How has altruism evolved?

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Rosie enrolled for the Cambridge Immerse Biology Programme in 2015. She is now studying Natural Sciences at Emmanuel College, University of Cambridge.

Altruism is a topic that I studied early on in my Evolution and Behaviour model. At first glance, it appears to go against everything that Natural Selection predicts. An altruistic act is one that benefits another at your own expense. As the main biological imperative of life is for genes to replicate themselves as much as possible to increase their frequency in the gene pool, why would animals act in a way to decrease their own fitness? In this situation, fitness can be measured in terms of the number of offspring an animal has since this reflects the number of times a gene has likely been passed on. The more offspring an animal has, the more successful its life has been.

There are 4 main types of social interactions with other members of an individual’s population – cooperation, selfishness, altruism and spite:

Looking at the above table we would expect most organisms only to act in cooperative or selfish ways, as these are the only ways that benefit them. Selfish actions only benefit that individual and have no effect on anyone else. Unsurprisingly, this type of behaviour is very common and includes things such as not sharing food.

Co-operation also has many advantages to an individual. Group hunting can be more effective than individual hunting and therefore result in a higher food yield per individual. Living in a larger group causes “the dilution effect”. This describes how being in a large group reduces the chance that a predator will catch an individual. Additionally, living with a larger group increases the vigilance in watching for predators.However, this only really helps if the organism that spots the predator alerts the others, which can be considered an altruistic act as it calls attention to itself and so is more likely to be targeted by the predator. A good example of this is in meerkat groups where adults will stand-watch at higher vantage points on their hind legs and alert the rest of the group if they see a predator.

If we consider altruistic acts at the level of an individual, it seems hard to imagine how altruism has evolved. An altruist in a group of selfish animals would be at a disadvantage as they would be willing to help others, but the others would not help them. This would, therefore, be strongly selected against and any genes for altruistic traits would rapidly disappear from the gene pool. In a primarily altruistic group, a selfish individual would have an advantage over the others as they may be the recipient of altruistic acts but not return them. This means that they would have a higher reproductive success and as a result the allele for selfish acts would rapidly increase in frequency within the gene pool. In Vervet monkey populations, monkeys will sound an alarm call to warn the others of the presence of a predator – this is an altruistic act as it calls more attention to themselves – making them more likely to be targeted by the predator. Surely a monkey with a selfish trait that did not warn the others would be at an advantage as it would be able to escape quietly, while the predator targeted the others? This was described by Darwin as “subversion from within” as any altruistic group will eventually give rise to a selfish individual which will be at an advantage and therefore increase the frequency of that selfish gene.

At the level of the group, however, it is possible to see that groups that contain all, or primarily altruists, would have an advantage over selfish groups in bad years where food is scarce – as a result the selfish groups might all die, leaving a greater proportion of altruists in the population. However, in good years selfish groups will do better than the altruists as they can put all their resources towards producing as many offspring as possible. Darwin proposed a theory for the evolution of altruism that suggested that although self-sacrificial behaviour is disadvantageous to an individual, it might be beneficial to the group. On its own, Darwins theory does not seem sufficient to explain the evolution of something that seems like such a paradox to natural selection.

In 1964 Bill Hamilton published his theory of “kin selection” – also called inclusive fitness. If we consider fitness to mean the number of copies of a gene you cause to be passed on to the next generation we can see that as relatives are likely to share some of your genes, it may be beneficial to help them reproduce (as well as yourself). By helping close relatives, you can increase the total number of copies of your genes that are passed on to the next generation.

If we imagine a gene that increased the likelihood of an organism acting in an altruistic way such as sharing food with everyone it came across, that would provide no advantage. However, if that organism only shared food with its relatives and that extra food helped them to increase their reproductive success then we can see how an altruistic allele could increase in frequency in a population. This theory does not rely on an organism being able to recognise its relatives as in social populations relatives will normally live close together. Cuckoos have evolved to exploit this lack of ability to recognise who is actually related to them, as they lay their eggs in Dunnock nests. At first glance, seeing a dunnock feeding and looking after a cuckoo chick might seem like an altruistic act on the part of the dunnock, but in fact this is simply because the dunnock does not recognise that the cuckoo egg is not one of their own – even when the cuckoo chick hatches first and pushes the other eggs out the nest.

Hamilton’s Rule defines the conditions under which a gene promoting altruism would increase in frequency in the population:

Where A is the altruist and B is the beneficiary. If A helps B, A will suffer a cost of decreasing the number of offspring it has but B will benefit and have more offspring than it would have done without A’s help. If B is closely related to A, then B’s benefit will also benefit A since they share some of the same genes. Relatedness is the probability that one gene in an individual is an identical copy, by descent of a gene in another individual. This will, therefore, be higher in siblings (a 50% chance of sharing the same gene) than in cousins (a 12.5% chance of sharing the same gene). If the inequality described by Hamilton’s rule is met, then it is evolutionarily beneficial for A to help B at A’s own expense.

This theory of kin selection works well to explain altruistic acts within social insect populations. For example, within bee colonies, sterile worker bees spend their lives looking after the queen bee, as this is the best way for them to try and ensure some of their genes get passed on since they cannot reproduce themselves. Worker bees have stings and will attack predators that get too close to their nest – in stinging the predator, the barb gets stuck in the predator and the bee dies. The evolution of this suicidal behaviour only makes sense if we consider that the nest is full of that bee’s relatives and in protecting them they will be able to reproduce to offspring with copies of the same gene as the worker ant.

This shows that altruistic behaviour is not the paradox of natural selection it seems. If we consider kin selection, we can see that the organisms still act in a way to maximise an alleles presence in a population. The conditions for the spread of an altruistic gene are given by Hamilton’s rule and when this inequality is met it is beneficial for an individual to behave altruistically towards the other – this explains the different levels of altruistic behaviour shown to different relatives. John Haldane once said “I would gladly lay down my life for two brothers or eight cousins.”

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