Why Two Sexes?

[Sunstone]

It is generally accepted within the evolutionary sciences that sexual reproduction became the dominant form of reproduction mainly because it was superior to asexual reproduction as a means of combating diseases and parasites. There's quite a bit of agreement about that.

But why did two sexes become the dominant number of sexes for a species to have? Why not three sexes, or 16 sexes (as are found in some species of insects) or 16,000 sexes (as is found in some species of fungi)? Why do almost all species have only two sexes?






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[ChristineM]

Not sure i could handle more than 4 or 5


I guess other than asexual reproduction 2 sexes is the most efficient use of cells.

 

[SalixIncendium]

I think the purpose of two sexes is to promote reproduction. There is pleasure in intercourse, which incentivizes species to breed. I'm not sure this exists in asexual reproduction. Is there any studies in population differences over time between sexual reproduction and asexual reproduction? I haven't really studied it.

Then again, I'm probably wrong.

 

[Polymath257]

This is a subject where a LOT of care is required. What, precisely, do you mean by a 'sex'? Remember that sex is NOT determined genetically in many species. Instead, environmental factors determine the sex of an individual, temperature being a very common trigger. Also, individuals in many species can change sex during their lifetime.

So there are actually *two* questions involved here: the first is why only two cells need to come together to produce a new embryo. The other is why most species divide the breeding population into only two basic groups to provide these reproductive cells.

For the first question (why only two cells come together to form an embryo), it is good to look at the cases where that fails. For example, as the OP pointed out, many species of fungi allow for large numbers of 'sexes'. But one aspect of this is that many fungi have cells that are syncitial: they have multiple nuclei in a single cell, often with differing genetic histories. So, it is possible for several different genetic strains to be *inside of a single cell*.

For the vast majority of multicellular species, though, syncitia are evidence of disease. Normal cells only have a single nucleus.

Next, is the very simple observation that cells tend to split into *two*, not three or four, daughter cells when they divide. In reproduction, this means that each sperm or egg gets half the total genetic load. And *that* means that it takes only two cells to get back to the full compliment of chromosomes.

Even in polyploid species, like many plants, this means that the number of chromosomes is usually a multiple of two. Triploidy is not very common.

So far, this means that, except for special cases where more than one nucleus exists in a cell, we expect only two cells to merge to form new embryos.

Next, we have the question of why there are only two main breeding strains in most populations. Why is it that even in species where sex isn't determined genetically, there are only two sexes? I suspect that this is primarily because the diversity benefits of sexual reproduction are present once two sexes are available, and that the increased benefit of three sexes isn't significant enough to disrupt that equilibrium.

Where are the outliers on this? Mostly in social insects, which already have rather strange inheritance structures. Past that, my understanding fades.

 

[A Vestigial Mote]

It is generally accepted within the evolutionary sciences that sexual reproduction became the dominant form of reproduction mainly because it was superior to asexual reproduction as a means of combating diseases and parasites. There's quite a bit of agreement about that.

But why did two sexes become the dominant number of sexes for a species to have? Why not three sexes, or 16 sexes (as are found in some species of insects) or 16,000 sexes (as is found in some species of fungi)? Why do almost all species have only two sexes?
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My guess is that the mechanics become more and more complex, and if anything, "nature" only "does" (anthropomorphizing here, for brevity) precisely what is found to be necessary... no more. Male/female DNA coupling produces higher amounts of genetic variation... which is found to be advantageous.

I would guess that genders beyond that very much require social conditions that are then able to drive evolutionary conditions. There exists a species of fish, for example, that could be interpreted as having 3 genders. There are females who lay eggs, and then 2 distinct forms of what we typically consider males. One form born large, who uses his physical prowess to defend the nest of eggs of his female, and one form born small and very fast, who uses his speed and agility to swoop into the nest and dust the eggs with his sperm while trying to avoid the large male guardian. Thus, each type is propagated. Some of the eggs are fertilized by the large male type - passing on his traits, and some of the eggs are fertilized by the small male type, who got the job done using stealth and speed.

Also, as someone else said - it isn't going to take more than 2 cells to start-up formation of a new embryo - it would be exponentially more complex to require more (timing, opportunity, etc.), and therefore it is extremely unlikely that more would become a requirement.

 

[Polymath257]

Not sure i could handle more than 4 or 5

I don't know, 4 or 5 might be fun.

 

[ChristineM]

I don't know, 4 or 5 might be fun.

ButUt more? I have to sleep sometime

 

[Double Fine]

Let's go back some time. Water showed up on earth around 4.3 billion years ago (about 200 million years after formation) and over the next 300 million years, the oceans were formed. First signs of life date back to between 3.8 and 4 billion years ago, and that was obviously single celled organisms.

It took these little guys around 500 million years to evolve the ability of photosynthesis - to "eat" carbon and to excrete oxygen. This continued for another billion years or so, before oxygen became so plentiful that we actually have atmospheric oxygen. How long are we into the story now? Well, we are at roughly 2.5 billion years ago; a full billion and a half into the history a life on earth.

A full 400 million years later, Eukaryotes evolved - cellular organisms that have both a nucleus and a membrane. A further 500 million years later and finally! Finally! Finally! we get to multicellular life. We are at roughly the 1.6 billion year ago mark. It took life on earth a staggering 2.4 billion years to evolve to a stage where they have more than one cell. This was all done with asexual reproduction.

Soon after multicellular life came about (200-300 million years or so) then sex was invented. Oh, how much do we owe to those horny little cells trying to get their rocks off. Yes, sexual reproduction entered the geologic column roundabout 1.2 billion years ago.

Since then, things happened and FAST. Because of more genes being added, offspring did not have to be near clones of it's parents anymore. Natural selection drives which genes from the mother and father are used and genetic information is stored through the generations as some sort of toolkit for the future. With a much wider gene pool, we saw evolution starting to explode.

We took a staggering 2.8 billion years to go from abiogenesis to sexual reproduction. After that? About 300 million years to the earliest plants. A hundred million years or so after that, the earliest animals. 100 million or so years after that, Cambrian Explosion.

Sexual reproduction using two genders really got life on earth into second, third, fourth and fifth gear, and that is why sexual reproduction is better and asexual reproduction.

Why not three genders? Okay, sure there are species with a phantom third sex (like males that can switch to female and vice versa) but:

- it is not needed. We saw evolution explode and being fine with just two. There is no need for a third.

- mating between animals would have been a lot more complex. In nature, the male is usually the suitor and the female usually the chooser. They find each other using scent, sounds, body language etc. How will it work if there were three? Can a male just screw a female and then she has to find the third gender? Do they have to do it together? How long does the male's sperm survive before the female needs to find the third gender for their part?

In short, other than it not being needed, it would make mating between animals really cumbersome.

Peace

 

[exchemist]

It is generally accepted within the evolutionary sciences that sexual reproduction became the dominant form of reproduction mainly because it was superior to asexual reproduction as a means of combating diseases and parasites. There's quite a bit of agreement about that.

But why did two sexes become the dominant number of sexes for a species to have? Why not three sexes, or 16 sexes (as are found in some species of insects) or 16,000 sexes (as is found in some species of fungi)? Why do almost all species have only two sexes?






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The chances of a contact between 2 organisms are a lot higher than the chances of a simultaneous or near-simultaneous contact between 3 or more.

 

[ChristineM]

What happened to my reply here???

Jesus it's all small

You wrote within the /quote so it thinks your reply is part of the quoted text.

Go back, edit it move the final quote with square brackets back to the beginning if your text

 

[Double Fine]

You wrote within the /quote so it thinks your reply is part of the quoted text.

Go back, edit it move the final quote with square brackets back to the beginning if your text

Fixed it thanks

 

[Double Fine]

I don't know, 4 or 5 might be fun.

In the mind it works. In real life though, nah, not so great

 

[Vouthon]

I concur with @exchemist that: "The chances of a contact between 2 organisms are a lot higher than the chances of a simultaneous or near-simultaneous contact between 3 or more."

But the topic raises a broader question for me about sexual dimorphism.

Sex is not as simple a concept to define as many think.

You have chromosomal sex and phenotypic sex (internal and/or external physiognomic, behavioural and secondary sex characteristics) for a start.

Then, we could have - or very well may not depending on the science - differently sexed brains with some evidence suggesting that males and females have dimorphic volumes of brain cortical gray and white matter.

For a majority of humans, that all lines up fairly nicely into a kind of neat "sexual dimorphism" or duality when all is said and done. I, for instance, am a genotypic, phenotypic and (presumably) neurotypic male whose gender identity is also masculine.

But for a large minority of human beings this is not and has never been the case. Every culture has people who don't confirm to the sex or gender binary.

There are people who are chromosomally male (XY) but due to complete androgen sensitivity are phenotypically and neurotypically female, as well as having a feminine gender identity post-puberty, who were born with vaginas, developed breasts and have all the secondary external characteristics of women.

Then you have "men" with Klinefelter syndrome who are phenotypically male but are actually XXY chromosomally, which means that genetically they aren't quite male or female because they have both the female genotype (XX) and contain the make one as well (XY). Some of these "men" develop a few of the secondary sex characteristics of women, despite having penises and testes.

And you have a lot of variations in between. For instance, some studies suggest that transgender people - while genotypically and phenotypically male or female - have brains ("neurotypes") that are more closely sexed to their perceived gender identity.

So....is sex really as simple as the apparent easy dimorphism would imply? For most of us, yeah it probably is, but across the board there are a fair number of intersexed and transgendered people and this all emerges naturally, nor has evolutionary adaption weaned these traits or anomalies away.

 

[Stevicus]

But why did two sexes become the dominant number of sexes for a species to have? Why not three sexes, or 16 sexes (as are found in some species of insects) or 16,000 sexes (as is found in some species of fungi)? Why do almost all species have only two sexes?

I have no idea, but it got me to thinking, it's oftentimes difficult for two humans to get together and get along and stay with each other for the long haul, enough to have kids and raise a family. Adding another sex to it would just make it all the more difficult and complicated, although it appears that some species might still be able to survive, as you've noted about some species of insects.

But, my goodness, 16 sexes? What if they get divorced? What a mess it would be - custody disputes, property division, alimony, child support. It seems it would be pretty chaotic.

 

[Polymath257]

I concur with @exchemist that: "The chances of a contact between 2 organisms are a lot higher than the chances of a simultaneous or near-simultaneous contact between 3 or more."

But the topic raises a broader question for me about sexual dimorphism.

Sex is not as simple a concept to define as many think.

You have chromosomal sex and phenotypic sex (internal and/or external physiognomic, behavioural and secondary sex characteristics) for a start.

Then, we could have - or very well may not depending on the science - differently sexed brains with some evidence suggesting that males and females have dimorphic volumes of brain cortical gray and white matter.

For a majority of humans, that all lines up fairly nicely into a kind of neat "sexual dimorphism" or duality when all is said and done. I, for instance, am a genotypic, phenotypic and (presumably) neurotypic male whose gender identity is also masculine.

But for a large minority of human beings this is not and has never been the case. Every culture has people who don't confirm to the sex or gender binary.

There are people who are chromosomally male (XY) but due to complete androgen sensitivity are phenotypically and neurotypically female, as well as having a feminine gender identity post-puberty, who were born with vaginas, developed breasts and have all the secondary external characteristics of women.

Then you have "men" with Klinefelter syndrome who are phenotypically male but are actually XXY chromosomally, which means that genetically they aren't quite male or female because they have both the female genotype (XX) and contain the make one as well (XY). Some of these "men" develop a few of the secondary sex characteristics of women, despite having penises and testes.

And you have a lot of variations in between. For instance, some studies suggest that transgender people - while genotypically and phenotypically male or female - have brains ("neurotypes") that are more closely sexed to their perceived gender identity.

So....is sex really as simple as the apparent easy dimorphism would imply? For most of us, yeah it probably is, but across the board there are a fair number of intersexed and transgendered people and this all emerges naturally, nor has evolutionary adaption weaned these traits or anomalies away.

And that goes to the point: biology is complicated. Life finds all sorts of ways to survive and it doesn't necessarily fit into rigid notions of how things 'should be'.

And, of course, we have the additional aspect of gender: the cultural aspects of sexuality that are another layer on top of the purely biological.

 

[Thief]

Why do almost all species have only two sexes?

and the alternate forms would be what?
besides make and female

multiple mates are sufficient reproductive opportunity

 

[Double Fine]

I concur with @exchemist that: "The chances of a contact between 2 organisms are a lot higher than the chances of a simultaneous or near-simultaneous contact between 3 or more."

But the topic raises a broader question for me about sexual dimorphism.

Sex is not as simple a concept to define as many think.

You have chromosomal sex and phenotypic sex (internal and/or external physiognomic, behavioural and secondary sex characteristics) for a start.

Then, we could have - or very well may not depending on the science - differently sexed brains with some evidence suggesting that males and females have dimorphic volumes of brain cortical gray and white matter.

For a majority of humans, that all lines up fairly nicely into a kind of neat "sexual dimorphism" or duality when all is said and done. I, for instance, am a genotypic, phenotypic and (presumably) neurotypic male whose gender identity is also masculine.

But for a large minority of human beings this is not and has never been the case. Every culture has people who don't confirm to the sex or gender binary.

There are people who are chromosomally male (XY) but due to complete androgen sensitivity are phenotypically and neurotypically female, as well as having a feminine gender identity post-puberty, who were born with vaginas, developed breasts and have all the secondary external characteristics of women.

Then you have "men" with Klinefelter syndrome who are phenotypically male but are actually XXY chromosomally, which means that genetically they aren't quite male or female because they have both the female genotype (XX) and contain the make one as well (XY). Some of these "men" develop a few of the secondary sex characteristics of women, despite having penises and testes.

And you have a lot of variations in between. For instance, some studies suggest that transgender people - while genotypically and phenotypically male or female - have brains ("neurotypes") that are more closely sexed to their perceived gender identity.

So....is sex really as simple as the apparent easy dimorphism would imply? For most of us, yeah it probably is, but across the board there are a fair number of intersexed and transgendered people and this all emerges naturally, nor has evolutionary adaption weaned these traits or anomalies away.

Ooh boy. I think I am going to get an awful amount of backlash on this, but here we go.

Okay so to summarise the claim here: you get a male neurotype to go with the male parts and a female neurotype to go with the female parts. And then we have those people that may have a male brain with a female part - which we call intergender, intersex etc. Have I got that broadly right?

You make a statement near the end, which I would like to address:

...nor has evolutionary adaption weaned these traits or anomalies away.

In nature, the "male brain" almost always goes with the male body and ditto for the females. Non-conforming individuals are eliminated; either they are killed or ousted by their pack/pride/clan, or they simply never pass their genes.

In humans, this is different. We don't kill the old and the weak, nor do we kill those not conforming to the neurotype (not in civilised countries, at least) and if you were born with a female neurotype in a male body, you could lead a safe and happy life in the Western world; you can even pass your genes on.

The Darwinian world is one of survival of the fittest (or in some cases, the luckiest) and in that world, individuals that don't conform simplyto the strictest of norms don't get to pass their genes, they get killed by predators, or their mothers abandon them.

Society however, have brought about protection and security to individuals not conforming to the norm; I myself have a cousin who born deaf and I know the struggle to get her into school. We raise the deaf, we raise the blind, we raise the physically weak - we don't abandon them like animals sometimes do.

And in the same vein, we raise gender dysphoric kids, we raise gay kids, etc. In the West at least, we don't abandon them for their non-conformance.

 

[Secret Chief]

In the mind it works. In real life though, nah, not so great

Think of the arguments over the TV remote.

 

[Secret Chief]

Ooh boy. I think I am going to get an awful amount of backlash on this, but here we go.

Okay so to summarise the claim here: you get a male neurotype to go with the male parts and a female neurotype to go with the female parts. And then we have those people that may have a male brain with a female part - which we call intergender, intersex etc. Have I got that broadly right?

You make a statement near the end, which I would like to address:



In nature, the "male brain" almost always goes with the male body and ditto for the females. Non-conforming individuals are eliminated; either they are killed or ousted by their pack/pride/clan, or they simply never pass their genes.

In humans, this is different. We don't kill the old and the weak, nor do we kill those not conforming to the neurotype (not in civilised countries, at least) and if you were born with a female neurotype in a male body, you could lead a safe and happy life in the Western world; you can even pass your genes on.

The Darwinian world is one of survival of the fittest (or in some cases, the luckiest) and in that world, individuals that don't conform simplyto the strictest of norms don't get to pass their genes, they get killed by predators, or their mothers abandon them.

Society however, have brought about protection and security to individuals not conforming to the norm; I myself have a cousin who born deaf and I know the struggle to get her into school. We raise the deaf, we raise the blind, we raise the physically weak - we don't abandon them like animals sometimes do.

And in the same vein, we raise gender dysphoric kids, we raise gay kids, etc. In the West at least, we don't abandon them for their non-conformance.

I'm curious as to why you expect backlash?

 

[Double Fine]

I'm curious as to why you expect backlash?

To say that some traits are "non-ideal" in nature might trigger some folks.