Name: Simulating Green Beard Altruism
Uploaded: 2025-11-19T00:48:37.000Z
Duration: 13 min 23 s
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If evolution is all about the survival and reproduction of the fittest individuals, then why are creatures sometimes nice to each other?

In this video, we're going to build a simulation to start looking at the biology of altruism.

Certain genes will code for certain types of behavior rules, and

we'll see which kinds work from a gene's perspective.

Each day, blobs will go out to the forest to eat some fruit.

Each tree has enough fruit to feed two blobs, but some of the trees have predators in them.

If the blobs are unlucky enough to visit a tree with a predator, they'll get eaten.

But if they don't get eaten, they'll head to one of the homes and reproduce, creating one or two new blobs in their place.

let's make this a bit bigger and let it run for a while to see how it goes.

Alright, so this little ecosystem is mostly stable.

Now let's add the ability to be altruistic.

When the blobs go to a tree with a predator in it, one of them will notice the predator and it'll have two options for how to react.

The first option is to just run away, leaving the other blob to die.

The second option will be to make a bunch of noise and warn the other blob, but

in the process attract the predator's attention with a 50% chance of getting eaten afterward.

Since these blobs are risking themselves to help others, we'll call this the altruistic behavior.

The blob's behavior will be controlled by a gene which has two versions or alleles.

The cowardly allele will be yella and the altruism allele will be blue.

Before we get to the simulation, it's worth saying that in real life, behavior is determined by a mind numbingly complex interaction between many genes and a creature's environment over time.

But this simple model is good enough to help us grasp some fundamental ideas.

Okay, let's start with equal numbers of each allele and see what happens.

This graph shows the frequency of each allele over time.

At the beginning, half the alleles are the blue altruism kind and stacked on top.

Half the alleles are the yellow cowardly kind.

These values change over time, but they always add up to 1 or 100%.

And speaking of the change, it looks like the altruism Allele goes extinct after just a few generations.

Maybe we made it a little bit too dangerous to be altruistic.

So let's lower the death chance for being altruistic from 50% to 10% to see if the altruism allele can do well on easy mode.

Okay, it took a bit longer, but the altruism allele still went extinct.

Now you might think, hey, wait a minute, we only tried this once.

So now let's repeat 30 versions of the same situation and see how that looks predict ahead of time what you think will happen.

Do you think the altruism allele will always go extinct?

Or maybe the first run was a fluke and the altruism allele will actually increase in frequency more often than not.

Our brains like to pretend we knew things all along.

So it's good learning and science practice to make a prediction before you see a result.

Okay, there's a lot going on in this graph.

Each thin black line is one run, and the thick black line is the average.

In some runs, the altruism allele lasted longer than others, but extinction seemed inevitable.

Well, one did hang on for all 400 generations, but

based on the other runs, it seems like it's pretty doomed in the long run.

So why did this version of altruism fail?

Well, this is a computer simulation, so we have a perfect record for everything that happened.

There were 225,817 blobs saved by warning calls.

85,781 had the altruism allele, and the other 140,036 had the cowardly allele.

And 24,000, 151 of the altruistic blobs died after giving the warning.

So overall, the altruistic allele saved a Net total of 61,630 copies of the altruism allele.

But since the total population size is limited by the predators, the total number saved isn't really what matters.

More blobs being saved just results in more blobs getting eaten later on.

What does matter, though, is the competition between alleles.

A successful allele for altruism would need to find a way to help itself more than it helps its competitor.

Well, one way could be to let the altruism alleles see each other so they know who to help.

But because DNA stays inside the blobs and it doesn't have eyes, we need some detectable feature to go with the altruism.

The classic example of this is a Greenbeard, and

that's a fun thing to put in the blob, so let's stick with that.

So instead of the allele for altruism from before, we have an allele for green beard altruism now.

When a blob with a green beard notices a predator, it'll only warn the other blob if that blob also has a green beard.

So let's see whether this Greenbeard strategy works in a simulation.

And let's go back to the 50% chance of getting eaten after giving a warning.

The green beard allele drove the cowardly allele to extinction, and decisively so.

But again, we should test it with many runs and with different settings.

I ended up testing the 50% and a 90% chance of getting eaten for 30 runs each.

In each case, what do you think will happen?

At the 50% death chance, it seems like our first run wasn't a fluke.

The Greenbeard allele does indeed have a massive advantage.

One of the runs got a pretty rough start, but

it still ended up taking over before too long.

Only 10% of the acts of altruism actually end up saving an extra blob.

So the random fluctuations are pretty dominant.

And even though the average shows an upward trend, the green beard allele does go extinct in several runs.

After seeing these, I was curious to try again.

But starting with only a 10% frequency for the green beard allele once again, what do you predict?

Will the graphs look the same, just shifted down to start at 10%?

In the 50% case, there are a few interesting things going on.

After a while, we see that the average goes completely flat, because in every simulation, one allele or the other has gone extinct.

Of the 30 runs, 11 had the green beard allele take over completely, and

Even though the greenbeard allele had an advantage and saw its frequency rise on average, it's still subject to the whims of luck when there are only a few copies.

And once extinction happens, it's permanent.

It's interesting to imagine what kinds of alleles might have appeared in the history of the Earth and could have done well, but

The second interesting thing is that it seems like the green beard allele's advantage depends on how many greenbeards there are.

Each run starts out with mostly random wobbling, but

when the density of greenbeards gets higher, the helping happens more often and they get more benefit.

And in the 90% death chance case, things are just really chaotic.

But we still do see an upward trend on average.

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Simulating Green Beard Altruism

process

scenario

inevitable

eliminate