Why is 2 sexes so common in nature?

[Another God]

This question isn't as obvious as it first seems. Think about it a bit: Why do we have 2 sexes?

 

[FZ+]

Because additional sexes will be too complicated, requiring the action of several individuals that is too unreliable and brings too little a benefit.

Because two sexes allow more capability for adaptation than one.

 

[Another God]

Originally posted by FZ+
Because additional sexes will be too complicated, requiring the action of several individuals that is too unreliable and brings too little a benefit.

Having 3 sexes doesn't mean u require 3 individuals to participate in reproduction. It just means that sex a can mate with eitehr sex B or sex C. Actually giving each individual more options.

Because two sexes allow more capability for adaptation than one.

You don't need sexes for gene swapping and recombination. You can have asexual mating.

 

[iansmith]

Originally posted by Another God
Why do we have 2 sexes?

Why should be more necessary? 2 is enough and it does the job

Originally posted by Another God
You don't need sexes for gene swapping and recombination. You can have asexual mating.

Gene swapping is not so easy and efficient for many metazoan.

 

[Another God]

But why have 2 sexes? Why do metazoan have so much difficulty in gene swapping? Why not have diploids which require to mate before reproduction, but have it so that any other member of the species is a potential partner?

The cost of sex is incredibly large if you think about it. In an evolutionary sense, to have two sexes, you need to have two individuals survive to reproductive age, eating x amount of food each etc, just so that one of them can have offspring. It would be twice as efficient (and twice as efficient in evolutionary terms is astronomical) if individuals could mate with anyone, and for every mating event, they literally swapped gametes with each other, getting both of them 'pregnant' (in whatever form that may be).

I know of a couple of reasons why sexes may have evolved, but I just want to see if there are other reasons out there, or if there is reason to ignore this perceived problem.

 

[Another God]

First thing to do, is to stop thinking in terms of humans. Humans are at the end of a very very long process of refining the 2 sex system, where we are very specialised in our roles. Take it back to the begining, back when 'pregnancy' didn't really exist. Think of creatures who form a zygote, and then the zygote grows of its own accord, free from requiring care from the parents. Plenty of organisms can do this, so an argument claiming that too much effort is required if both organisms are 'pregnant' or both will need to take care of young isn't such a good argument. both individuals may be fertilized, both may lay an egg, or drop a seed, or divide (whatever the scenario), and both may carry on with their lives as if nothing happened.

I say that twice as effective is astronomical, becuase when u study evolutionary theories (Quantitative genetics, mutation rates, selective pressures etc) you quickly realize that slight advantages become very large very quickly (in evolutionary sense.) So a situation where one species has the ability to have twice as many offspring as a competing species is literally astronomical.

The Dimorphism is a good point. There is in fact a species of Octopus which has a 2m long female, and a 2cm long male. Quite obviously these two sexes don't directly compete with each other, so it would be much easier to support the populations. (Each species' biggest threat comes from its own species since they are all competing for exactly the same resources. Having half of the species different, is an advantage.)

 

[Innexplicable]

Asexual mating is unproductive beacuase adaptation, and evolution is harder, also the possibility to grow larger is neerly impossible. Having two sexes allows for natural selection to take place, selecting the best genes, and it also let's us evolve fast in certain situations like in a bottleneck effect. Having 1 sex, carrying both sexual organs (the egg and the sperm) would be illogical, since one most have the best trait of anyone sex. Some species however have the best traits of one sex, and have the other one on standby incase there are a depleted amount of males or females.

 

[Another God]

Originally posted by Innexplicable
Asexual mating is unproductive beacuase adaptation, and evolution is harder

I don't understand how it is either unproductive or how evolution is harder. What do u mean exactly by asexual mating? I mean any process whereby two diploids form haploid cells internally, then swap those haploids to form new recombined diploids... Exactly the same as sexual reproduction, but without the need to match up in an exclusive pairing system. there isn't even any need for different sexual organs.

also the possibility to grow larger is neerly impossible.

Why is that? Evolution still works as normal. 'Sexual Selection' would also still exist, but it wouldn't be male selecting females or females selecting males, it would be a population wide selection from inside. Only the most attractive individuals get mated with. Same concept, less exclusivity.

Having two sexes allows for natural selection to take place, selecting the best genes, and it also let's us evolve fast in certain situations like in a bottleneck effect. Having 1 sex, carrying both sexual organs (the egg and the sperm) would be illogical, since one most have the best trait of anyone sex. Some species however have the best traits of one sex, and have the other one on standby incase there are a depleted amount of males or females.

Natural Selection takes place with one sex, two sexes, or 42 sexes. Natural Selection even takes places where there is no genetic exchange. I don't get this point either.

 

[iansmith]

Originally posted by Innexplicable
Asexual mating is unproductive beacuase adaptation, and evolution is harder

Bacterial and archea still exist and are present in some extreme enviroment. Talk about evolution problem.

Originally posted by Another God
The cost of sex is incredibly large if you think about it. In an evolutionary sense, to have two sexes, you need to have two individuals survive to reproductive age, eating x amount of food each etc, just so that one of them can have offspring.

Is the advantage outweigthing the cost in term of evolution. I think so.

Originally posted by Another God
It would be twice as efficient (and twice as efficient in evolutionary terms is astronomical) if individuals could mate with anyone, and for every mating event, they literally swapped gametes with each other, getting both of them 'pregnant' (in whatever form that may be).

I will answer by this

Originally posted by Another God
Each species' biggest threat comes from its own species since they are all competing for exactly the same resources.

Twice as efficient but could cost more than twice as much

There are still species that are hermaphrodite. Best example are snails. How good is there genetics diversity?

Also if you could mate with everyone could that create problem.

 

[Monique]

For the question why we need sexual reproduction: only during meiosis do recombinations take place, reshuffling the genomes. On overage 40 of these recombination events during meiosis. This reshuffling adds to our genetic diversity.

 

[selfAdjoint]

And we need to shuffle to keep up with the bacteria and viruses that infect us in speed of hereitary change.

 

[Monique]

The reason that microorganisms don't need meiosis (in my opinion) is that they divide very very fast and are able to adapt to their environment much quicker than mammals do. They can take up genes from their environment and loose genes when they don't need them.

Our reproduction process is very slow on the other hand, and thus we need to overcome the static nature of reproduction.

You have got two chromosomes, each carry FG (F=faulthy,G=good). The faulty gene is always passed on, no matter what. Reshuffling during meiosis will cause the following chromosomal arrangements: FF and GG. Now, if GG gets passed on you've got more healthy offspring. FF should die before being able to reproduce.

 

[Monique]

I just noticed I overlooked the following comment by Another God:

I don't understand how it is either unproductive or how evolution is harder. What do u mean exactly by asexual mating? I mean any process whereby two diploids form haploid cells internally, then swap those haploids to form new recombined diploids... Exactly the same as sexual reproduction, but without the need to match up in an exclusive pairing system.

So asexual mating already includes meiosis.

Well, then I have got the following question/statement: Plants evolved sexual reproduction independently from animals..

Again, slow reproducing organisms. Apparently combining two different genomes gives one 'astronomically' improved odds for survival, because it adds to genetic diversity. When a disease would strike not all organisms will die (as would happen due after cloning).

 

[Another God]

Male and Female genomes are not that different. The only difference is the x and y chromosome, and that is a human only phenomenon. Some species don't bother about the Y chromosome etc. So having two sexes does not increase diversity. It increases phenotypic diversity, and that is an important point, because it can lighten the competition between the members of the same species, but it doesn't increase genetic diversity all that much at all.

I must stress though, like, really really stress, that having 2 sexes is not because of meiosis. We can have meiosis without sexes involved. We can have meiosis with 3 sexes involved, 4 sexes etc. Why make it so particular and exclusive so that if you want to mate, you have to find the one which is opposite to you? Something which happens time and time again to be only half of the total population?

Twice as efficient but could cost more than twice as much

There are still species that are hermaphrodite. Best example are snails. How good is there genetics diversity?

Also if you could mate with everyone could that create problem.

I don't understand how it could cost twice as much.

Hermaphrodites change from male to female right? If they still mate with one another, than I doubt their genetic diversity would be any different to most other animals. If they self fertilise, than that is different. I am talking about sexual reproduction happening without sexual exclusivity. Hermaphrodites almost do it...but still with some sort of exclusivity (often age dependent or male-female ratio, which is just as good as having no exclusivity).

How could being able to mate with everyone create problems?

 

[Another God]

Originally posted by selfAdjoint
And we need to shuffle to keep up with the bacteria and viruses that infect us in speed of hereitary change.

I'll say this a few more times, because no one seems to have caught onto it yet: You don't need to have two sexes in order for meiosis => recombination to occur.

Sexual reproduction can be done without need of two exclusive sexes. Why are 2 sexes so predominant?

 

[Another God]

Originally posted by Ambitwistor
Any way you cut it, reproduction has a cost in one way or another; you don't get offspring for free. If you did, everything would reproduce all the time. (Sure, that that kind of growth couldn't be sustained, but so what, if reproducing doesn't cost anything?)

Things do try to reproduce as much as they can but don't they? It still depends on mating rituals, mate selection (is that other individual good enough for me or not? Will my offspring survive after I invest whatever I invest into them? etc) Every organism does want to have as many offspring as possible, but it also doesn't want to waste its own time/effort/life in doing so.

Having the ability to have more offspring is not necessarily a good thing. Take it to the logical conclusion: why aren't all organisms designed to have thousands of offspring, like insects? It doesn't work well for everybody.

I know that, but being able to produce offspring in times of hardship is advantageous over not being able to. Of course over population could be a problem, but there are other mechanism that ahve evolved to stop that: But this doesn't change the fact that being able to reproduce in harder circumstances, is better than not being able to. That is what evolution works on...the adaptability and ability to continue reproducing no matter how hard the times are...

How often have you heard the saying "I wouldn't have sex with you if you were the last man on earth"? Well, what if there was one man left on earth? How would the race continue to reproduce?

 

[Another God]

Here's a thought for you all now that I have you all seemingly hooked.

Have you ever wondered why the Y chromosome is so much smaller than the X chromosome?

 

[iansmith]

Originally posted by Another God
Hermaphrodites change from male to female right?

Hermaphrodites have both sexual organs. i.e. it is a male and female at the same time. some species are capable of self-fertilization whereas some requires 2 individuals. Therefore, both partner becomes pregnant.

Genetic diversity becomes important. If your genetic diversity as a species does not increase due to hermaphrodites, then what is the point of having numerous individuals becoming pregnant. Numerous offspring will flood the environment and as you said

Originally posted by Another God

Each species' biggest threat comes from its own species since they are all competing for exactly the same resources.

So limiting the number of individuals that becomes pregnant, limits the number of new individuals and it limits the competition.

Why 2? 3 is too many. Can you imagine the complexity of 3 different kind of sex.

Originally posted by Monique
The reason that microorganisms don't need meiosis (in my opinion) is that they divide very very fast and are able to adapt to their environment much quicker than mammals do. They can take up genes from their environment and loose genes when they don't need them.

In term of generation, it takes about the same time for microorganism to adapt to the enviromnent than us. It is just that micororganism take 10 minutes to few hours to have a generation whereas most meazoan take weeks to years. The mutation rate in bacteria and archea is sligthly higher than eucarya. 10-4 to 10-7 for bacteria and archeae and 10-8 to 10-12 for eucarya.
As you point out microorganism can uptake genetics elements and are more prone to modification transposons and viruses.

 

[iansmith]

Originally posted by Another God
Have you ever wondered why the Y chromosome is so much smaller than the X chromosome?

Have you ever wonder why only mamals have this chromosome dependent sex determination?

 

[Another God]

Originally posted by Ambitwistor
Isn't that my point? Organisms don't actually reproduce as much as possible, because there are costs involved in doing so. In fact, it may be advantageous to have half the population incur different costs than the other half.

I think this is a good point...its got me thinking...


[/quote][/b]
Who said anything about not being able to produce offspring? When two organisms (successfully) mate, offspring are produced, regardless of whether the offspring physically come from one of them or both of them.
...
It wouldn't. But even if he could reproduce asexually as well as sexually, I wouldn't give good odds of the race surviving anyway, if conditions were so harsh that it was already narrowed down to one remaining organism, especially when all you're doing is perpetually producing genetically identical copies (other than the occasional mutation) of that one organism. [/B][/QUOTE] The first part, the point of either of the individuals being able to have offspring as opposed to one exclusive half of the species being able to is to avoid the problem outlined with the 'last male on earth'.

As for the last male on Earth scenario, I never said the species was on the brink of extinction, I said there was only one male...which of course, in the situation of any 2 sex species would put it on the brink of extinction. Secondly, the scenario doesn't even have to apply to worldwide, extinction event ideas. Populations can get separated at any time, and if that population happens to have incredibly few females, then the species is put into immediate risk.

but most importantly...

But even if he could reproduce asexually as well as sexually, I wouldn't give good odds of the race surviving anyway, if conditions were so harsh that it was already narrowed down to one remaining organism, especially when all you're doing is perpetually producing genetically identical copies (other than the occasional mutation) of that one organism

please read the other posts aswell:

I'll say this a few more times, because no one seems to have caught onto it yet: You don't need to have two sexes in order for meiosis => recombination to occur.

Sexual reproduction can be done without need of two exclusive sexes. Why are 2 sexes so predominant?

In other words, I am not talking about self fertilisation, or about budding, or cloning. I am talking about sexual redproduction, without needing two exclusive sexes.

 

[Another God]

Originally posted by iansmith
Genetic diversity becomes important. If your genetic diversity as a species does not increase due to hermaphrodites, then what is the point of having numerous individuals becoming pregnant.

I am not talking about hermaphrodites, I am talking about there being no sexes. There would still be sexual reproduction.

To make this easier, imagine Gay men could have babies, and lesbians could have babies, as well as normal heterosexual people having babies. There. That is what I am talking about. Except in a real world scenario, there wouldn't be any need for a penis and a vagina (and the various internal differences), as there would be one sexual system common to both.

So limiting the number of individuals that becomes pregnant, limits the number of new individuals and it limits the competition.

Competition from within doesn't count against the species in terms of evolution though. If internal competition is the worst competition, then the species is doing great evolutionarily. Think about it...I'm sure you know what i mean.

Why 2? 3 is too many. Can you imagine the complexity of 3 different kind of sex.

I can imagine a complex version, but I am sure you can imagine the same simple version that I am thinking of. Some organisms exist in which there are many sexes. They don't require all of the sexes to be involved, what they require is one from a sex other than themselves.

So you are sex 12, you can mate with sex 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 13, 14, 15, but you cannot mate with sex 12. In other words, your options are opened up to something like 14/15s of the total popultion, rather than our rather limited 1/2 of the population. The point of sexual reproduction is not to sample every 'sex' of the species, the point is to recombine the DNA from two individuals in a new way.

 

[Another God]

Originally posted by iansmith
Have you ever wonder why only mamals have this chromosome dependent sex determination?

No i haven't. But you left out Insects. They also have a xsome dependenve for sex. They do it differently (ie: no Y at all), but they still do it.

I suppose it has to do with being Diploid for starters. Can u give me an example of a Diploid species which doesn't have a xsome dependence?

 

[Monique]

So you are asking why do we need different sexes at all. Why not be able to fuse your gamete with anyone other's gamete.

So tell me, which organisms DO use this mechanism. Some microorganism must, but I am not a microbiologist. Plants don't, plants are sex-dependent or hermaphrodite.

I think it is logistics, how to get the two gametes together from one organism to the other, and how do you prevent self fertilization if the same cell types are able to fuse. All gamete cells would fuse with themselves during production.

It would require a self-recognition program in gametes, which didn't develop.

 

[Another God]

here is one reference from my lectures:

Nature. 2002 Dec 12;420(6916):664-6. Related Articles, Links


Sex releases the speed limit on evolution.

Colegrave N.

Institute of Cell, Animal and Population Biology, University of Edinburgh, Edinburgh EH9 3JT, UK. n.colegrave@ed.ac.uk

Explaining the evolutionary maintenance of sex remains a key problem in evolutionary biology. One potential benefit of sex is that it may allow a more rapid adaptive response when environmental conditions change, by increasing the efficiency with which selection can fix beneficial mutations. Here I show that sex can increase the rate of adaptation in the facultatively sexual single-celled chlorophyte Chlamydomonas reinhardtii, but that the benefits of sex depend crucially on the size of the population that is adapting: sex has a marked effect in large populations but little effect in small populations. Several mechanisms have been proposed to explain the benefits of sex in a novel environment, including stochastic effects in small populations, clonal interference and epistasis between beneficial alleles. These results indicate that clonal interference is important in this system.

Science. 1998 Sep 25;281(5385):2003-8. Related Articles, Links


Sex and conflict.

Partridge L, Hurst LD.

Galton Laboratory, Department of Biology, University College London, London NW1 2HE, UK.

Evolutionary conflict occurs when the deterministic spread of an allele lowers the fitness either of its bearer or of other individuals in the population, leading to selection for suppressors. Sex promotes conflict because associations between alleles are temporary. Differing selection on males and females, sexual selection, and differences in transmission patterns between classes of nuclear and cytoplasmic genes can all give rise to conflict. Inert Y chromosomes, uniparental inheritance of cytoplasmic genes, mating strains and sexes, and many features of sexual behavior may have evolved in part as a result of evolutionary conflict. Estimates of its quantitative importance, however, are still needed.

grrrr...frustrated. My damn library website doesn't seem to want to give me electronic access to any online journals or databases... I can't find stuff let alone look up the articles themselves.

meh, i'll look around some more on google i think

 

[Another God]

OK, I feel like a lot of this conversation has been wasted on irrelevant crap, so, sorry about that, but I am going to clean it all up right now, and we can start from there...

OK, so we all know that sexual reproducation has its advantages, there is growing hard evidence that organisms which sexually reproduce are fitter than non-sexual reproducers of the same species etc. The question must then be asked: Why have 2 sexes? Why not have a situation where any individual can mate with any other? And if there are going to be sexes, why not have 3, 4, 5 or 6? (where u can mate with any individual except for individuals of your own sex)

The primary reasons cited in my lectures on this topic consisted of
1. Possibly to lessen the competition between the two sexes
2. Possible answer = Male sex is a parasite on the female sex.

1 => I mentioned an example of the octopus where the male is 2cm, the female 2m. Obviously these two sexes aren't using the some food resources etc, so there is no competition between them.

2 => This was the best answer to the question that my lecturer had heard of. the idea being that in the begining, all you needed was one individual to get slightly better at donating DNA than at receiving it, or at using the DNA it did receive etc, and then the division is set up. Over time, these better donators get better and better at donating, at the cost of receiving DNA, and they will tend to always get genes into the next generation (because they are so good at donating their DNA). The 'females' in this scenario would initially still be able to mate with all members, but over time the dedicated giver genes would start to take root in the population, and those capable of receiving will just about always be receiving from these givers. As such, if a mutation arose in the receivers, limiting their ability to give, then nothing would suffer. There would still be reproducation. in the end, each become particular specialised into their role.


Having just explained that out, I realize that the 'new insight' into it that I thought I may have gained from this thread, is actually very similar to the parasite theory anyway.

Someone said something about there being 2 sexes because 'its simple with two', or something like that, which made me realsie : Of course, you need two individuals to have sex, and in each instance, one needs to be giving the DNA, and one needs to be recieving. (this is the case in everything from E.coli upwards so it would seem) Now, even in E.coli I don't think you could say that they have sexes, but at any particular time, one is F+ or F-, and it can only mate with its opposite. Same with Yeast. Yeast has the gene for sex a, and the gene for sex alpha in its genome, but only one of them is expressed. So exchange only ever occurs between opposites. And that is reason enough for the obvious 2 sex scenario to be set up.

...and from that point on, I think the parasite idea explains it well enough. You only need one individual to be particular good at giving etc, and then it runs away.

 

[iansmith]

Originally posted by Another God
I suppose it has to do with being Diploid for starters. Can u give me an example of a Diploid species which doesn't have a xsome dependence?

Sex determination for reptiles is temperature dependent and there other factor but X has no play.

http://eebweb.arizona.edu/courses/Ecol483_583/bowden%20sex%20determ%202000.pdf

Originally posted by Monique
Plants don't, plants are sex-dependent or hermaphrodite.

Plant are hermaphrodites but I don't know if they can all self-fertilize and I don't if all type of plant are hermaphrodites. They have some asexual mechanism of spreading. You can easily clone a plant at home. You cut part of it then you can grow it.

 

[Monique]

Why not have 6 sexes? Wouldn't that be terribly inefficient?

I like the parasite idea, but that would require a nurturing relationship with the growing egg in order to have an evolutionary advantage. It is a disadvantage to be pregnant, being non-pregnant will enhance your survival.

Wouldn't it be very bad for a population if every individual was burdened by the possibility of having to provide nutrients to parasitic offspring?

I am still interested to find out what the reproductive cells of lower organisms look like, are they all so specialized as sperm and egg cells? Or are there organisms where they essentially look the same?

 

[iansmith]

Originally posted by Monique
I am still interested to find out what the reproductive cells of lower organisms look like, are they all so specialized as sperm and egg cells? Or are there organisms where they essentially look the same?

In lower animals such as jellyfish, the dimorphism is striking but in protozoan and fungi the dimorphism is not obvious and it does not appear to have specialization specialization. The buddign yeast has some interresting feature for their sexual cycle.

 

[StarkyDee]

Hi everyone- I don't like the idea that we think of carrying offspring as a burden from 6base reproduction. Let's say that we evolved by having 6 or 7 different sexes from which we could mate. Whose to say these combinations wouldn't evolve the species into genetically better ways of reproducing by using less nutrients for the offspring. Really I would think the possibilities of a 6 or 7base model would have exponentially successful adaptive qualities compared to our 2base reproductive model. By saying adaptive, I mean becoming smarter(using more than 10% of our brain) and more muscule development(stronger faster). Could having more sexual hybrids for mating prove successful? in my opinion yes. maybe we just haven't developed far enough yet.

 

[Tyro]

Asexual reproduction IIRC, as described during my biology lectures [zz)] will not produce the same level of genetic diversity as sexual reproduction. In the short term, it may be more energy efficient, but in the long run, you risk getting the entire species wiped out by a single nasty disease. I think a similar problem is plaguing (literally) banana trees now, which are genetically identical because of the way they are cultivated (grafts, I think).

As for three or more sexes, it is possible, but not entirely feasible. For a start you may need a more complicated genetic structure, a triple helix, for example. It probably would not work out to be a 3-sex-but-any-2-will-do thing because all three sexes must be involved for the benefits (increased genetic diversity and genetic defect protection).

My guess is that if a three or more sex creature were found, it would be in a very hostile, mutagenic environment where genetic constitution is important.

 

[Another God]

Originally posted by StarkyDee
using more than 10% of our brain

We use all of our brain.

 

[Another God]

Originally posted by Tyro
Asexual reproduction IIRC, as described during my biology lectures [zz)] will not produce the same level of genetic diversity as sexual reproduction.

I said:

I'll say this a few more times, because no one seems to have caught onto it yet: You don't need to have two sexes in order for meiosis => recombination to occur.

Sexual reproduction can be done without need of two exclusive sexes. Why are 2 sexes so predominant?

IOW: I am not talking about asexual reproduction.

For a start you may need a more complicated genetic structure

That isn't necessarily true