effect can be used to explain certain altruistic behaviors amongst individuals who have low relatedness but share the altruistic genes.
Check out this Wikipedia article for a more detailed explanation of the Green-beard effect.
How Does Altruism Spread Amongst Non-Relatives?
So we've discussed how altruistic alleles can spread amongst relatives and Greenbeards. That's pretty simple and fairly intuitive. As to how they spread among non-relatives, well that's a good question. We will now look at three different methods that explain this phenomenon: by-product mutualism and reciprocal altruism.
Practice the Boomerang Effect yourself!
By-product mutualism is not quite altruism in the traditional sense, however its effects do appear to be altruistic or cooperative. This phenomenon requires a population of selfish individuals who seek to maximize their own individual fitness. By-product mutualism occurs when these selfish behaviors have the side effect of benefiting the population itself! So the selfish actions appear cooperative but they are really not.
An additional aspect to consider is the Boomerang Effect. With the Boomerang Effect, the actor of a selfish trait harms itself to enact the trait. The name refers to how a boomerang can come back to hit the person who throws it. This study on black bird nest defence has a good example of the boomerang effect.
Reciprocal altruism occurs when an individual acts in a manner that temporarily decreases its own fitness while increasing another's fitness, with the expectation that the other individual will act in a similar manner at a later time.
To explain reciprocal altruism, we will need to look at something called the Prisoner's Dilemma. However for this we will need to consider evolutionarily sustainable strategies and payoff matrices.
The Prisoner's Dilemma
With the Prisoner's Dilemma consider the following scenario: two criminals are apprehended by the police and are in questioning. However, the police do not have enough evidence to tie them to this crime and need confessions to convict them. The prisoner's do not know this and believe that the police do have the evidence. If a prisoner defects and blames his fellow criminal then he is offered a reduced sentence and the other prisoner is convicted with a harsher sentence. If both cooperate with each other then they both go free. If both defect and blame each other then they both receive the original sentence (neither better nor harsher). this is visualized in the payoff matrix above.
As you can see, cooperation is not an ESS since it is highly like for defection to invade (notice how Temptation is higher than the Reward for cooperating). So how does this explain altruism?
The Iterated Prisoner's Dilemma
Oftentimes, however, it is seldom that an individual will only encounter another individual just once. Instead, the Prisoner's Dilemma will be iterated, or happen repeatedly. So imagine that ten rounds of the Prisoner's Dilemma occur and that there is a new strategy that we'll call tit for tat (an English expression for equivalent retaliation). With the tit for tat strategy, the individual will cooperate the during the first round and repeat what the other participant does for the additional rounds. The payoff matrices below visualize this:
As you can see above, both always defectors and always tit for tatters are evolutionarily stable strategies. So if the tit for tat strategy is occurring then it will remain!
What Are The Limitations of Reciprocal Altruism?
So altruism or cooperation can occur through reciprocal altruism, but is it likely? What are the limitations of this method? Well, for reciprocal altruism to occur, there must firstly be a high probability of multiple interactions between individuals. Also, for the tit for tat strategy to take place, individuals must be able to remember or identify defectors.
Additional Evolutionary Biology Help:
- What is an ESS? - Evolutionarily Stable Strategies Explained
More information about payoff matrices and ESS.