Reframing the evolutionary benefit of sex

by paulfchristianoSideways View2 min read14th Sep 201922 comments

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EvolutionBiologyLogic & Mathematics
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Automatically crossposted from https://sideways-view.com

From the perspective of an organism trying to propagate its genes, sex is like a trade: I’ll put half of your DNA in my offspring if you put half of my DNA in yours. I still pass one copy of my genes onto the next generation per unit of investment in children, so it’s a fair deal. And it doesn’t impact the average fitness of my kids very much, since on average my partner’s genes will be about as good as mine. (ETA: but see the discussion below, in which case the costs might be much bigger.)

But the trade has transaction costs, so I’m only going to do it if I get some benefit. In this post I’ll tell a particularly simple story about the benefit of sex. I think this is basically equivalent to the standard story, but I find it much clearer. It also makes it more obvious that we don’t require group selection, and that the benefit is very large.



Why doesn’t sex change the average fitness of my kids? The possibility of a “lucky” kid who gets the better genes from both of us is offset by the possibility of an unlucky kid who gets the worse genes from both of us. If the effects of genes are linear, the average fitness will be exactly the same as the parents. In practice I expect it to be slightly lower because of convexity and linkage disequilibrium.

But sex increases the average fitness of my grandchildren, because my fittest children will be responsible for a disproportionate fraction of my grandkids. More precisely, if my if an organism with fitness dX has (1+dX) kids per generation, then the total fitness of my grandkids is E[(1 + dX)^2] = 1 + 2 E[dX] + E[dX^2]. So increasing variance by 1 unit is as good as increasing average fitness by 0.5 units.

Reproductive decisions are naturally a tradeoff between average fitness and variance. Sex slightly lowers the average but increases the variance. If you try to get the same amount of variance with random mutations, you’ll have to totally tank your kid’s expected fitness, because your current genome is well-optimized, and you’ll also pass on fewer genes to the next generation (since some of yours got destroyed). In fact, it’s hard to think of any way to get similar benefits without exchanging genes.

Variance becomes linearly more important over time. For the fitness of my grandkids, 1 unit of variance is worth 0.5 units of fitness; for great-grandkids, they are equally valuable; for great-great-great-grandkids variance is twice as valuable. I don’t think we have to look too many generations ahead before the variance bonus from sex outweighs the costs. For example, if genetic fitness differs by 5% between my offspring, and sex reduces fitness by 1%, then sex breaks even within 6 generations.

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From the perspective of an organism trying to propagate its genes, sex is like a trade: I’ll put half of your DNA in my offspring if you put half of my DNA in yours. I still pass one copy of my genes onto the next generation per unit of investment in children, so it’s a fair deal. And it doesn’t impact the average fitness of my kids very much, since on average my partner’s genes will be about as good as mine.

Wait, you seem to be assuming that both parents invest equally in the offspring, but in most (vast majority?) of species, one sex invests more than the other. In some species the male makes virtually no investment at all, and what you say here clearly do not apply to those species.

From an evolutionary perspective, males and females presumably get an equally good deal on the margin (or else the sex ratio would shift). That need not need to look like a productive investment in order to justify this basic picture (e.g. the story would be the same if males fought for territory and protected mates against other males).

If the low-investment sex doesn't add value of any kind then it would change this picture. E.g. if males just compete for mates and then do nothing beyond mate, then females would get an advantage by cloning themselves. Maybe plants actually fit into this picture most straightforwardly of all.

(This would actually also happen if males invested 50%, if they couldn't track paternity and females could secretly fertilize themselves.)

In the case where you get nothing in return, it seems like you are taking a 50% fitness hit from sex by passing on half as many genes. So if differences in fitness between kids were 5%, it would take about 300 generations for sex to break even. If fertilizing yourself is a complicated adaptation, then maybe that's enough to stick with sex (after it evolves in some more equitable species) but it's pretty different from my claim about breaking even in 6 generations. And in the case of plants or other hermaphrodites presumably there is an easier gradient to more self-fertilization, so that's even more puzzling and maybe this would bring me back to the more usual view where there is something more to be explained (either evolution is surprisingly forward-looking, or we need some story about why the advantage is bigger than it looks).

either evolution is surprisingly forward-looking

Interestingly, this relates to our discussion about episodic vs non-episodic learning algorithms. In this case it seems clear that evolution is not episodic and assuming very large population sizes ought to maximize long-run inclusive fitness. So the puzzle here is that if it takes 300 generations for sex to break even, then if a mutation caused some member of a sexual species to start reproducing asexually, the sexual population would crash to 0 before it could recover.

My idea for solving this (which I just thought of now so take it with a grain of salt) is, because a sexual species can maintain a genome against a much higher mutation rate than an asexual species can (see past discussion), an asexual species needs to have a much lower mutation rate (i.e., much more machinery to prevent/repair mutations) to survive. When an asexual population arises from a sexual species, it doesn't have the extra protective machinery and therefore quickly succumbs to accumulation of harmful mutations, perhaps before the sexual population can go extinct.

Or if it does drive the sexual population extinct first before itself going extinct, if most species are sexual then a phenotypically nearby species can just come occupy the now vacated niche.

Evolution isn't episodic. In some sense the question motivating the OP was whether many of the important phenomena can come from being episodic with an appropriate utility function (something like exp(fitness) instead of fitness).

In some sense the question motivating the OP was whether many of the important phenomena can come from being episodic with an appropriate utility function (something like exp(fitness) instead of fitness

I don't understand this. Want to elaborate?

[+][comment deleted]2y 0

Next time I'm having sex, instead of thinking "Why am I doing this? This is a terrible way of propagating my genes. I wish I was in Chernobyl," I'll think "Just be patient, in six generations it will all be worth it."

I'm not sure why this is pinned. This seem like a wrong explanation.

You can get varriance with mutations of germinal cells, and indeed this does happen. Not to mention gene transfer (e.g. I'll keep 99.99% of mine and take 0.01% of yours), which over the last decade has been observed in hundred of eukaryotes.

Sex in primitive species is not a varriance inducing mechanism it's a hidden-trait preserving mechanism.

The chromosomal setup that allows for sex means we can have hidden traits that only show up in a small % of offsprings (e.g. simple story is you need 2x SNPs which are present only on one of the chromosomes for each parent => 25% occurance). This allows for e.g. sikle cell anemia to be a thing that can be selected for when malaria incidence is high in the population, and selected against when incidence is low. So you have "fitter" hunters in no-malaria times and fitter malaria-resisters in loads-of-malaria times. But you don't get rid of either genotype entirely, because it can be carried around and only manifsted in a small % of kids (which are going to be selected out early on) until it's needed, and then the small % with the adaptive mutation are heavily selected for.

This "hidden trait" explanation is what the chromosomal setup we have provides over bacteria.

Now the question remains something like "Why not carry around 4x different ones in your germinal cells all the time but only "activate" two while allowing all 4 to mutate" ... and the answer is something like 1) might be harder to evolve, since you want to keep the embrional stage as simple as possible, that's when most mistakes in growth happen 2) that requires carrying around extra DNA 3) asymetric selection has benefits 4) actually <this weird species of fungi> 5) group selection

But you only need 1) in order to account for this, and as in most cases I assume that's probably the real explanation, instead of a just so story, it was the path of least resistance.

So no, varriance is not what you get via sex.

You can get varriance with mutations of germinal cells, and indeed this does happen. Not to mention gene transfer (e.g. I'll keep 99.99% of mine and take 0.01% of yours), which over the last decade has been observed in hundred of eukaryotes.

Mutations of germline cells come with huge fitness penalties. Taking 0.01% of your genes introduces an extremely small amount of variance.  And unilaterally replacing 50% of my genes with yours is equivalent to a 50% drop in fitness (!). The bacterial method itself also doesn't seem to work in humans (because you need to have the genes during development, and less importantly you need to spread them throughout your body). So it seems to me like sex adds very significant benefits over these alternatives.

Is your view here coming from some quantitative estimate or further reasoning you didn't include in the comment, or is it reflecting the consensus view in some field? In either case, it would be great to see a pointer. If this is just your guess based on the reasoning in the comment, that's fine and I'm happy to leave the argument here (or with your rebuttal).

Mutations of germline cells come with huge fitness penalties. Taking 0.01% of your genes introduces an extremely small amount of variance.  And unilaterally replacing 50% of my genes with yours is equivalent to a 50% drop in fitness

99.9% of those genes will be identical, hence why I used the 0.01% number, if you want to induce more mutations you can (see bacteria) and if you want to introduce more mutations in a controlled way (i.e. not break anything important) you also can, and humans actually already do this, as do most multi-celular eukaryotes (see B cell selection in response to antigens, for example)

The bacterial method itself also doesn't seem to work in humans (because you need to have the genes during development, and less importantly you need to spread them throughout your body). So it seems to me like sex adds very significant benefits over these alternatives.

I'm not sure if it works in humans, but humans did not invent sex. Simple multi-ceulular eukaryotes did. In those species HGT works just fine and is indeed a contributor to evolutionary variance that got them to where they are. I had a great paper on this but can't find it, see for example: https://onlinelibrary.wiley.com/doi/pdf/10.1002/bies.201300007

It's not a big thing in practice, because sex works better, so it makes sense to protect against HGT in all but a few niche cases (transfer from bacteria-like organels such as mitochondria)

Is your view here coming from some quantitative estimate or further reasoning you didn't include in the comment, or is it reflecting the consensus view in some field? In either case, it would be great to see a pointer. If this is just your guess based on the reasoning in the comment, that's fine and I'm happy to leave the argument here (or with your rebuttal).

It reflects a consensus view in the field in so far as nobody has tried to justify sex in terms of variance in genetics since the 70s (see the very wikipedia article cited) and even that tried to justify it was being variance under very specific conditions that allow it to be less dangerous and/or with a higher chance of varying in a "right" direction.

Variance is for sure an advantage. But it's not the advantage. Since there are 1001 ways you can introduce variance.

So both under common sense and under all accepted models I know of the intuition presented here is wrong.

And you don't need much to figure that out, again, just try to answer the question:

If variance is the main reason we have sexed reproduction than why not do it bacteria-style, or more realistically why not do it B-cell-selection style but for germinal cells?

That also means the variance could be better controlled by environment, you could literally say "I want exactly x% variance".

Additional questions to ponder might be:

  • Why always 2 sexes (outside of weird fungi)
  • Why is the sex stable (outside of a few niche animals)

Those will also point you in the right direction of actually figuring out why we have sexed reproduction.

Again, this is not to say variance is not a benefit. Much like "locomotion over flat ground" is a benefit of crab legs. But "variance" is something boring that can come about 1000 different ways. The reason crab legs are interesting and special and worth evolving is showcased when looking at how crabs move on steep terrian. Similarly, the reason sexed reproduction is interesting and special and worth evolving only becomes obvious when thinking of things that you can only accomplish using it (or at least that are much more costly to accomplish otherwise, and that no other lifeforms do in other ways)

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If the above doesn't make sense please tell me and I'll try to rephrase

If you want a good pop-science book that goes over how biologists think of the role of sexed reproduction you could check out "The Red Queen: Sex and the Evolution of Human Nature". I read it like 4 years ago, when I knew nothing on the subject, and while in hindsight it has some bad points (and I don't agree with the central hypothesis) it does a good job of showcasing the different views in the field in a way that you can get with limited knowledge of the molecular level processes involved.

I'm not sure why this is pinned.

Not sure what you mean by this?

As in, given that the information here is seemingly false, I'm unsure why it appeared under the "Recommended" section of the website.

Ah, that section is a random sample of posts that you haven't read, weighted slightly exponentially by their karma. So any frontpage post can show up in that section.

Oh, I though they had to be hand picked to some extent to get there, in that case I guess it makes sense.

I still hold my original cirticism of the ideas presented but that's unrelated.

This post gives an elegant explanation of the evolutionary benefit of sexual reproduction that was new to me, at least. I like it, although I also wish some expert added their thoughts.

Not quite sure where this would fit into your views. A numbers of years back I came across an article about snails. Those that lived below of certain depth of water were unisex -- self replicating. The same species of snail, when living in shallower waters displayed male and female and reproduction required the exchange of genetic materials as you mentioned.

The theory on why that was observed was at a certain depth the snails were not confronted with a lot of the diseases they faced in the shallower waters. Sharing the genetic code improved their ability to fight the diseases, IIRC.

On this picture, the claim is just that sex is worthwhile if it adds enough variance to make it worthwhile, so you could either reduce costs or increase variance.

(This picture could be wrong though, if the average of two organisms is just more fit on average than the parents and it's not about variance at all.)

(This isn't really affected by Wei Dai's concern above.)

How did you set up crossposting? IFTTT?

I didn't set it up, the LW team did.

Please ask them to auto-crosspost ai-alignment.com too, unless you're not doing that intentionally? (I have some comments on your latest post there and would rather discuss here than on Medium.)

(Already asked them if they could crosspost ai-alignment.com to alignment forum. I was just being reactive for this one though, they asked if I was OK cross-posting from sideways-view. I can manually cross-post most recent post.)