Intraspecific Group Arm Race


[Image By Ciar – Own work, Public Domain,

We all know the basics behind Darwinian evolution. Individuals from a species are born with different traits and the fittest members have more offspring, which share some of these “winning traits”. Antelopes become faster, giraffes become taller, etc.

Nevertheless, selection pressures do not always affect a species as a uniform group. In fact, the most interesting examples of competition arguably occur between different groups in the same species. This tells a more complicated story than the monolithic interspecies competition (e.g. prey vs predator), or even than individual fitness against the rest of the species (tall giraffe vs all other giraffes, male vs other males). That is why I call it Intraspecific Group Arm Race.

Today I would like to talk about how these strange pressures appear and what they entail. As usual, this post is not a summary of a current consensus, but a personal reflexion.

The most famous example of intraspecific group competition must be the evolutionary arm race between males and females mallards. Sexual arm races exist in dolphins, flies, beetles and many other species, but ducks are the most popular example for online news (and I just know the following paragraph should increase traffic to my blog tenfold…).

Mallards have complicated sex lives. You see, single males like raping females, and females do not like being raped by males. Therefore they both evolved complicated genitals: males have long, corkscrew shaped penises; females have even more gigantic, corkscrew shaped vaginas. (Maybe I should write this once more to be sure to attract the internet crowds?)
The race for complex genitals occurs because single males have an interest in fathering at least some ducklings rather than zero, while females have an interest in choosing the best father for their offspring (and not just the most rapey one). Thanks to their convoluted vagina, females can control who fertilizes the eggs: their active cooperation is required for the male to succeed. The sexes both continuously evolve to trump each other’s adaptations. Most birds don’t even have penises, so the evolutionary pressure must definitely be strong.

Arm races occur whenever you have competition. But for intraspecific group arm race to start, you need to satisfy these four requirements:

  1. You need groups that coexist through a generation
  2. These groups must belong to the same species
  3. They need to compete for a limited physical or abstract resource
  4. The offspring must be able to end up in any of the competing groups at birth OR be able change groups during lifetime

A group is defined as an ensemble of individuals who are on the same side of the competition for a resource at some point in time: male/female/asexued, cheater/cheated, adults/kids. In the ducks case, 1. the 2 groups are single males vs females. 2. They are all mallards (rule 2 is a corollary of rule 4, but let us still write it explicitly). 3. They compete for a choice: who gets to choose the genetic makeup of the ducklings? The cost of losing for the unpaired male is not passing his genes on. If he can father even 1 duckling, that is the difference between the life or death of his entire lineage! The female on the other hand, always passes her genome on: for her the cost is spending energy to raise ducklings that have unwanted genes and might not all survive and reproduce. 4. The ducklings can be born male or female: they can end up on any side of the competition. This last condition prevents one side from “winning”: an ideal genome would give you strong male offspring AND strong female offspring, otherwise you’re sentencing half of your own descendants to losing the game.

Groups do not have to be as classical as male vs female.
Take human babies vs human parents. Have you noticed that babies cry a lot (Yes, you have…)? Let’s be honest, a lot of the time it seems that they cry for no specific reason and nothing but time will calm them down.
One theory, not uncontroversial but backed up by interesting data, is that babies cry day and night both to exhaust their parents and to extend the post-birth infertility of their mother (linked to breastfeeding). The end goal being that no other sibling should be born too early so the baby can have the parents’ full attention and grow with all the physical and emotional resources available (like milk, cuddles and peekaboo parties). Human babies and human adults compete for the adult’s energy: the conflict arises from the fact that the baby fares better if the parents don’t make another baby too soon, but the parents fitness would be increased by 1. Not being exhausted and 2. Having as many babies as possible. Babies should evolve to be as noisy and stressful as possible, while parents evolve to discriminate unmotivated crying and ignore it. But the arm race is contained within the species: the baby is supposed to in turn join to the “parents” group. An ultra effective crying baby would turn into a very miserable parent. Again, the tug-o-war is balanced.

This brings us to the critical idea of unbalanced costs. Two competing groups have to mitigate the costs for their opponents cum future offspring, but what about the costs at the scale of the species? There is no baked-in rule preventing an arm race from being costly to the species. Some scientists say that a successful mutation must increase the fitness of the whole species, otherwise the mutation would be weeded out. I think that something much more interesting happens.

Can a mutation be beneficial for an individual and damaging for its species? Yes. Some very aggressive male ducks end up drowning unlucky females. From their point of view, there is no loss: if the female resists so much that she dies, the male probably wouldn’t have managed to fertilize the egg anyway. Remember, for those single males, just one duckling is the difference between life and death of their entire lineage. It is highly beneficial for them to be overly aggressive, and to spread their aggressive genes. At the scale of the species though, the cost is high. These female ducks could have raised many generations of ducklings. But no individual acts with an entire species in mind: even on the brink of extinction, the aggressive males would likely not be able to “just stop drowning uncooperative females”, as females would not be able to “just stop being uncooperative and accept unwanted genes”. There is no magical stop switch, “think about the species before yourself” button: arms races can go wrong.

And yet. Restraints can evolve and enhance stability. I will always remember Justin Werfel’s talk about evolved death. A major dominant theory is that death has no evolutionary advantage, and therefore no species will ever evolve to have its members die younger rather than older. After all, if you die, you loose on occasions of passing your genes on.
But during the last 15 years, evidence for the evolution of death has accumulated. More generally, in a space where resources are limited, a population that has self restraint mechanisms will survive better than a population of reckless individuals. Imagine a group of cows devouring all of the grass at once, not allowing it to grow back. They face immediate reward as they fatten but long term starvation as the grass just dies from exhaustion. Now imagine if one of the cows has a mutation for restraint. Maybe it’s a dwarf cow, or maybe it dies after only a few years. Anyway, the grass were this cow and its descendants live will be healthier than elsewhere, and on the long term they will, as a group, survive where the others starve and die.

Restraints are counter intuitive in a classical evolutionist view, because evolution is not supposed to look into the future. If I can have more kids than you right now, I am fitter than you and will supplant you. But this view fails to take into consideration environmental feedback, which can be quite immediate. The fasting cows only survive because they share the same physical space, allow their own offspring to inherit it and protect their turf from outsiders. If the voracious cows could just come up and eat what the fasting cows have saved up, this would not work.  The environment must be inherited and protected just as the genes are.

So there is no need for evolution to “look into the future” to favor less costly arm races, and there is no guarantee that the arm race offers any advantage to the species either.

This becomes particularly interesting when you start considering other, more abstract groups satisfying the conditions for intraspecific group arm race. Take cheaters and vigilantes. The transmission does not even have to be genetic. When you find out a new way to cheat, you have an immediate advantage. But you also become able to spot people who cheat in the same way as you. So if you teach your offspring or other in-groups how to cheat, they also have an advantage but they cannot use it against you or their siblings. The more successful your cheating strategy is, the more offspring you transmit it to, and… the less successful it becomes. The greater honeyguide is a good example of cheating dynamics. These birds lay an egg in other birds’ nests. But they have also evolved to notice if another honeyguide has already left an egg there, and destroy that egg. So they have to evolve eggs that look more and more like the parasited bird species, to avoid being crushed by their fellow cheaters; but they also have to become better and better at spotting suspicious eggs… Here the two groups are simply the 1st honeyguide to lay an egg vs the ones that come after. Whether a bird will belong to one or the other group is pretty much random. All conditions are respected for a perfect intraspecific group arm race. Yet there is a big loser in the story: the parasited bird. None of its own offspring survives, only the honeyguide chick. And if the competition between honeyguides is so harsh already, it must mean that almost all nests get parasited, several times in a row! The bird could well get parasited to extinction.
Unless, that is, a mutation for restraint appears first and the conditions for its protection are satisfied…

We have seen that these interesting competitive dynamics can happen for concrete, classical groups defined by their genes, only redistributed once per generation; to groups defined by their age, with a total permutation halfway through the generation; to the more abstract who-came-first groups that change every time a honeyguide visits a nest. This framework seems highly robust and its preconditions are so loose that it seems likely to have a large, underestimated influence on the evolution of species.

What about leaders vs followers? 1st child vs following siblings?

How many other groups can you find out?


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