Cross-posted from substack.

"Everything in the world is about sex, except sex. Sex is about clonal interference."
– Oscar Wilde (kind of)

As we all know, sexual reproduction is not about reproduction. 

Reproduction is easy. If your goal is to fill the world with copies of your genes, all you need is a good DNA-polymerase to duplicate your genome, and then to divide into two copies of yourself. Asexual reproduction is just better in every way:

Build costly DNA-manipulating machinery that chops the DNA into pieces to produce gametesJust copy yourself bro
Scout the perilous wild for a mate, and perform a complicated ceremony so your gametes fuse with each otherJust copy yourself bro
Only pass 50% of your genome on to the next generationJust copy yourself bro
Differentiate into two types, making it twice as hard to find a matching gameteJust copy yourself bro
Make all kinds of nonsense ornaments to satisfy the other sex's weird instinctsJust copy yourself bro

It's pretty clear that, on a direct one-v-one cage match, an asexual organism would have much better fitness than a similarly-shaped sexual organism. And yet, all the macroscopic species, including ourselves, do it. What gives?

Here is the secret: yes, sex is indeed bad for reproduction. It does not improve an individual's reproductive fitness. The reason it still took over the macroscopic world is that evolution does not simply select for reproductive fitness.[1] 

Instead, the evolution of sexual dimorphism is a long sequence of strange traps, ratchets and outer-world eldritch cosmic forces that made it somehow inevitable. So let's talk about those things your parents never told you about.


The birds, the bees, and the fission yeast

What bugs me is that, not only most people have absolutely no idea why sexual dimorphism exists, but they seem entirely fine with that. Our lives are punctuated with all sorts of frankly weird practices related to it, but the reasons we ended up there remain obscure even to many biologists.

So I figured I would write up a summary of some popular theories. This way, when time comes, you can explain to your children the long evolutionary trajectory that culminated in VR ChatGPT cat-girlfriends.

(Note 1: As always with evolutionary biology, everything in this article is subject to uncertainty, controversy and mystery. Always keep in mind the Golden Rules of biology: all models are wrong; everything has exceptions; don't talk about fungi; mitochondria is the powerhouse of the cell.)

(Note 2: As this is a bottomless topic, I’ll have to make some cuts. I know you’re burning to learn about Pseudobiceros hancockanus’ penis fencing, but I can’t cover everything.)

First, let's get something out of the way.

Something something diversity-generation

I often hear vague explanations about sex being a way to generate genetic diversity. I don't find it compelling. If you want genetic diversity, you can do it in much easier ways than turning into a sexually-reproducing dimorphic species. One of them is, just increase the mutation rate, bro.

Bacteria are good at this. E. coli comes with a whole toolkit of DNA-polymerases with various degrees of accuracy. When everything is going fine, they use the most accurate one to faithfully replicate their genomes. But, in case of particularly bad stress, the bacteria start expressing error-prone polymerases, which increase their mutation rate. Who knows, if the mother cell is going to die anyway, some of the mutant offspring might stumble upon a solution to escape the bad situation.

All that is to say, raw genetic diversity cannot be the whole picture. It has to be a specific kind of genetic diversity.

Part 1: the evolution of sex

Most of the articles I read about the evolution of sex ask "what are the advantages of sexual reproduction?", then proceed to explain what are the advantages of sexual reproduction. The problem with this approach is that, if sexual reproduction really had such clear advantages, nobody would do asexual reproduction any more. But, to this day, asexual species are still very much around and successful. What we need to know is, "in what ways does sexual reproduction give access to new evolutionary niches?"

So, what do all sexually-reproducing organisms have in common? For context, sex evolved about 1-2 billions years ago, before multicellularity, but later than photosynthesis. It's very closely associated with the eukaryotes, a clade that appeared when archaea started eating bacteria, and the bacteria turned into organelles like mitochondria. In fact, mitochondria seem to be closely related to sex: to my knowledge, organisms with mitochondria always use sexual reproduction, as if mitochondria made sex necessary. Could there be a link between these two?

Before answering that, let's examine what sex does to your genome.

Genetic hitch-hikers and clonal interference

Let's start with an innocent, asexual bacterium. It reproduces by dividing itself into two daughter cells, who then proceed to do the same, and so on. Sometimes, a mutation occurs somewhere in the DNA. If it's a bad one, it will soon vanish from the population. But if it's a good one, the mutant will reproduce quicker than its siblings, and the descendants of this mutant will eventually take over the whole population. That's evolution 101.

What happens if new mutations occur at a faster rate than natural selection can sort them out? Two bad things can happen.

The first one is clonal interference. This is when a second beneficial mutation occurs in an unrelated cell before the first good mutant has time to take over. If the second mutation is even better than the first one, then it is that mutant that will take over, and the first mutation will be lost forever. That's too bad, because it would have benefitted the species.

From Peabody et al. 2017. Mutation C is clearly good, but the even better mutation D appears somewhere else and drives C to extinction.

The second problem is genetic hitch-hiking. This is when a beneficial mutation occurs in the same lineage where a detrimental mutation just occurred. If the good mutation has a larger effect than the bad one, the mutant will still grow in frequency, and the bad mutation will extend to the whole population.

That's quite a big problem. Since it's easier to break things than to improve them, the majority of possible mutations are bad ones. Thus, if a cell finds one beneficial mutation, it will often come with a bunch of detrimental hitch-hikers, and there's no way to get rid of them. This is called Muller's ratchet and in some conditions it can make fitness decrease as a result of natural selection.

The Fisher-Muller model[2]

Enter sexual reproduction. Instead of getting all the mutations from the mother cell, the newborn cell receives an assemblage of random pieces of each parents' genome. As you repeat the process, you end up with many different possible configurations. Among them, hopefully, there will be newborns with all the good mutations, and none of the bad ones.

From Peabody et al. 2017. Here a cell with "AC" mates with a cell with "ABD", creating a glorious "ABCD" mutant, and C doesn't go extinct.

In effect, sexual reproduction parallelizes natural selection, as each variant gets tested separately in a different individual.

That’s the theory. Does it work in practice? McDonalds et al. (2016) had yeast evolve with and without sex for 1000 generations and sequenced them at regular points in time. In the following plot, the blue alleles are bad, the orange ones are good. It's pretty spectacular:

The asexual lines caught a lot of bad mutations by genetic hitch-hiking, while the sexual lines managed to purge all of them while retaining the good ones.

And, despite the cost of sexual reproduction, the sexual lineage (orange) was able to evolve more efficiently, and over the long run its fitness improved much faster:

This is a typical example of second-order selection.[3]

I can hear you complaining, "this is entirely unrelated to mitochondria". Where do they fit in the picture? And why do organisms without mitochondria get away with the old asexual reproduction scheme?

Hot, hot DNA

Remember, we are ~1.5 billion years ago, and photosynthetic organisms have just released massive amounts of oxygen into the atmosphere, leading to the Great Oxidation Event. Some bacteria are starting to use this oxygen for breathing, turning them into little living powerhouses. Then an archaeon swallowed one such bacterium and made it into its very own intracellular powerhouse of the cell, starting the era of the eukaryotes. Unfortunately, heavy respiration releases a lot of oxidizing chemicals, and instead of going to the environment, these chemicals now accumulate in the eukaryote's cytoplasm, creating a lot of DNA damage. The mutation rate goes up.

Asexual reproduction works marvels in the low-mutation, high-selection regime: a mutation occurs, if it's good it takes over, if it's bad it disappears. Then the next mutation arises. Clonal interference is not a problem because mutations are rare enough they get selected one by one.

But if the mutation rate increases due to heavy respiration, both clonal interference and Muller's ratchet become much worse:

Fig. 1
From Peabody et al. 2017

Thus, the increased mutation rate due to mitochondria is probably what kick-started the evolution of sex

This opened the door for entirely new opportunities: as genome size increases, Muller's ratchet gets worse and worse. Sex makes it possible to have larger genomes, packing more genes, and allowing for more complexity. This paved the way for exciting stuff like multicellularity.

Part 3: not my type

Now we have a cool mechanism for shuffling the genome, but this mechanism doesn't include any mating types yet. That is, we have only one type of individual, who can mate with any other individual. But this is very uncommon in nature. Basically every species we know differentiates into separate mating types, like "male" and "female", that cannot reproduce with themselves. Even baker yeast, who don't have any apparent male-female distinction, still have simple molecular components to switch between two mating types, so they can only mate with yeasts of a different type. (Yes, I know about snails. Hermaphrodites do have exclusive mating types, they just happen to be carried by the same individuals.)

Why aren't yeast pansexual?

There are many reasons why mating types could evolve, and it's not clear which one(s) really happened. Here's a good review[4], I'll just go over the ones I find most interesting:

Selfing-prevention: exclusive mating types prevent an individual from mating with itself, also known as selfing. Selfing is easy, as an individual's own gamete are already around and immediately accessible. That would completely defeat the point of sex, not to mention inbreeding depression.

The molecular explanation: to have two gametes fuse together and combine their DNA, you probably need some kind of ligand-receptor pair. It could be two surface proteins that bind to each other and cause the membranes of the two cells to fuse. Obviously, this doesn't work well if both cells are identical, as their ligands/receptor pairs would constantly try to bind to each other within the cell's own membrane. Or gametes might release pheromones to attract each other – clearly this doesn't work if the gametes are attracted to their own pheromone. Hadjivasiliou and Pomiankowski, 2016 present a lot of evidence of this happening in various bizarre microscopic creatures. 

Organelle competition: eukaryotes have organelles like mitochondria or chloroplasts, who come with their own DNA and replicate independently. Now imagine a rogue, mutant mitochondrion that replicates much faster than the rest, instead of doing its powerhouse job properly. Nothing prevents this mutant from replacing all the good hard-working mitochondria until the cell is barely functional. But if you have exclusive mating types, you can have a mechanism such that only one parent will transmit its organelles to the progeny (e.g., in humans, the mitochondria come exclusively from the female). Now, there is no competition between individual organelles – the fitness of an organelle is locked to the fitness of the entire organism.

It takes 23,328 to tango

Ok, but why only two mating types? The male/female yin/yang mars/venus duality is something we take for granted, but in terms of evolutionary stability, it sounds like the worst possible arrangement. Say we start with two mating types. If a mutation creates a third type, it will be much easier for the new mutant to find a partner, as it can mate with everyone else in the population instead of only half of it. So it should rapidly invade until it reaches the 1/3-1/3-1/3 equilibrium. Then we can keep adding new mating types – here the optimum seems to be "as many as possible". The mushroom Schizophyllum commune gets it, with its 23,328 different mating types (which makes me wonder what discussions are like in their gender studies departments). But the two-sex binary is by far the most common arrangement in nature. Why don't we all have an interesting sex life like Schizophyllum?

The counter-balancing force here is genetic drift, the variation in a gene's frequency due to random sampling between generations. As you add mating types, the number of individuals of each type becomes increasingly small, and it's increasingly likely that all individuals of a type will be lost to random sampling.

In practice, the number of mating types depends on how many generations of asexual reproduction happen between two sex events, as this governs the relative importance of genetic drift compared to the benefits of an extra mating type. As it turns out, our unicellular ancestors probably did a lot of asexual generations between two mating events, so the number of mating types was pushed to the minimum of two. (In contrast, Schizophyllum, the sexy mushroom, uses sexual reproduction all the time, so it makes sense for it to be so non-binary.)

Note that we are not dimorphic yet. The "males" and "females" might express different receptors and secrete different pheromones, but they still have basically an identical body. The next transition, again, sounds absurdly complicated: you'd have to wire an entire gene regulation program so the population differentiates into two types, which means covering every cell in the organism with appropriate receptors so each tissue knows in what way to develop. Preposterous.

This is when the room temperature drops, rain starts pouring, the old wooden beams scream ominous screeches, and a band of contrabasses starts playing Arnold Schoenberg. Here enters our old friend, Moloch.

Part 3: symmetry-breaking

The next step in our journey takes us from two equivalent sex types producing symmetrical gametes, to males producing swarms of tiny motile minimalistic sperm cells and females producing huge oocytes packed with covid-survivalist levels of food.

How exactly the transition happens is hard to model, because it was certainly influenced by the spatial structure of the environment or the non-linearities in the function “material resources in a gamete → fitness”. 

But we can get a rough idea by considering a species with two types of gametes, whose size is controlled independently by different sets of genes. Each type of gamete can be either big and packed with resources, or small and massively-produced. We will see that, even if we start with everything symmetric, this configuration is unstable and the symmetry will inevitably break to create dimorphic organisms, where each sex produces either big or small gametes. 

What follows is very tedious, but I will do it so you don't have to. (Really, you can skip this and just trust me.)

Consider a diploid amoeba with two genetic loci, M and F. M controls the size of male gametes, and F the size of female gametes. Each locus has two possible alleles: B (big) and S (small). Therefore, a haploid gamete can have four possible genotypes: MB/FB, MS/FB, MB/FS and MS/FS).

What happens if an MB/FB population encounters a tiny MS/FB mutant population? In general, small gametes have a huge advantage. Say a big gamete contains 1000 units of resources, and a small one only 1 unit, but there are 1000 times as many of them. Two big gametes mating together make a 2000-resources zygote, while a big gamete mating with a small one makes a 1001-resources zygote. It means the small gametes can mate with 1000 times as many big gametes, but the zygotes’ resources are only reduced by ~50%, a pretty good deal. So we get a few MB-MB/FB-FB diploids (from MB-FB mating with itself), a lot of MB-MS/FB-FB from the wild-type mating with the swarm of MS/FB gametes, and a tiny amount of MS-MS/FB-FB from the mutant mating with itself. Therefore, the next generation of gametes will have a lot of MS/FB gametes, who will then mate together until they have completely taken over.

This works just as well for MB/FS. What happens if a population of MB/FS encounters one of MS/FB? We get the haploids MB-MB/FS-FS, MS-MS/FB-FB and MB-MS/FB-FS. This gives us a new type of gamete: MS/FS. This one is extremely good at fecundating other gametes, producing MS-MS/FS-FS diploids. However, these diploids cannot mate with each other, as a zygote made from two small gametes wouldn’t have enough resources to be viable. So the MS/FS gametes cannot possibly take over the population. We are left with MS/FB and FB/MS, who are tied, until one of them takes over for an unrelated reason (like a beneficial mutation somewhere).

You can do the same for other kinds of mating, like MS/FS vs MB/FB. You’ll see that, eventually, individuals with one big and one small gamete type always win. 

At this point, the advantage of having many small gametes isn’t so much to produce more viable offspring, but to keep all the eggs for oneself and prevent competitors from fertilizing them. On the collective level, this is far from optimal, since a lot of the small gametes are wasted. It harms the offspring, as they have to start with half as much resources than if the two parents' gametes were big. But the collective optimum turns out to be unstable.

Again, natural selection selected against reproductive fitness – an isogamous species would win over an anisogamous one. But here, we are not talking about the competition between two different species. We are talking about intraspecies competition: an organism versus its own mutants.

And that’s it, we have evolved anisogamy, sexual dimorphism for gametes. From now on, we define the "male" as the type who makes the numerous small gametes, and the "female" as the one who makes the scarce overpowered oocytes.[5]

Can this dimorphism extend to macroscopic traits, like human breasts or peacock tails?

Part 4: the dimorphification

Here we enter the "look at this funny lizard I found" part of biology.

Before we get to sexual selection, the good old natural selection still plays a big role in generating dimorphism. Now the symmetry has been broken, the species as a whole can optimize the way it does sexual reproduction by specializing males and females' bodies in different ways. Various appendages appear to streamline the process. But I'm sure your parents already explained that part perfectly well.

[Small aside: the oldest known sexually-reproducing organism is the algae Bangiomorpha pubescens. If you think it's funny that the first sexually-active species is called this way, you will be delighted to learn that the first animal known to practice internal fertilization (that is, with a dick) is a fish called Microbrachius dicki. It is named after its discoverer, Robert Dick. Biologists have no sense of humour, this just keeps happening.]

As Darwin himself pointed out, once sexual dimorphism exists, a second kind of selection applies: sexual selection.

The Bateman

Now that female gametes are the limiting factor, the two sexes face new incentives:

  • For males, what happened at the gamete-level roughly happens again to individuals. They produce gametes in excess: the best way to succeed is to claim as many partners for oneself as possible.
  • For females, if you are already guaranteed to reproduce, mating with more people will not necessarily improve reproductive success. Devoting more resources to the survival of the zygote after fecundation is more important. This includes mating with the best males.
  • As a result, what started with gamete size ends with society-level dynamics where males face intense competition, to the point of fighting each other, resulting in a wide variation in reproductive success among males.

This is called Bateman's principle, the Red Pill people love to bring it up, and it is kind of wrong.

First, Bateman's original research is pretty weak. Second, while it's true that the males are most often the ones facing intense competition, it's far from universal – in many species the males (despite producing the smaller gametes) are responsible for all parental care and in these case, it's big muscular females who fight for males. (I made up the "big muscular" part).

Overall, there is no clear pattern about how the two sexes diverge. Things can go all over the place. Perhaps you think humans' mating rigamarole is weird (a typical kenjataimu experience), but really, we are relatively tame. There are extremely unholy things like Sacculina carcini's parasitic castration cycle[6], Bonellia viridis whose micro-males are so disposable they don't even have a mouth to eat with, or (god forbid) Pseudobiceros hancockanus's penis fencing.

Some filthy, digusting sea stuff
Bonellia viridis (source). This is the relatively normal female. The male, not depicted here, is the one that is weird.

Fisherian runaways

Imagine a species of birds who benefit from having elevated feathers on their foreheads for some reason. Maybe it's for dusting off cobwebs in the nest's ceiling, I don't know. This evolutionary niche impacts selection in two ways – direct selection, and sexual selection. Let's think in terms of genes:

  • If a variant makes a bird's top feathers grow larger, it will be directly selected for,
  • If another variant makes birds sexually attracted to birds with thick head-plumages, it is also selected for. This is because carriers of this variant will often mate with individuals who carry the head-plumage variant, producing offspring that carries the two variants together. While the sexual-attraction variant gives no advantage by itself, it will rise in the population by hitch-hiking with the adaptive plumage variant.

It sounds like all is good: females will instinctively be attracted to the males with the optimal amount of head-feathers and everybody will win. But we fall prey to another empyrean pagan god and our path runs into another ratchet.

Here's the problem: at the beginning, before selection for thick feathers happens, all the males in the population are below the optimum. There is some variation, but no one comes close to the optimal coiffure. So what instinct do you think will be evolved first?

  • Being attracted to the optimal amount of feathers, which is unknown and doesn't occur anywhere in the population yet, or
  • Being attracted to as much feathers as physically conceivable, the more the hotter?

You bet.

Gif of a bird parading.

These two instincts are functionally equivalent – in both cases, the female will pick the male with the most feathers available among the current population. The hitch-hiking will work just as well, and the much simpler "moar feathers" instinct will be selected for.

And now we are in trouble. Let's say this goes on until the average male has just the right amount of feathers. At this point, all the females have acquired the instinct to find head-feathers super hot, and they still want moar. Having more feathers remains a fitness advantage, not because it helps clear up cobwebs, but because it attracts more sex partners. So the average head plumage will continue to expand, way beyond the optimal amount, until every male bird has turned into an 18th-century macaroni.

This whole thing is called the Fisherian Runaway, after our beloved eugenicist Ronald Fisher. And we cannot go back: if a new allele makes a female attracted to a less insane amount of feathers, she will mate with less conventionally-attractive males and will have less sexy sons, so the allele will encounter a barrier when it's time for her son to find a mate.

The optimal bird: "why don't females mate with nice guys like me?"

A possible example in humans is the boob. Other primates don't have boobs – they are flat most of the time and only swell for lactation. Maybe, at the beginning, swollen boobs was a sign of fitness, then human males got really into swollen boobs, then human females started padding them with fat to appeal to the males' instincts, leading to the persistent round boobs we witness today – even if the pad of fat isn't actually very useful for lactation.

(Note that this is one of many hypothesis about the evolution of breasts. It’s a highly controversial subject and an active research topic.)

Evolutionary biologists discussing boobs

Sexual selection can lead to all kinds of seemingly implausible phenotypes. If you want more, this review by Michael Ryan has a lot of funny shit ("sand pillars built by male crabs that approximate refugia to females, fins of male fish that mimic food, and male moths that mimic bat echolocation calls"). And let's not forget Basolo's classic Science paper about the sword-less ancestor to swordfish being attracted to human-made swords.

Epilogue: are aliens sexually dimorphic?

If there are other intelligent lifeforms in the galaxy, it sounds likely to me that they evolved through natural selection. It's also likely that they are relatively complex organisms, since they must have evolved some form of intelligence. They may be very different from us, but could they still be sexually dimorphic?

I would guess it's plausible. None of the evolutionary transitions that led to sexual dimorphism are obvious, but they seem almost impossible to escape. Based on what has been observed on Earth, sexual reproduction is virtually necessary to evolve into a complex fully-fledged multicellular organism.[7][8] Even the very first step, about genetic hitch-hiking, could apply to biological systems completely different from DNA. It's about searching through the space of possible sequences for the fittest one, and how much information you get every generation.  So I would guess dimorphism is more frequent than, say, action-potential-based neurons or sound-wave communication. But, by the time we meet them, they might have engineered themselves into yet another stage of evolution, and none of this will be relevant any more.


  • Unicellular organisms used to divide asexually, selecting one mutation after the other
  • Photosynthesis opened the possibility of oxygen respiration, then an archaeon ate a respirating bacterium to make it its personal powerhouse. That generated a lot of oxidative damage on the DNA, leading to a lot of clonal interference between good mutations, and a lot of bad mutations hitch-hiked with the good ones
  • This problem could be solved by mating with other individuals and shuffling homologous DNA to generate new individuals with random combinations of the existent mutations, allowing to purge the deleterious ones
  • This improved evolvability and made it possible for macroscopic organisms to develop
  • For various reasons this system worked better with exclusive mating types, and genetic drift pushed the number of types to two in most cases
  • While initially symmetrical, intra-species competition made symmetry unstable and the two types diverged into big gametes and large gametes
  • The asymmetry extended to the species' bodies in general, through natural and sexual selection
  • Genes coding for sexual preferences started to hitch-hike with beneficial genes
  • As the sexual preference instincts were misaligned with the optimal levels of traits, this led to Fisherian runaways, and organisms developed ridiculously exaggerated traits
  • And this is how I met your mother.
  1. ^

    Here I define reproductive fitness as the average ability of your genes to reproduce. That's it. This may be different from the way you define fitness in general. For a deeper discussion, I recommend Hannah Kokko's great review on the stagnation paradox.

  2. ^

    This is also referred to as the "Vicar of Bray" parabola, but I never understood why.

  3. ^

    An alternative way to look at it is in terms of the distribution of fitness among a group. Sexual reproduction may decrease the immediate average fitness, but it also increases the variance of fitness, as it creates individuals with all the bad variants, and others with all the good variants (this is not the same thing as having more genetic diversity!). In conditions with only the few individuals with the highest fitness reproduce, then having a high variance in fitness means it's more likely that the fittest individual will be from your offspring.

  4. ^

    When the intro of a review ends with "we finally attempt to validate or refute these theories using data on fungi", you know things are about to get wild.

  5. ^

    Male and female are therefore not defined by the presence of an Y chromosome. Many species (like birds) use something completely different. Many other species don't use sex chromosomes at all, and differentiate in males/females based on environmental clues like temperature.

  6. ^

    Quoted for posterity: "The female Sacculina larva finds a crab and walks on it until she finds a joint. She then molts into a form called a kentrogon, which then injects her soft body into the crab while her shell falls off. The Sacculina grows in the crab, emerging as a sac [...] on the underside of the crab's rear thorax, where the crab's eggs would be incubated. Parasitic Sacculina destroy a crab's genitalia, rendering the crab permanently infertile. [...] The male Sacculina 'larva' looks for a female Sacculina on the underside of a crab. He then implants his cells into a pocket in the female's body called the "testis", where the male cells then produce spermatozoa to fertilize eggs. When a female Sacculina is implanted in a male crab, it interferes with the crab's hormonal balance. This sterilizes it and changes the bodily layout of the crab to resemble that of a female crab by widening and flattening its abdomen, among other things. The female Sacculina then forces the crab's body to release hormones, causing it to act like a female crab, even to the point of performing female mating dances. [...] When the hatching larvae of Sacculina are ready to emerge from the brood pouch of female Sacculina, the crab [...] shoots them out in pulses, creating a large cloud of Sacculina larvae. The crab uses the familiar technique of stirring the water to aid in flow."

  7. ^

    I don't count volvoxes or slime molds as "fully-fledged", and I don't want to hear about mycorrhizal fungi.

  8. ^

    A long time ago, scientists thought they had discovered an exception: a strange rotifer called Bdelloidea, who doesn't seem to reproduce sexually at all. All the individuals ever observed were female, so either the males are hiding very well, or they are reproducing asexually. But it turned out that they used to be sexual, then decided that the future is female and went for parthenogenesis. If the theory is right, this comes with a big cost in evolvability, so we'll see how they handle global warming. Likewise, there are many strictly-asexual plants, but all of them have made the switch recently. It doesn't look like asexuality in plants in very stable over long periods.


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I often hear vague explanations about sex being a way to generate genetic diversity. I don't find it compelling. If you want genetic diversity, you can do it in much easier ways than turning into a sexually-reproducing dimorphic species. One of them is, just increase the mutation rate, bro.


I think the math is actually pretty clear on this one - sexual selection is an asymptotically more effective optimization algorithm from information theory first principles. If this weren't true I wouldn't expect sexually reproductive species to be so dominant, given we evolved from asexual ones.

There happens to be a chapter called "Why have sex?" in MacKay's *Information Theory* on this topic. In his simplified models, the rate of information gain / good genes discovered per generation is much larger with recombination than without.

Intuitively, mutation in asexual organisms involves randomly changing genes, and if the organism you're starting from is high-fitness, randomly changing genes is much more likely to be bad than good. So every generation you have to overcome this immense amount of mean regression just to break even. Jacking up the mutation rate makes the problem worse. In sexual selection, you get good genomes basically for free (meaning you can support a much higher mutation rate too!)

That's not to say asexual reproduction doesn't work - clearly it does - but it seems to only be viable for small genomes. Past a certain genome size, the asymptotically better scaling of information gain outweighs the constant transaction costs of matching.

I came here to mention the MacKay :) it's a great chapter

As you add mating types, the number of individuals of each type becomes increasingly small, and it's increasingly likely that all individuals of a type will be lost to random sampling.

In practice, the number of mating types depends on how many generations of asexual reproduction happen between two sex events, as this governs the relative importance of genetic drift compared to the benefits of an extra mating type. As it turns out, our unicellular ancestors probably did a lot of asexual generations between two mating events, so the number of mating types was pushed to the minimum of two. (

I am confused. If I think "Genetic drift means that many-typed populations tend to collapse towards fewer types as some types randomly die off," which is putting your hypothesis in my words, I feel like this predicts that we should see some sort of distribution over N-typed species -- with a bunch having 2 genders, a smaller bunch having 3, a smaller bunch having 4, etc. But instead it seems like they almost all have 2. Which I suppose is what you'd expect if the genetic drift effect was VERY strong. But it doesn't seem plausible to me that it would be that strong. Moreover, wouldn't this effect be weaker in larger populations? (idk just attempting to reason about something I have no expertise in here!) Also what does # of generations of asexual reproduction have to do with it? And why does it matter that our unicellular ancestors had only two mating types; surely that was long enough ago that we'd have time to evolve three or four types by now?

I have the same concern as Daniel Kokotajlo, but for a different reason. 

Mr Malmesbury considers the gradual extinction of genders beyond 2, but he never mentions the injection of fertile new genders into the population through mutations. 

In order to make the case convincing case for unique suitability of exactly two genders, we should look for reasons why systems with three or more genders would be unstable. Here is a hint: consider a third gender entering into an established species with two genders: one with a huge gametes, the other with tiny gametes. Where does the gamete size of the new entry fit in?

The number of generations controls how long your experiment lasts. The longer (or more generations), the more drift you have, so the more likely for a given gene (or in this case - genders number) to take over. This effect will be weaker in larger populations, but unless you have an infinite population, given enough time (or generations), you'll end up with the 2 sexes (except for fungi, of course, as always). Eukaryotes first appeared 2.2 billion years ago. For comparison, the Cambrian explosion, with the first complex life, was only ~500 million years ago. That's a lot of time (or generations) for things to stabilize.

There are multiple mating types around. Mammals have the XY/XX chromosome thing going. Birds have a different chromosome set (denoted as ZW/ZZ). Some families use egg temperature to determine sex. Some fish have one male, and if it disappears, the next ranking individual becomes the male. Insects also have totally different mechanisms. But there are usually only the two sexes (apart from fungi), probably for the efficiency reasons outlined in the OP.

I believe there are also single-celled eukaryotes which have more than two mating types.

I think the key is that you have to have a system where a third mating type makes sense. Having fallen into the basin of attraction of anisogamy, and then later sexual differentiation of reproductive anatomy, it's much harder to develop a new sex that could reproduce with existing males and females (but not itself).

The way the fungal system that leads to the claim of over 20,000 mating types for Schizophyllum commune is similar to how our pheremones (purportedly?) work; you just want to find someone who smells different (i.e., has a different set of MHC) from you, and there are many ways to have different MHC combinations. If someone develops a new one -- good! They smell different than everyone (except their own children) and so they never end up stuck with a distantly related potential mate who nevertheless smells too much like them, and this improves their reproductive success until the new MHC is widespread in the genepool.  Additionally (in the case of MHC but not mating types) there is purportedly actual immune benefits which drive this, in addition to the generally beneficial encouragement of "out-crossing".

But isn't there selection pressure for a two-sex species to evolve into a three-sex-species and so forth? why is the equilibrium 2 instead of 3 or 5 or different for different species? I guess you are saying the force of genetic drift is so strong that it overcomes the force pushing towards more sexes in pretty much every species ever... but since genetic drift is by definition a pretty weak force, I think that means you are saying that the pressure towards more sexes is extremely weak. Why is that? Is it not that beneficial to be able to mate with 100% of the population instead of merely with 50%?

The "random sampling" that causes genetic drift is applied once every generation, asexual or not, so the optimal number of types depends on the ratio of generations that are asexual vs sexual. The Constable & Kokko paper has a mathematical model to quantify how many asexual generations you need for 2 being the optimum, and it turns out that most isogamous species are well into that regime.

That being said, you're entirely right when you ask "why is the equilibrium 2 instead of 3 or 5 or different for different species?" – Constable's model and empirical data is only for isogamous species like baker's yeast. It seems plausible that our isogamous ancestors were in the same regime, and then anisogamy evolved and kind of locked us into a 2-types configuration. But that's mostly speculation, I don't think we have any clear empirical data that confirms this hypothesis. That's still open to investigation.

Another thing I didn't mention is that the organelle-competition hypothesis naturally leads to 2 types, so it could simply be that.

Arguably you do in fact see this, at least as to 3-type species. Many bees, ants and other eusocial insects have queens, drones, and workers. Workers are often described as sterile females, but it seems like that's us 2-typers imposing a 2-type frame on what are clearly 3 types.

Workers are only sterile in the most eusocial of species. In others, being a worker vs. queen is something of a choice, and if circumstances change a worker may start reproducing. There isn't a sharp transition between cooperative breeding and eusociality. 

Even in very eusocial haplodiploid species (so ants and bees, but not termites), unmated workers may reproduce after the death of the queen. They can only produce sons, but it's still reproduction. .  

In others, being a worker vs. queen is something of a choice, and if circumstances change a worker may start reproducing. There isn't a sharp transition between cooperative breeding and eusociality. 

Yep. Once the old naked mole-rat queen dies, the remaining female naked mole-rats have a dominance contest until one girl emerges victorious and becomes the new queen.

Drones are haploid and develop from unfertilized eggs. Queens and workers are diploid.

Stingers are ovipositors, and obviously workers have them. (It makes sense if bees and ants evolved from parasitoid wasps.)

There are degrees of eusociality, and workers are only mostly sterile. When they do reproduce, they lay eggs. Queens may suppress this tendency with dominance behaviors, pheromones, and may eat eggs laid by the workers.

For these reasons it makes sense to call the workers female like the queens.

Workers don't mate. I don't think they count for purposes of this discussion.

organisms with mitochondria always use sexual reproduction


Or at least their ancestors did. You mention Bdelloidea in a comment, which are one of the inevitable exceptions (as you mention in the introduction, which I very appreciate, as "everything in biology has exceptions" is something I often find myself saying), but they are descended from eucaryotes which did have mitochondria.

The opposite seems true, though - true sexual reproduction seems to be exclusively by eukaryotes. So you could also say that sex makes mitochondria necessary. There seem to be a couple of good jokes there...

One other pedantic note to add to this generally excellent article is that non-eukaryotic organisms also have methods to mix their genes, what with bacterial conjugation or viral recombination, without the dimorphism. 

I came here to say this; there are many species of Eukaryotes that seem to reproduce exclusively asexually. I know Malmesbury said not to mention Fungi, but I'm a mycologist so it's what I do. The lesson there seems to be the asexuality evolves fairly easily from sexuality, and is adaptive when you have a good genome which is well adapted to a relatively stable environment.  But it's also kind of a dead end; you don't usually see large groups of related species which are all asexual (with the possible exception of Glomeromycota, although their genomes suggest they are in fact getting some action).  Instead, the closest relatives of asexual species are often sexual species.  I believe the same is also observed in plants.

The n°1 reason why I said not mention fungi is that I'm absolutely not a mycologist and I wouldn't be able to talk about them. So I greatly appreciate that you do it! Typically, I had never heard of glomeromycota, despite them apparently being involved in symbiosis with 80% of plants. I like to think that I have a decent understanding of the living world, and then I'm constantly reminded that I don't, and probably nobody does...

My understanding is pretty much what you said - when the going is good, then go asexual (e.g. strawberry runners, grasses or Asian knotweed), but also try for seeds, There are a couple of species of plants that have lost the ability for sexual reproduction, but I can't recall them right now. That being said, various plants used by humans can be pretty much exclusively reproduced asexually and so have lost the ability for sexual reproduction, specifically because they have very stable environments. The obvious examples are seedless fruits (bananas, grapes), but ginger and garlic are interesting plants that have been propagated from cuttings or bulbs for thousands of years and so lost the ability to produce seeds (with the normal caveats).

Aphids are also an interesting example, where the previous year's eggs hatch in the spring as females, which then clone themselves as fast as possible - when there's too many of them they will create clones with wings, and when autumn comes around, they will create male clones to then go through the normal sexual reproductive route. Which is also an example of the stable/unstable environment issues you mentioned.

This is clear, beautifully written, and funny, thanks for taking the time to create it.

Just to add a point to the section on Fisherian runaways, this is a wild guess, but I suspect that runaway sexual selection has several possible causes. One is the "overshoot effect" you describe, where sexual selection pushes the evolution of a beneficial trait and this continues past the optimum point. Another (this is where I'm guessing) is that while the fitness of mates is linked through their offspring, it's not identical, and desirable traits in a mate are not necessarily the traits that would benefit the individual's genes the most. For example, if an organism invests some energy into helping its kin, that benefits its alleles, but not those of its mate. Unlike the overshoot, this effect should persist even in equilibrium. Direct or kin selection pushes in one direction for the benefit of one's own alleles, while sexual selection pushes in the other direction for the benefit of the alleles of your prospective mate. These forces settle at some equilibrium point.

For example, take a hypothetical bird species where males being large and strong is genuinely helpful for protecting their nest (increasing offspring fitness and thus the joint fitness of the male and their mate). However, having large and strong offspring is more of an energy investment for the parents. They need to feed their babies more worms while they're growing, and therefore they're limited to having fewer offspring if those offspring need to be big. Thus, individual / kin selection might want males to be smaller than sexual selection would want them to be. As a prospective mate, a female would not particularly care how many siblings a male has, but she would care about how well he can protect their nest. The equilibrium point should lie somewhere in between the point chosen by sexual selection and the point chosen by individual / kin selection. The males will tend to be a bit larger than they would be if females selected mates at random, and they'll tend to be a bit smaller on average than the size that the females prefer the most.

This theory also generates the prediction that being ungenerous to one's own kin should be attractive. (Generosity to the kin of one's mate should be attractive, though.) This doesn't seem true in humans, as far as I can tell. It's important to take note of these contradicting bits of evidence.

This theory also generates the prediction that being ungenerous to one's own kin should be attractive


Isn't that what makes "Romeo and Juliet"-like stories "romantic"? When one forsakes one's own genetic clan to elope with the mate, it signals extreme devotion to the mate and is therefore attractive.

Hm, my background here is just an undergrad degree and a lot of independent reasoning, but I think you're massively undervaluing the whole "different reproductive success victory-conditions cause different adaptations" thing. I don't think it's fair at all to dismiss the entire thing as a Red Pill thing; many of the implications can be pretty feminist!

I don't think it matters that much that Bateman's original research is pretty weak. There's a whole body of research you're waving away there, and a lot of the more recent stuff is much much stronger research!

You don't necessarily have to talk about sexual competition at all. You can just say, for instance, that female reproductive success is bounded - human women in extant hunter-gatherer tribes typically have one child and then wait several years before having the next. If a woman spends twenty years having children and can only have one child per four years, then she's only going to have five children. Her incentives are to maximise the success of those five children and the resources she can give each child. Meanwhile, a man could have anywhere between zero children and... however many Genghis Khan had, so his incentives tend much more strongly towards risk-taking and having as much sex as possible.

Of course there's massive variation between species; there's massive variation in how every and any trait/dynamic plays out depending on the context and environment. But we can generally come up with reasons why particular species might work the way they do; for example, I've heard the hypothesis that the fish species with very tiny males are adapted for the fact that finding a conspecific female to mate with in the gigantic open ocean is basically random chance, so there's no point investing in males being able to do anything except drift around and survive for a long period until they stumble across a female. Humans aren't a rare species in a giant open ocean, so male humans don't have to really rely on just stumbling across female humans through sheer luck after weeks of drifting on the currents.

You don't have to bring "males face more competition than females" into it at all. You can just say "whichever parent has higher parental investment is likely to have stricter bounds on reproductive success, so they'll adapt to compete more over resources like foods, while the low-investment parent competes more over access to mates". Then when you look at specific species, you can analyse how sexual dimorphism in that particular species is affected by the roles each sex plays in that species and also the species' context and environment.

Sometime when it's not 2am, if it'd be helpful, I'd be happy to pull out some examples of papers that I think are well-written or insightful. Questions like "ok, so, if males maximise their fitness by having as many mates as possible, what the heck is going on with monogamy? Is there even any evidence that human men in extant hunter-gatherer tribes have much variation in their reproductive success caused by being good at hunting or being high-status or whatever? For that matter, what the heck is going on with meerkats?" are genuinely interesting open research questions, I don't really think they're associated with the red pill people, they're things the field is approaching with a sense of curiosity and confusion, and also I would really like to know what the heck is going on with meerkats.

You're right. Honestly I wouldn't be able to talk about this in detail because this is getting far from the things I know best (full disclosure, my own research is on bacteria). The few papers I've cited give some general patterns, and my general point was "things can go in many different ways depending on the specifics, and even the well-known Bateman principle isn't universal".

That's unfortunately all can do: there's a whole world of things to say about how sexual dimorphism actually develops in metazoans, but it takes years of learning to get a deep understanding of what's going on.

Definitely post the papers you're thinking about! If you feel like making a new post about that, I can't encourage you enough to do it. This post was by far my most successful, so it looks like a lot of people are interested in the topic. I'm sure many people would enjoy your contribution (at least I would).

As for the Red Pill thing, I kind of regret mentioning it – I just thought it was funny, but it's not really that funny or useful. Maybe I should edit it out.

OK, "top level post on the biology of sexual dimorphism in primates" added to my todo list (though it might be a while since I'm working on another sequence). Now that I know you're a bacteria person, this makes more sense! I'm a human evolution person, so you wrote it very differently to how I would've. (If you'd like an introductory textbook, I always recommend Laland and Brown's Sense and Nonsense.) I don't know as much about the very earliest origins in bacteria, so that was super interesting to read about! 

The stuff about adding a third sex reminded me somewhat of the principle that it's unstable to have an imbalance between the sexes; even if it would be optimal for the tribe to have one male and many females, an individual mother in such a gender-imbalanced tribe would maximise her number of grandchildren by having a male child. I've seen this used as the default example to prod undergrads out of group-selection wrongthink, so I'm curious if this is as universally known as I think it is. 

I can't tell you whether to edit the post, but I think it's very common for people to act/joke/suggest as though the study of human evolution inevitably leads towards racist/sexist conclusions. In part, this is because fields like anthropology have a terrible history with sweeping conclusions like "women just evolved to be weaker" and other sins like "let's make a categorization system that ranks every race", which has certainly been used by unscientific movements like Red Pill. But anthropology has done a lot to clean up its act, as a field, and I don't think this is ground we want to cede. The study of human evolutionary biology has only ever confirmed, for me, that discrimination on grounds like race and sex is fundamentally misguided. So I try to push back gently, when I see it, against the implication that studying sexual selection or sexual dimorphism will lead towards bigoted beliefs. Studying science will generally lead away from bigoted beliefs, because bigoted beliefs are generally not true. (I don't intend this to come across as harsh criticism, either! I just want to explain why I think this is worth caring about.) 

And yes, many people will upvote this kind of post because I think folks appreciate the virtue of scholarship. We have a lot of people who can write down their ideas on how to improve your thinking, but fewer who can come in from a specific field and explain the object-level in detail. You deserve many upvotes for citing your sources! 

The Yanomamo maximize the number of females in their tribes by kidnapping them from other tribes sucker enough to feed & raise females rather than males (which they could have used to raid females from other tribes).

Count me as a vote for sharing those papers, here or in your own post. 

I am quite interested in curating this, however, I do think there are a bunch of kind of important questions and objections in the comments. I would probably curate this post if you respond to them, even if I don't find the responses that compelling, because I do think this post seems quite good, but I want to get one more level of sanity-check on the content by the commenters.

What do you mean by curating? So far I've tried to answer the questions and objections when I saw them, are there some I've missed? (Obviously I don't pretend to be able to answer everything). Also, do you think there are some clarifications that I should add to the main text?

Curation is a thing where the post shows up at the top of the frontpage for 1-2 weeks, and we send out the post to something like 30k people who are subscribed to get updates whenever we curate a post.

This is one comment that seems good to respond to: 

That sounds exciting! I hadn't seen Elisabeth's comment, I just wrote a reply. Do you think there are modifications I should make to the main text to clarify?

The reason it still took over the macroscopic world is that evolution does not simply select for reproductive fitness


Here I define reproductive fitness as the average ability of your genes to reproduce

I think this is false by definition? The thing evolution selects for is the ability of genes to reproduce. How are you using the terms here?

(Sorry I missed your comment)

Here by "reproduce" I just meant "make more copies of itself" in an immediate sense (so reproductive fitness is just "how fast it replicates right now"). For example, in Lenski's long-term evolution experiment, some variants were selected not because they increased the bacteria's daily growth rate, but because they made it easier to acquire further variants that themselves increased the daily growth rate. These "potentiating" variants were initially detrimental (the copy number of these variants decreased in the population), and only after a long long time they took over the population. So, according the definition of reproductive fitness I used, they lead to a lower reproductive fitness – the reason they were eventually selected for is not that they're good for reproduction, but that they're good for evolvability. Of course, you can say that eventually they increased in copy number, but that would be defining "reproduction" in a different way, that I find less intuitive.

Now, is that other definition (how gene copy number increases over the long term) what evolution ultimately selects for? I'm not sure. To quote Kokko's review on the stagnation paradox:

"Trees compete for sunlight and attempt to outshade each other, but when each tree consequently invests in woody growth, the entire forest must spend energy in stem forming and—assuming time or energy trade-offs—will be slower at converting sunlight into seeds than a low mat of vegetation would have been able to. Every individual has to invest in outcompeting others, but the population as a whole is negligibly closer to the light source (the number of photons arriving in the area is still the same). This is why in agriculture, externally imposed group selection to create shorter crops has improved yields."

She gives other examples. In these cases, the number of individuals tend to decrease over time, even in the long run. 

This was very interesting, thanks for writing it :)

My zero-knowledge instinct is that sound-wave communication would be very likely to evolve in most environments. Motion -> pressure differentials seems pretty inevitable, so would almost always be a useful sensory modality. And any information channel that is easy to both sense and affect seems likely to be used for communication. Curious to hear your thoughts if your intuition is that it would be rare.

This depends on the size and distances involved, but it's a good intuition. You need a mechanism to generate the pressure differentials, which can be an issue in very small organisms, which can be an issue.

Small and sedentary organisms tend to use chemical gradients (i.e. smell), but anything bigger than a mouse (and quite a few smaller things) usually has some kind of sound signals, which are really good for quick notifications in a radius around you, regardless of the light level (so you can pretty much always use it). Also, depending on the medium, sound can travel really far - like whales which communicate with each other over thousands of miles, or elephants stomping to communicate with other elephants 20 miles away.

Admittedly, I haven't read about the problem of sex since '90s but back then the argument against the naive "sex is good because it allows all the good genes to get into a single organism" was that that made sense from the point of view of the species, but not necessarily from the point of view of the individual -- while the natural selection works on the individual level.

In particular, when a female has a choice to reproduce either sexually or via parthenogenesis, in the former case she loses 50% of the fitness (because half of her genes get recombined out). Thus, the advantages of the sexual reproduction must outweight this huge drop in fitness. Even worse, it must outweight it quickly. "Your progeny is going to be better off after 100 generations" is not going to work, because when your fitness drops by 50% you'll die out in few generations.

Anyway, if the newer research found a solution to this problem, it would be interesting to hear about it.

Promoted to curated: I think this post overstates some claims, in-particular I think it underweighs some information theoretic arguments for sexual selection, and I do have a feeling that this post tells an overly neat story that leaves out a bunch of important bits, but I overall still feel like I learned a lot of useful things from this post, and it was clearly very well written.

Thank you for writing this! 

I always love bio stuff and this is a fantastic post! I think on the 'why sex?' question, it's not selfish gene enough.

Genes, not organisms or species, are the replicators, the predominant unit of selection. If you're a replicator (gene) in a community of specialised cooperating[1] replicators (genome) sharing a vehicle (organism), horizontal transfer mechanisms (generalisation of sexual transfer) look like neighbour-replicators whose specialised job is to facilitate orderly migration of potential new neighbour-replicators. Of course you want (some nonzero quantity, usually, of) this[2]! (Especially to adapt to changing environments, including evolution of conspecifics.) Every lineage of life exhibits some horizontal transfer, whether sexual or not.

The real questions are then 'why sex-in-particular?' and 'why sex-exclusively?', which this post touches on to some extent quite well. For interested readers, MacKay's Information Theory, Inference, and Learning Algorithms has a great chapter 'Why Have Sex?', as another Oliver already pointed out.

  1. Usually cooperating! Well, at least sort of sometimes cooperating?! ↩︎

  2. Virus and other intracellular parasitic replicators look like roving invader/pillagers from this perspective, breaking the orderly guarantees provided by horizontal transfer ↩︎

Can you say more about why the Bateman logic holds or doesn't hold in different situations?

According to this paper, the "root" factor is how much effort each parent invests in caring about offsprings, as in some species the male is the primary caregiver. But that's really hard to measure and check empirically, so they instead measure "the maximum number of independent offspring that parents can produce per unit of time", and they find very good agreement with which sex faces the most intense competition.

On notable exception is the hippocampus, where the males both face intense competition and invest more resources in the offspring. Because of course it had to be hippocampi.

According to this 2009 paper, seahorses aren't actually an exception, and the males are indeed the more choosy one, at least in this one experiment. (They're an "exception" to Bateman's principle in the sense they have the smaller gamete, but this is explained away by their greater parental investment.)

In general, yes, parental investment can "outweigh" gamete size in some situations, and typically this ends up happening in cases where investment in offspring isn't as strongly physiologically sex-linked as it is in most mammals which allows different strategies to evolve more readily.

For instance, in birds, the egg is quite large, but because incubation is quite time consuming (as is feeding) and this can be shared between parents, you end up with more bi-parental care, as a male can increase his reproductive success by staying and incubating. Because there are many species of bird with bi-parental care, this opened up the pathway for evolution of the jacana, where the male does all the parental care, and the male is rate limited by how many eggs he can incubate and is more choosy. 

In mammals, since most of the energetic investment is in gestation and lactation, both of which only females can do, and you end up with bi-parental care being more rare than in birds. One notable exception being humans, which have an exceptionally long childhood that extends long after weaning. 

Insects in general and fruit flies in particular were a particularly bad species to detect Bateman's principle in, because they're r selected rather than K selected. In K selected species, genetic quality of the mate matters much more because of how few offspring there are; in species where the goal is to produce as many offspring as possible, genetic quality and therefore mate quality and ergo choosiness has a much smaller impact. 

I've read that male humans are capable of lactation but it takes very weird circumstances (possibly involving drugs) to make it actually happen.

Let me just quote Wikipedia: "A seahorse [...] is any of 46 species of small marine fish in the genus Hippocampus." Because I spent a few confused minutes trying to figure out how males could face more intense competion in a brain part.

Fun fact: in the field of optimization there are heuristics which are modeled after evolutionary principles. These "evolutionary algorithms" also work with populations, offspring generation through mutation and mating, selective pressure, diversity preservation, and so on.


As a rule of thumb, these algorithms also work better when sexual reproduction is used. For example, a standard theoretical benchmark are monotone functions on bit-strings, where each gene takes only two values zero and one, and flipping a zero into a one gives higher fitness in all situations and environments. This seems like an easy situation to optimize, but asexual algorithm don't find the optimum (where all genes are one) for exponential time (in the number n of genes) if the mutation rate is large. Algorithms which use mating have no trouble. This comes from Muller's ratchet. [1]

More remarkably, even if the mutation rate is arbitrarily small, asexual algorithms don't find the optimum for exponential time if they use populations, where in each round they produce a new offspring and prune the least fit individual. Again, algorithms with mating don't have these problems. This also comes from a version of Muller's ratchet, but on population level. Essentially, if there is a beneficial mutation then other mutations have time to accumulate until the beneficial mutation has taken over the whole population, and this takes long enough to accumulate very many bad mutations, even for extremely low mutation rates. [2]



(free preprints available on arxiv)

I laughed, I thought, I learned, I was inspired  :)   fun article!

Thank you very much for this excellent post!

Would you be able to give a more detailed explanation of Organelle competition? I'm afraid I didn't understand at all how having different types prevents it.

It's not so much the different types in themselves that prevent competition, but having multiple types make it possible to have a mechanism that forces all organelles to come from only one pre-selected parent. If all organelles come from the female, then a rogue mitochondria cannot take over by making more copies of itself or by poisoning other mitochondria, because the only way to make it to the next generation is to be in the female gamete, period. In other words, there's not much an organelle can do to increase in frequency, aside from improving the overall fitness of the organism. Does that make more sense?

But by making more copies of itself/poisoning other mitochondria isn't it more likely to end up in the female gamete?

Not quite, if it's less efficient at doing the normal mitochondria work, it puts a big burden on the cell, who is then less likely to reproduce.

But wouldn't that be the case for any organelle, even one which is inherited from both parents?

I would guess that when organelles are inherited from both parents, the traitor organelle is disadvantaged by its burden on the host, but advantaged by it's ability to be the predominant organelle in the offspring. If the cost-benefit is favourable, then the traitor organelle will take over. OTOH, if only one parent transmits the organelle, the advantage disappears but the burden remains. So I'd expect that it makes it more difficult for traitor mitochondria to invade. Hopefully that makes sense!

Great article!

Maybe homologous recombination should be mentioned as the reason why "the newborn cell receives an assemblage of random pieces of each parents' genome". Just mixing chromosomes would not be enough to stop muller's ratchet. 

A fascinating eukaryotic exception is the white-throated sparrow, which functionally has four genders in an equilibrium where the tan-striped males mostly mate with white-striped females and vice versa. (I first read about it in Joan Strassmann’s book Slow Birding; the Wikipedia page for White-Throated Sparrow also has some introductory info. It seems to involve a chromosomal inversion.)

Part 3: not my type

should probably be "Part 2" instead

McDonalds et al. (2016) had yeast evolve with and without sex for 1000 generations

... wait, yeast can reproduce sexually or asexually?
looks at paper

OK, they figured out how to get yeast to have sex? Seems wild.

I've always felt the Fisherian runaway hypothesis begs the (second order) question:

The first order question (for the scenario here) is - Why don't the male bird head plumage continue to grow indefinitely longer from generation to generation? This one is easy. At some point the plumage would become so impractical as to make mating impossible. 

The second order question is harder: Why is it that some species get away with remarkably impractical features (the peacock comes quickly to mind), while other species appear to be pretty close to a local maximum in adaptation? 

I suppose that a scarcity of predators and a generous environment ought to be part of the story for the most flamboyantly maladaptive species. But has this been empirically verified? And are their other considerations at work?

That sounds plausible, but I've not looked into the empirical research on that topic so I can't tell you much more! 

Suggestion: Maybe move the summary to the top (as a sort of abstract or tl;dr).

This is a really great post, thanks for writing it! I learned interesting things, and love your writing style. Man, biology is weird

A possible example in humans is the boob. Other primates don't have boobs – they are flat most of the time and only swell for lactation. Maybe, at the beginning, swollen boobs was a sign of fitness, then human males got really into swollen boobs, then human females started padding them with fat to appeal to the males' instincts, leading to the persistent round boobs we witness today – even if the pad of fat isn't actually very useful for lactation.


It seems odd that humans have such extensive female ornamentation, something which is barely ever seen among other species.  This goes beyond boobs.  Why is it women who wear makeup and not men? It's an invariant across different cultures that women are the ones who care and put more effort into their appearance. 

What's the state of the field's thinking on this? Is it an open mystery with some crackpot theories, similar to the mystery of the existence of gay people?

First, let me express my congratulations and thanks for an extremely thorough and thought provoking writeup.

My question to Malmesbury is: the theories expressed are very elegant but it is far less clear just how well these theories are backed by evidence.

Granted, definitive evidence is probably never going to be possible given that single celled creatures don't really leave fossils, but the inherent risk in any elegant theory is "turtles all the way down".

It is also less clear to me why there cannot be intermediate means of "sexual" reproduction - in particular, the core concept of mitochondria being vital to sexual reproduction + female lineage of mitochondria seems to represent a divergent angle than pure "fitness". 

If successful procreation and presence in the ecosystem is the true measure of genetic success, then all the sexual fitness games are just froth around our mitochondrian overlords.

Thank you for the write-up! I wanted to ask a couple of questions:

  1. I remember reading a hypothesis that mitochondria directly caused sex, i.e. made host cells fuse into one, because it was beneficial for them in case they were trapped in a weak or dying host. This was supported by the finding that genes from mitochondria are involved in the initiation of cell fusion. Is this hypothesis still around or is it dead?
  2. I used to think archaea and bacteria cannot really swallow other cells, as with them having better things to do with their membranes. And the bacterium ending up inside an Arche could more likely be a result of the Arche lovingly wrapping itself around the bacterium because the latter produces delicious metabolic waste for it. Can you comment this?

It seems odd for mitochondria to be causing the mutation problem sex is supposed to solve, when mitochondria themselves don't reproduce sexually.

Restricting mitochondria reproduction to one mating "type" does not by itself prevent a "selfish" mitochondria from arriving. If one mitochondria develops a new mutation, it is now competing against all the other mitochondria in that same organism without the mutation (like a cancer). But in fact the restriction goes beyond merely the "type", as all the somatic cells are dead-ends for mitochondria.


Robin Hanson has a worthwhile post on why some organisms are exclusively male rather than being hermaphrodites capable of male & female mating "types".

Nikolas Lloyd has an evolutionary theory on why human females have breasts instead of teats (well before they even get pregnant).

I think there's an obvious problem with the theory of runaway sexual selection: once a trait gets deleterious, there will be selection for different preferences. As Lloyd theorized, initially there would be a preference against large breasts (it would indicate not being immediately fertile), but the trait could still get going because the male who mated with such a woman anyway would turn out to be making the right move (as it no longer signalled that). And in other species of animal, large breasts would be deleterious in females. They're possible in humans because our females no longer have much need to outrun anything while while carrying such encumbrances (but there are still limits to that, which is why fantastically large breasts of the sort some men prefer are usually the product of surgery rather than genes).

Somatic cells are generally dead ends evolutionary. Your toe cells aren't going to do much reproducing. Also, mitochondial (or in general organellar) DNA is split between the actual mitochondria and the cells containing them. Biology is fun!

The argument for mitochondria is that they cause the cell environment to be more toxic (what with them being the cell's powerhouse). This in turn is going to provide a lot of selection pressure. In the same way e.g. global warming is causing a lot of selection pressure.

Runaway sexual selection has limits. This is also sort of the point. If you can carry around massive breasts, tails, noses or whatever and still be very prosperous, that means you're good. Where "prosper" can mean running away from lions if you're an antelope, or be the top of the village pecking order if you're a human. Like a short pro basketball player. If they're short, but still at a pro level, that's someone you want on your team. This is known as the handicap principle, and can be explained via signaling mechanisms.

The distinction between "somatic" and "germ" cells only exists for sexually reproducing species.

I wonder if it's worth having a follow-up post, outlining how sex concerns the life history strategies associated with the production of big/small gametes, and this is what makes you M/F, rather than actual presence of gametes (since e.g. post menopausal women don't produce gametes, but are still female, and emphasising strategy makes sense of the intuition that even the vast majority (though not all) of 'intersex' people are still pretty clearly M or F).


  • You have two "part 3" and no "part 2"
  • The final summary says "the two types diverged into big gametes and large gametes"

Selfing-prevention: exclusive mating types prevent an individual from mating with itself, also known as selfing.

The "Selfing" link goes to the Inbreeding Depression page, I believe in error. Feel free to delete this comment if I'm wrong or if I'm right and you fix it.

Thank you for spotting this, I fixed it.

When I inevitably have to answer why I didn't duplicate myself to my future offspring, I will link them to this post; thank you for this gem.