Clarifying the math behind kin selection

question

I have a problem understanding the actual math behind kin selection.

So I have read the selfish gene and thought that I understand kin selection.

But then someone made this argument to me:

"Kin selection is nonsense, because a human should sacrifice his children to save the life of 10 fruit flies, (as a fruit fly shares around 50% of genes with a human), or some amount X of fruit flies, or maybe 5 chimpanzees (who share 98% of genes with a human)"

And while this seems ridiculous to me... I couldn't debunk it.

The math must be more complicated than simply adding up the gene copies, because if you just add up the amount of copies there would always be some amount X of fruit flies that you should prefer over your own children.

So I was thinking... it makes no sense, because some genes don't exist in fruit flies even if you have a trillion fruit flies.

So my question is: what is the actual math behind kin selection? how does it work? how do you actually calculate the value of an individual of any species?

I would also appreciate book recommendations that explain the math behind it in more detail than the selfish gene.

in evolutionby Ubermensch42133785

100%

22d

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22d

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CaptainLoggy

22d

The basic math problem here is that genetic overlap and relatedness score for the purposes of inclusive fitness, while loosely related, are NOT the same thing. Your siblings have a relatedness score of 0.5 (on average), a fruit fly would have something to the tune of 10^-11, which isn't the 50% or whatever figure of genetic overlap.

T_house

22d

It's kind of amazing to me that someone could know about kin selection and come up with that argument in their head without stopping to think whether either a whole lot of very smart people have made an inconceivably stupid error OR they haven't really understood it properly themselves

Ubermensch42133785

22d

Why so aggressive? My point wasn't that kin selection theory is wrong. My point was that I couldn't debunk the argument against it and hence I require a deeper understanding of the math behind it. You are essentially making an argument from authority here. "just believe the theory because some smart guy came up with it" that's not how science works, we should always question and try to falsify existing theories. And instead of being obnoxious why don't you just try to answer the question

T_house

22d

I meant your friend, not you.

ETA: and not blaming you for not being able to refute it on the fly when confronted with it. I meant that person has taken the time to think of it, decide they are right and definitely haven't come up with a flawed premise, and do no more investigation before scoffing at the decades of work that's gone into the study of it - which is, of course, not what science is about.

Ubermensch42133785

22d

can you provide some resources how the math actually works in detail? maybe a book/article? would be appreciated

SKazoroski

22d

This gives a simple explanation of how the math works. As you can see, the relatedness score already gets as low as 0.125 for a cousin (a person you share a pair of grandparents with).

fluffykitten55

22d

This assumes that there is no relatedness between any of the 4 grandparents, which is not generally true and substantially is/was not in human bands.

Larry_Boy

22d

Relatedness is complicated/mildly dysfunctional. If you follow the standard definition and have a deep enough pedigree everyone becomes highly inbred and has relatedness 2 to everyone else (yes 2, relatedness is weird). This is obviously not useful. Somehow relatedness has to account for mutation, drift, and population structure, which are ignored in the standard definition. Barring a better theory you have to assume that the individuals that start your pedigree are unrelated individual sampled from a panmictic population of infinite size.

SKazoroski

22d

It was just a simplified explanation of how the calculation is done. Real family trees are going to be messier than the made-up example family.

theglandcanyon

22d

A gene that inclines you to sacrifice your life for ten fruit flies is going to rapidly die out of the population. A gene that inclines you to sacrifice your life for three of your children, if it is very rare in the population, increases its share of the next generation, because each child has at least a 50% chance of carrying it, whereas it is rarer than that in the general population.

As its prevalence in the population goes up, the gene variant will experience less of a benefit from the individual sacrifice, but still gets some benefit.

brfoley76

22d

There's a couple major problems with your friend's argument. First "gene" is just shorthand for "genetic variants in a population."

The correct perspective for kin selection is to look at it from the "point of view" of a genetic variant in a population. A variant will increase in frequency if more individuals with that variant successfully reproduce.

So assume I have a mutation for stubbly whiskers and there is an identical mutation in fruit flies. Assume the mutation also makes me love flies with stubbly bristles. No matter how many fruit flies with stubbly bristles I select for, if I don't personally reproduce, the mutation will go extinct in humans immediately.

If the same behavior occurred in humans (like if I was motivated to help stubbly bristled people and they helped each other) the mutation could plausibly increase.

That's the intuition behind kin selection.

All your brothers and sisters share (on average) half your genetic variants. So any genetic trait that increases the number of brothers and sisters you have will increase in the population, as long as it doesn't negatively affect your own fitness too much.

Imagine there was a variant such that it increased a shark's chance of surviving infancy by double, with a potential small cost to the mother. Would it increase in the population?

If the cost is close to zero for the mother (no change in the number of future brothers and sisters for baby shark) then yes it will increase. If the mutation is for "eat your mother to death from the inside out" the cost will be at the expense of all future brothers and sisters and the mutation will quickly go extinct.

Again, the point is, it relates to frequencies in a interesting population. This is how evolution works.

DoctorBeeBee

22d

That person is misunderstanding what sharing X% of genes with a different species means, versus sharing genes with other human beings. We share 100% of genes with other humans, as in we all have the genes for building human bodies as opposed to building chimpanzee or fruit fly bodies. So when talking about two different people, you both have, say, the same genes for building human eyes, but you have variations of those genes that make your eyes different colours for example - that's an allele, not a different gene.

Atypicosaurus

22d

There are different types of DNA similarity. The fallacy used here is called ambiguity.

The kind of DNA similarity we use in kin selection is not in fact DNA similarity. It's something like degree of relationship. The reason is that it's not about the absolute similarity of the DNA. It's about the chance of a phenotype , caused by a genetic variation to be shared. A genetic variation can be minuscule on the DNA level yet can cause major phenotype. Like, the sickle cell anaemia is 1 base different from the normal variant.

So in fact sharing a genotype is a term that has no meaning outside of a species. You can't have the same genotype as a chimpanzee yet alone a fruit fly. Using terms when not applicable is a fallacy.

So when we're talking about kin selection we think about the chance to share a genotype with another individual of the same species, and the chance is higher the closer related they are. So a genotype making an individual sacrificing themselves (this is the phenotype) must do it so that the sacrifice ensures kin survival. Otherwise it's just a suicide gene.

iskshskiqudthrowaway

22d

Are you looking for Hamiltons rule?

rb-c>0

Relatedness coefficient x benefit - cost > 0

cubist137

Evolution Enthusiast

22d

"Kin selection is nonsense, because a human should…"

"Should"? Yeah, no. Kin selection isn't a "should" kind of thing. It is, rather, a process which either does operate, or else does not operate, on a breeding population. "Should" and "ought to" are personal preferences, and they simply don't enter into it!

Ubermensch42133785

22d

uhm you are missing the point. I didn't imply a moral obligation. the point of my argument is that kin selection theory predicts that creatures will act in ways to maximize genetic copies.

Lets make the argument again:

if kin selection theory predicts that humans should sacrifice their lifes to save the lifes of 10 (or some other amount) of chimpanzees, but we are not observing that. this kind of falsifies kin selection theory.

but maybe kin selection theory doesn't predict that. hence I asked for a mathematically accurate model of kin selection theory.

cubist137

-1

Evolution Enthusiast

22d

You didn't imply a moral obligation. The argument that you "couldn't debunk" implied a moral obligation, when it explicitly asserted something about how human beings should behave.

Ubermensch42133785

22d

again you are missing the point. I didn't use the word should in a moral context here. I used it to mean "kin selection theory predicts that creatures will behave as if they were trying (should) to maximize copies of genes"

cubist137

Evolution Enthusiast

22d

I am unaware of any critter that takes kin selection into account before deciding how it's going to act. Am not at all sure that any argument which is built around the premise (implied or explicit) "If I act in accordance with kin selection…" is even wrong.

OrnamentJones

22d

All right cracks knuckles

The one fundamental thing about the evolution of cooperation is that for a cooperative allele to beat a cheater allele (we're talking about one gene only here), the cooperators need to distribute the cooperative benefit /sufficiently disproportionately towards the cooperators/.

What is sufficient? Depends on the system. How can organisms do that? An easy way to do that is to interact with relatives, because relatives are very likely to share your genotype and so then you automatically get the "cooperate only with cooperators" result (depending on how close relatives you interact with).

That's kin selection: maintaining cooperative behavior is relatively easy if it involves interacting with close relatives.

Edit: also, the math is kind of a mess because Bill Hamilton (the guy who came up with this idea) was absolutely fucking incomprehensible both in person and in writing, and then his results were overly simplified into a "rule" (which I have generalized and explained verbally above already)

wibbly-water

22d

Okay so it feels like part of the problem comes from the fact that maths causes instinct, not logic.

From my understanding; while you may have a point that mathematically speaking you should be able to find a number of fruitflies worth saving mathematically speaking (though as another commenter pointed out that with relatedness scores, fruitflies are many many many times smaller relatedness so it would reuire billions upon billions upon billions) - your instincts don't know that.

Never in human evoltionary history has "my child or countless hordes of fruitflies beyond reckoning" ever come up. If a similar thing has come up (I dunno - say with other animals like dogs) its still rare enough that its not a factor in evolution. Perhaps it is occuring now with ecological destruction and people favouring the ecosystem / planet over other people... I guess our wider kin instincts could kicking in and trying to make sure either we as a species or even us animal life as we know it. Perhaps kin selection (both the maths and the instinct) is partially why we try to conserve animals.

But at the end of the day the maths will always say that the closer related to oneself another being is (child, family, tribe, species, etc etc) the more genes it has in common so at least some altruism will help our genes propagate long-term. Thus we get programmed with at least some altruism as an instinct that fires and misfires in ways that roughly, but don't exactly line up with the maths.

anthro4ME

19d

You can't use math to prove or disprove a metaphor, "selfishness" implies intent, and genes aren't capable of that.

GoOutForASandwich

22d

It is about the % shared by common descent. The reason it’s not just shared genome is because mutations that are not altruists would be selected and take over, so the gene in question would quickly become part of the genome not shared. Close relatives are more likely to have the same set of altruism related genes.

Carachama91

22d

Kin selection is about the variation between individuals of one species. By aiding your kin, you are helping the individual species genetic variation within you to survive. Saving a chimp or a fruit fly is not preserving any of your individual, human genetic variation. This all gets down to fitness, which is basically the number of copies of your personal genome that gets passed to the next generation. It is an intraspecific measurement not an interspecific one. 10 fruit flies mating is never going to recreate your genome.

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