The basis of evolution is to pass on traits that improve an individual’s chance of survival and reproduction (fitness) to the next generation. An important example of one of these traits is social behaviour; how do individuals interact together to optimise their fitness? Sociality in nature can range from bees, where the majority of the hive work selflessly for a single reproductive female, or polar bears, where the male doesn’t even stick around to raise his own cubs. Many decades of research on evolutionary genetic modelling have provided some answers on how social structures evolve for different species, as well as the evolutionary conundrum of altruism.
If the individual’s ultimate goal is to increase their own fitness, how do instances of cooperation and altruism, where an individual increases the fitness of others at its own detriment, evolve within nature?
The answer lies in the relatedness between individuals within the population, otherwise known as their kinship. The degree of kinship between two individuals is associated with the proportion of the genome that is inherited Identical by Descent (IBD), descent referring to being inherited identically from a recent common ancestor. The coefficient of relatedness (r) is calculated by finding the most recent common ancestor and determining the probability that both downstream individuals receives the same copy of any gene (allele).
The straightforward way to envision your genetic information being passed on is through reproduction, but W. D. Hamilton saw that the more abstract way is to increase the likelihood of your close relatives passing on their genes. This was termed inclusive fitness. Hamilton’s Rule predicted whether a trait/allele (A) would be beneficial to an animal community in terms of inclusive fitness, and be favoured by natural selection. It states that the frequency of allele (A) will increase in the population if the benefits (b) to related individuals (r) outweighs the cost (c) to themselves.
A prominent example of altruism in found in prairie dog communities. A single scout warns the colony of an oncoming predator by emitting a piercing alarm call, at the detriment of pinpointing its location to the predator. Interestingly, this is a sex-biased trait involving females 65% of the time, even though they only comprised 30% of the population. How did the gender disparity co-evolve with this social behaviour?
The answer comes back to the genetic relatedness between individuals, involving the migratory patterns of adult males. Males move from prairie to prairie in search of more reproductive opportunities, while females stay put and care for their young. This leads to females having a much higher average relatedness within the population. This higher relatedness in Hamilton’s Rule outweighs the cost to the individual.
Whereas altruism involves related individuals, reciprocity involves unrelated individuals. Some species have evolved the social behaviour of assisting cohabiting individuals of the same species. If individual A pays a cost to help individual B, but the cost is recovered at some point in the future when B helps A, then selection may favour this behaviour.
Game Theory, a strategic decision-making tool that utilises pay-off matrices, can be utilised to interpret reciprocal behaviour. The Prisoner’s Dilemma is one such tool that models altruistic behaviour of cooperation and reciprocity.
In this model, the evolutionary stable state (Nash Equilibrium) is the strategy where A can’t benefit from switching their position while B stays constant, and B can’t benefit from switching his position while A stays constant. In the case of The Prisoner’s Dilemma, the equilibrium state is for both prisoners to defect and rat the other one out.
So how can cooperation evolve if the stable state is defection?
And how do others exploit this system?
While the above outcome assumes one-off games occurring, where the scenario is isolated to a single instance, cooperation evolves with the opportunity for future reciprocation. A repetitive scenario allows cooperation to become the evolutionarily stable state.
The Free Rider problem is a behaviour that threatens the spread of cooperative altruism. Anti-predator mobbing behaviour in birds has proven to be an effective model to illustrate this phenomenon. An experiment was designed to test whether prey were more likely to help partners mob when they had helped them in the past, which turned out to be the case, and were less likely to help mob with the birds that had been caged and hadn’t mobbed previously.
Another kind of evolved social behaviour is conflict. Conflict not only occurs between competing mates, but is also the case between genetic relatives (parent-offspring conflict). Conflict for a wide range of species draws on Game Theory to strategise whether to fight or flee.
Say an arctic wolf encounters a seal stranded on ice. However, a similarly size male is approaching the doomed seal up ahead. Each encounter, wolves choose their behaviour based on whether to be aggressive (fight) and risk injury (cost), or be cautious (flee) and concede the valuable (v) resource. In the associated pay off matrix, the size of the competitor male will factor in to the magnitude of injury that it could inflict, while the volume of meat from the seal weighs into the benefit of engaging in the conflict.