It is exactly 49 days before Christmas, and the songs have already started in our house. And the tinsel. I’m terrified that my housemates are going to lead me down a slippery slope into a White Wonderland before December has even rolled around.

But I insist that I’m not a Grinch when it comes to Christmas. It definitely is a magical time. I like the joy of present giving, the food, the merry cheer. There’s a magnetic sense of warmth and frivolity in the days that surround it. Without 25th December and New Year to boot, we would have to fight our way through winter, that ice-cold blight on our year, with no respite. Christmas is like coming up for air.

I’ve always maintained, though, that the best thing about Christmas is seeing family. You know how it is. People are working so hard all year, in their own little worlds, building up their own lives, that you barely get time to see anyone. “I’d love to see you, but all my weekends this month are booked up!” “Sorry, it’s just work is so stressful right now.” It sometimes seems impossible to navigate everyone’s social calendars, just to see the people you love. But Christmas solves that by providing a few days of nationwide relaxation and indulgence, where you can finally get brothers, sisters, parents, grandparents, cousins, aunts and uncles in the same room, to eat and laugh and catch up. Those of you with large, extended families full of distant relations you barely know might feel differently about this, but my immediate family is a small, tight-knit unit, and so I love them and always look forward to seeing them all together when the end of the year rolls around.

Christmas is usually the one time I get to see family all year.

My question is this. Why does our love for family members run so deep? And why are those relationships qualitatively different from others? There’s a Spanish proverb that translates to “an ounce of blood is worth more than a pound of friendship.” Think about your own life. The love you have for a sibling is very different from the love that you might have for a friend or partner. Why is that? What is it about being genetically related to someone that causes such love, and means you get together every Christmas?

Hold that thought, because we’ll get back to it. Now, I’m broadly interested in a biological paradox. The question is: how does altruism evolve? That is, how could a behaviour that imposes a cost on an individual, and offers a benefit to a conspecific, ever evolve, given that individuals that have the behaviour are less likely to survive and reproduce than others? Any genetically programmed behaviour like that should be whittled out by natural selection quite quickly. It just doesn’t seem to be adaptive to help anyone at a personal cost to yourself.

Take bees, for example. Imagine a colony of bees, filled with thousands of individuals. Their social organisation is uniquely interesting, in that individuals in the colony are specialised for certain roles. The males are drones, who have a very simple job: find a mate and then immediately die. However, the females are more interesting. They are split into the workers and queens. Worker bees, the drone’s sisters, are the most common kind, and they do everything required to keep the colony alive and well. They clean the hive, guard the colony from intruders, and forage for pollen and nectar (they can communicate with one another the location of these by performing the fascinating waggle dance). Importantly, worker bees are sterile. They do not reproduce; they leave that up to the rarer queen bee. Queens do not do any of the jobs of the workers. Instead, their sole purpose is to populate the colony, laying approximately 1500 eggs per day in peak season. They also live for about 5 years, compared to the six week lifespan of the workers.

This social organisation is an example of eusociality, and can be seen in other insects like ants and wasps. The behaviour of the workers can be seen as a kind of altruism, right? They behave at a cost to themselves (never reproducing, working their asses off day and night) and for the benefit of another (the queen and the drones, who seems to reap all the rewards). What’s even more interesting is that the workers will actively police and kill any other workers who decide to go against their role and reproduce for themselves. It’s as if they choose sterility together.

Charles Darwin was baffled by this asymmetry in division of reproductive labour. How could it ever evolve? What are individual worker bees getting out of this? Under his view of survival of the fittest, worker bees were certainly not doing very well. They had to shoulder all the work of maintaining the colony, and they could never reproduce. Therefore, their behaviour should never evolve.

Just a quick aside. It’s tempting here to think that worker bees are acting for the benefit of the colony as a whole. That somehow, individuals are adapted to help the entire group, because that will in turn benefit themselves in the long run. This kind of thinking (that traits evolve by group selection because they are for “the good of the group” or “the good of the species”) is fallacious. It is incorrect because it misunderstands how natural selection acts. Imagine that you had two groups, A and B. Group A is full of altruistic individuals who help each other at an expense to themselves, and Group B is full of selfish individuals. Yes, Group A will probably have higher fitness than Group B, if you average across the whole group. But natural selection doesn’t work on averages, it works on individuals, since they (not groups) are the ones who reproduce. In Group A, if a single selfish individual is born into the group, they will be in the perfect position, as they can take advantage of their group’s altruistic tendencies without paying any cost themselves. Therefore, they will have higher fitness than the rest of their group. Because natural selection favours strategies with higher fitness, selfishness will spread rapidly, as it’s the best strategy for an individual to follow (it will “invade” the population). Depressingly, within-group altruism falls apart, despite its between-group benefits, and group A and B both end up full of selfish individuals. Even if, in the long run, it would benefit all individuals to work together and be altruistic as a group, natural selection is short-sighted and would halt this teamwork before it could even get started.

Back to our bees. So, if it’s not group selection causing the workers to be so altruistic, what is it? Here, it helps to talk about genetics. Darwin developed his theory in the 1800s, before we had any knowledge of the genetic underpinnings of traits. Now we know all about inheritance, the genome and DNA, we can shift the attention of evolution from the individual to the gene. Darwin thought of natural selection as “the differential survival and reproduction of individuals”. Nowadays, we think of natural selection as “the differential survival and reproduction of genetic material”. In short, a trait will evolve when the alleles underlying it propagate more successfully than other alleles. This is a subtle but, as we will see, crucial insight. (For more detail about this, see Richard Dawkin’s The Selfish Gene, a really incredible book that turned the way I see the world completely upside down).

A gene's eye perspective can inform our thinking about altruistic behaviour.

Let’s move our thinking about evolution from an individual level to a genetic level. Imagine you have two alleles, Allele A and Allele B (recall that alleles are different possibilities of genes at the same locus, like alleles for having green vs. blue eyes). These two express different traits in the individuals that hold them (here we’re assuming haploid genetics, for the sake of argument). As an example, let’s say that Allele A causes an individual to express the behaviour “find food for myself”. Allele B causes an individual to express the behaviour “starve”. In a population with just these two alleles, it is easy to see which alleles will survive: only the individuals with Allele A will live long enough to reproduce. Allele A will propagate, and this will be natural selection.

Okay, but how does this help us with our puzzle about altruism in bees? Well, let’s change what these alleles do. Instead of eating behaviour, they now express themselves as possible helping behaviours towards relatives. Allele A now causes an individual to express the behaviour “help individuals who also have Allele A, at my own expense”. Allele B now causes an individual to express the behaviour “do not help anyone”. Suddenly, we’ve thrown an interesting dynamic into the mix. The propagation of alleles is now not as simple as the eating behaviour example. At first glance, it seems that Allele B will have higher fitness than Allele A, since A expresses itself in an individual as a self-damaging, individually costly behaviour. However, counter-intuitively, Allele A might actually spread quite well, because even though it expresses a costly behaviour in an individual, it is helping copies of itself in other bodies. Thus, in an entire population, Allele A could still propagate and the behaviour could evolve. Selfish genes may create altruistic individuals.

This is called kin selection, and it bypasses the issues that Darwin had with eusociality by recognising that alleles can improve their own fitness in one of two ways:

  1. By expressing a trait in an individual that improves the survival and reproduction of that individual, thereby helping themselves directly (direct fitness)
  2. By expressing a trait in an individual that improves the survival and reproduction of individuals that also share the allele (indirect fitness)

By looking at adaptations at an individual level, as Darwin did, we are only concerning ourselves with one aspect of fitness, direct fitness. And usually, as we see in nature, adaptations are expressed this way. They help the individual that has them. I have eyes that see, legs that walk, opposable thumbs that grab… this helps me survive and helps my genome propagate itself. However, there are a subset of adaptations that can only be explained by taking indirect fitness into account, such as eusociality, as we will see.

Clearly, relatedness matters here. Since an individual can never “see into” another’s genome, you can never truly know whether you’re helping someone who shares the same allele as you. Instead, you use the probability that the other individual shares it. In diploid species like us, we know that we are on average 50% related to our parents, due to inheritance from both mother and father, and also on average 50% related to our siblings. Further out, we are 25% related to grandparents, aunts, uncles, nephews and nieces, and 12.5% related to cousins. So Allele A from before is less likely to evolve if it’s interacting with cousins than with siblings, since it can be less certain that the altruistic behaviour is benefiting a copy of itself.

This led Bill Hamilton to create his rule for the evolution of relative-directed altruistic behaviours: altruism can evolve when rB > C, where C = the cost to the individual performing the act, B = the benefit to the recipient of the act, and r = coefficient of relatedness between interacting individuals. The cost-benefit ratio of helping must exceed to coefficient of relatedness in order to evolve. For example, two brothers are 50% related. A helping behaviour that imposes a cost C = 0.5 can evolve if the behaviour provides the other brother with a benefit greater than 1 (B > 1). Try plugging those numbers in to see that this works.

Let’s relate this back to our nice worker bees. Their altruistic behaviour, where the costs to themselves at first seem substantial enough to stop the evolution of the behaviour in its tracks, need to be scrutinised in the context of relatedness. And like a puzzle falling into place, we realise that bees actually have a curious sex-determination system that ensures that workers are 75% related to their brother drones, who do all the mating. That’s a very high relatedness coefficient; higher than that of your sibling, and think of how much you’d do for them! Plugging that number into Hamilton’s rule shows us that a costly altruistic behaviour, such as forgoing your own reproduction to help the reproduction of related drone brothers, can indeed evolve, providing the benefit to the drone is substantial enough.

Paradox solved! Darwin’s confusion stemmed from his lack of understanding of genetics and relatedness, but we have showed that kin selection can drive the evolution of altruistic behaviour in eusocial species. But what about species like us, where relatedness between individuals is lower (maximum 50%, omitting identical twins)? Since genetics underlies all of evolution, Hamilton’s rule has actually been shown to apply not just to eusocial species, but to everything natural selection does, in all species. That includes our ancestors, our closest cousins (the great apes) and us. Let’s stop being scientific for one second and get introspective. Think about your family. You’re probably closest emotionally to your parents and your siblings. Yes, that’s maybe partly due to being raised by them / being raised together (this is in fact a cue to relatedness) but your genetic relatedness means a lot here. Altruism between these close relatives is very common. Then, “emotional closeness” will tend to decrease as you extend your way outside your closest relatives.

Accordingly, studies have shown that our prosociality towards relatives seems unique: we don’t tend to expect reciprocity from relatives, we are often nepotistic, we take sides with family in disputes, and we are more likely to look after related than unrelated children. Interestingly, a study of relatives receiving a share of an estate from a will showed that the size of the share was broadly associated with their relatedness to the deceased (Smith et al. 1987). However, most of this evidence is correlational only. In an experimental study, Madsen et al. (2007) found that individuals held a painful ski-training exercise for longer if it was providing a monetary benefit to more closely related individuals.

Regardless of whether you believe this evidence or not, or the extent to which you agree that our evolutionary past shapes our current behaviour, it is clear that relatedness is important in our modern relationships. Here, I suggested that kin selection has shaped our psychology to direct altruism towards kin, and to develop emotional closeness in accordance with this, explaining why “an ounce of blood is worth more than a pound of friendship”. Although we are certainly not akin to eusocial bees in their altruism, humans tend to love their families, and the fact that this is so widespread across many cultures suggests that it is an evolved tendency. Hamilton’s rule provides insight into why this might be the case.

So why do we get together at Christmas? There are many reasons. Tradition, culture, a general sense of community… these are all valid reasons. But I believe an important factor is the deep love that families share for one another, which has evolved due to kin selection. Personally, I love the idea that evolution (which with terms like “survival of the fittest” and “struggle for existence” has, to some people, become synonymous with competition and nastiness) can produce the love and care that we see in human families. That’s what I’ll be thinking about when I head home for my roast dinner this Christmas.

But still, 5th November is far too early for Christmas songs. Just saying.


References

  • Madsen, E. A., Tunney, R. J., Fieldman, G., Plotkin, H. C., Dunbar, R. I., Richardson, J. M., & McFarland, D. (2007). Kinship and altruism: A cross‐cultural experimental study. British Journal of Psychology, 98(2), 339-359.
  • Smith, M., Kish, B., & Crawford, C. (1987). Inheritance of wealth as human kin investment. Ethology and Sociobiology, 8, 171–182.