Why Sex? And why only in Pairs?

Understanding the purpose of sex is a fundamental unresolved problem in evolutionary biology. The difficulty is not that there are too few theories of sex, the difficulty is that there are too many and none stand out. To distinguish between theories, we ask: Why are there no triparental species with offspring composed of the genetic material of three individuals? A successful theory should confer an advantage to biparental sex over asexual reproduction without conferring an even greater advantage to triparental sex. Of two leading theories (red queen and mutational), we show that only one is successful in this sense.

That is a new Economic Journal paper by Motty PerryPhilip J. Reny, and Arthur J. Robson.  Of course the core question is a classic example of thinking at the margin.  The core conclusion is that mutations continue to rise with the number of sex-participating partners, but in simple Red Queen models the limiting features of the genotypes is the same whether there are two, three, or more partners.  The argument on pp.2739-2741 is not readily blog-summarizable, and I do not grasp it fully, but at the moment I have the following intuition.  If a parasite attack comes, the species needs only move away from the targeted genome to continue reproducing, due to some all-or-nothing assumptions about the nature of the attack.  This differs from the mutational game, where there is always some marginal (expected value) gain from moving yet further away from the initial nature of the species.  Playing a game against an identified opponent brings a better-specified and more stable and less varying response strategy than playing a game against an as-yet-unidentified opponent.  That isn’t how the authors put things, but…

Since we don’t observe much three-party reproduction (hardly any in fact), that suggests the Red Queen model is more likely to apply.

For the pointer I thank TEKL.


To say sex is for procreation is like saying novels are written to make movies . Well how come no movies are written to make novels? https://en.wikipedia.org/wiki/Category:Novels_based_on_films ...

Sex is mostly for philosophical reasons. To understand life and history and geography.


Sex is only for procreation. That is entirely the point. That it is pleasurable is not the purpose nor the intent, if we can talk about the intent of something that has evolved.

In fact it is so much the intent that religions that prohibit, and presumably reduce, sex for purposes other than procreation out compete those that do not. Even without any form of birth control.

"That it is pleasurable is not the purpose nor the intent, if we can talk about the intent of something that has evolved."
It is a coincidence.
"In fact it is so much the intent that religions that prohibit, and presumably reduce, sex for purposes other than procreation out compete those that do not. Even without any form of birth control."

For example, a bunch of Saudi Islamists outcompeted thousands of Americans a few years ago. I bet lots of them used birth control. Yet Saudi lobby keep outcompeting Iranian lobby.

"It is a coincidence."

What? Sex being pleasurable is the most obvious example of an evolutionary incentive in existence.

Is it an established fact that religions which prohibit non-procreative sex out-reproduce others? I see no evidence of that since A) effective contraception is so new a technology evolution has not had a chance to work through its results and B) a religion may ban something but that does nor mean the behavior does not carry on regardless since religions (and even governments) have limited means to enforce such bans, especially when they apply to very private behaviors (the "war on drugs" is an obvious case in point).

There's 2741 pages in it?

Why do so many require two for reproduction when most lifeforms reproduce without any genes from a second individual?

In a number of species, adults can either self fertile, or change gender in adulthood to solve the search problem of finding an opposite mate.

Clearly, bisexual reproduction is a degenerate evolution from a self sufficient individual species to one dependent on at least one additional individual. And many raise offspring with the aid of others, but not necessarily of opposite gender.

Economists don't seem to understand nature. Or evolution. Life is not transactional, but simply large numbers of random events that seem to have direction and order, but merely fill out the space allowed by laws of nature.

"Fooled by Randomness"

Since we don’t observe much three-party reproduction (hardly any in fact)

Surely some one will supply an example of three-party reproduction.

You're right, biologists call organisms with more than 2 parents polyploids. More than 2 sets of chromosomes comes from multiple fertilization.

Hardly common? Wheat, corn...

"You’re right, biologists call organisms with more than 2 parents polyploids. More than 2 sets of chromosomes comes from multiple fertilization."

This is not right at all. There are no sexual life forms with more than 2 "parents" (though certain organisms like slime molds blur the boundaries). Further, there are many organisms that are polyploid that have a single parent.

Have you not heard of a threesome?

One example (SFW):

I read "in Pairs" as "in Paris" and expected a different discussion

Me too. I wondered what level of craziness could this be

Me too! I believe it is because of the idiosyncratic capitalization.

Because they will always have Pairs.

I did too. That's weird. I was trying to figure what I'd been doing if it only happened in Paris.

That's an interesting about why three-party sex doesn't occur. I always thought of it as a coordination problem: it's significantly harder to get three organisms together. If you imagine early sex as just random collisions, it's highly unlikely that three bodies will simultaneously collide. (Though one can imagine, two colliding, sticking together, and then waiting for the third.)

By the way, why is this published in an economics journal and not an evolutionary biology journal?

Because they 'deferred' publishing it. [didn't make the cut]

“Since we don’t observe much three-party reproduction (hardly any in fact)”

There are artificially produced three parenets baby, https://www.nature.com/news/genetic-details-of-controversial-three-parent-baby-revealed-1.21761

Though that only with a third person’s mitochondria.

There are also naturally produced three parents baby in the full sense that the baby has two different sets of DNA, very rare but possible. the results of two rare events occuring sequentially.

First it is possible to have three parents fracternal twin, https://en.wikipedia.org/wiki/Superfecundation


“””Superfecundation is the fertilization of two or more ova from the same cycle by sperm from separate acts of sexual intercourse, which can lead to twin babies from two separate biological fathers.”””

“””one study estimated that it might occur in as many as one in 400 (0.25%) twin births in the US. Another study reported that among non-identical twins whose parents had been involved in paternity suits the frequency was 2.4%. “””

Then in some rare occasion the two zygotes can fused into one, https://en.wikipedia.org/wiki/Chimera_%28genetics%29


“””An animal chimera is a single organism that is composed of two or more different populations of genetically distinct cells that originated from different zygotes involved in sexual reproduction. If the different cells have emerged from the same zygote, the organism is called a mosaic. Chimeras are formed from at least four parent cells (two fertilised eggs or early embryos fused together). Each population of cells keeps its own character and the resulting organism is a mixture of tissues. Cases of human chimerism have been documented.[1]”””

Although fascinating, chimerism this is not really 3-party reproduction. It is, rather, just the fusion of two (or more) different cell populations. In some cases, this happens soon after sex/fertilization - but in other cases the merger happens without connection to sex (or even involving an embryo at all)...

It is the possibility for the natural fusion of three parents fraternal twins.

The resulting organism might be a blend of tissues from 3 parents, but this is not really 3-parent reproduction. *After* sex/fertilization is finished, 2 different embryos fuse. We could, more grusomely, just wait 9 months and swaps organs between 2 newborns half-siblings. In either case will achieve a chimera, but that does not really mean we have observed 3-parent sexual reproduction.

Three party reproduction isn't that uncommon in fungi.

I think your confusing fungi having more than 2 sexes with fungi having 3-party reproduction. Fungi sexually reproduce by the fusion of the genetic contribution of 2 parents

Why Sex? And why only in Pairs?

Because while the male amoeba was keen, the female amoeba said she was not that sort of girl? At least not with another female amoeba.

The fertilization process is most efficient with two parts (sperm and egg.) Think of a reproductive process that required two different sperms and one egg (or two eggs and one sperm.) It's harder to get three things together than two.

It's like a pair, which is optimal. Adding more elements just makes the access control process more cumbersome.

Didn't Newton conclude that there is no solution to the three-body problem?

With gestational surrogates, three-party reproduction is common in humans already: one provider each of sperm, egg and womb. The child gets genes from the sperm, genes from the egg, and the microbiome of the gut from the womb.

Re: three party sexual reproduction

Normsl cell division (mitosis) doubles the DNA and separates an equal amount into two cells. Meiosis evolved once, as an adaptation of this process, to replicate DNA and divide, then divide again, to make 4 cells with 1/2 the normal DNA content.

I could see how you could adapt this process to higher powers of 2, but not powers of three. Everything from meiosis, to mitosis, to DNA replication, to the structure of DNA itself has powers of two baked in.

Triploidy is a red herring -- its formally possible for it to be the result of fertilization by two different sperm (or pollen granules), but it's almost always either the result of a failure of one of the two divisions of either female or male meiosis, or duplication of a single sperm genome after fertilization. Thus only two parents, and the resulting triploids are either not viable, infertile, or undergo a duplication to become hexaploid and then participate in regular old powers of two sex.

Powers of three would likely require an entirely different form of life with a different genetic material. There would be significant advantages to asexual reproduction by powers of three vs powers of two:

1 -> 3 -> 9 -> 27 -> 81...
1 -> 2 -> 4 -> 8 -> 16...

Such that the cost of sex for powers-of-three sex (2/3 of population are excess males and male*s that cannot produce offspring on their own) would be greatly magnified relative to regular powers-of-2 (1/2 population is male). Powers of three sex would have to confer a significantly greater advantage to evolve.

Three-party sex would provide a greater incentive for the female to cut the two "males"(or whatever name the genders which don't carry/sit-on the offspring is called) out. This is known to happen in nature in our two-person regime:


The parthenogenic animals don't take over because of the problem of genetic diversity. But two's enough for that. In fact, a three-person-sex species wouldn't be much more genetically diverse in practice than a two-person-sex species. To see why, ask yourself how many "great-grandparents" you have in the fifteenth generation back: 2^15 = 32768. Now, many species have local breeding populations much lower than that. Maybe as few as a couple thousand. So many of these great-grandparents are in the family tree in many places, the animal is "inbred." This will be the same if the calculation is 3^15. In both cases, it is restricted by the local population size.

Furthermore, one of the "males" will have an incentive to duplicate the effort of the other one. That way, he can pass down double his genes.

The solution lies in law, not biology.

The law (in its majesty) can declare that one has two mothers, or three fathers- or any number of parents of any sex.

Law is obviously superior to mere biology.

Why do humans not have cyclical periods of estrus/rut like many mammals? Having minds that are not clouded by passion all the time should be an advantage. Look at all the dumb things that have been done as a result of amatory feelings. Being free from this drive for eleven months a year would permit much progress.

BTW, considering how difficult it is for many people to find one other person while having a 24/7 sex drive, having to find a third person would require a huge amount of energy. Eartworms are hermaphrodites because it is hard to encounter another earthworm while tunneling; their current arrangement makes every encounter potentially productive.

This is a pseudo-question if ever there was one. We don't have to explain why a species hasn't evolved in a certain way. That is a fact. I am organized vertically (one hole on top, one on the bottom with most organs in between) because my ancestors were worms. A theory of sex T is not a more likely to be the correct explanation of the evolution of two parent sex because it explains the non-fact of the non-existence of three parent sex. Otherwise, theory T could be supplanted by theory T2 that provides a better explanation for why there is no four parent sex. And, so on. This would turn biology into speculative metaphysics. One theory is better than another because it provides us with a speculation on why we did not evolve differently than we had. We didn't evolve a certain way because we evolved the way we did. Those are the biological facts that require explanation. The actual question is why sex given its costs and the obvious real benefits of asexual reproduction (which actually exists). What is the benefit of sex that is conferred to out-compete asexual reproduction. Nothing about three parent sex (which doesn't exist) answers this actual scientific question. That is probably why this biology paper appear in an econ journal.

I don't know about you, but I have *two* holes on the bottom.

I also have legs. We are not actually worms. We evolved from worms.

Is anyone else curious about why that article had three authors?

Perhaps we just haven't gotten there yet. DNA is double stranded and two double strands would be the next most likely, but it doesn't mean a three stranded helix of another kind would be impossible, just more complex and difficult to develop.

If you draw a black box spanning generations, two-ancestor sexual reproduction can emulate any other ancestor count you wish to name.

Exactly - the authors seem to use one-time reproduction by ancestor-less parents as their implicit model.

Why sex? and Why pairs? are interesting questions, but I'll add a third: Why Economists?

Mathematical biology is already a field, and includes people with much better mathematical or biological backgrounds -- or both! -- than any economist is likely to have. You'll note that this article appears in an economic journal, not a mathematical biology journal.

What is it about economics that makes economists so willing to accept ersatz recreations of other fields?

There's this really cool Principle of Having a Bigger Advantage in One Field than Another that I read about in an art history journal one time. Maybe economists should look into that.

The references article seems to consider sex as a single event in which 2 sui-generis organisms combine rather than the result of successive generations. Sexual organisms are *already* the result of many, many "parents" - I have the genetic contribution of my 2 parents, who embody the genetic contribution of my 4 grandparents, who embody the genetic contribution of my 8 great grandparents. Thus, a sexual organism doesn't need more than 2 parents because it already received the variation of scores of ancestors from the 2 parents it has.

And, second, 2 partner sex is just one form of genetic mixing - one that is ubiquitous because of common descent among higher organisms. Biological historical contingency explains that part.

Sex evolved to defeat parasites.

"Sex" is too large a category to have one driver - and parasitism is only one (likely minor) component:

The changes to cells to create germ cells (sperm and egg) may have been a result of competition between colonial cells:
You would have an advantage if you could convince your competitor cells to produce cells that leave the colony rather than dividing in-place and crowding your space.

A single cellular organism that experience some damage or loss of genetic material could "save" itself by combining/swapping material with a genetically intact partner.

Sex evolved in single-celled before there were separate male & female parents to bring together - so our main intuitions about "sex" are usually not based on the original version...

> Sex evolved in single-celled before there were separate male & female parents to bring together – so our main intuitions about “sex” are usually not based on the original version…

Precisely what I'm referring to. Perhaps I'm oversimplifying? I thought it was the same basic evolutionary pressure regardless of scale except that in larger phenotypes, it has the "advantage" of distributing beneficial mutations even faster because the replication rate is lower than for small creatures.

Thanks - I now have a good question for the family biologist ( doesn't everyone have one?::) over the holidays :)

Maybe it is something to do with the planet you live on...

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