Why is 2 sexes so common in nature?

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

 

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.

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

 

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 percieved problem.

 

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 realise 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.)

 

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.

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.

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.

 

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?

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??????

 

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?

 

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.

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?

 

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?

 

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 seperated at any time, and if that population happens to have incredibly few females, then the species is put into immediate risk.

but most importantly....

please read the other posts aswell:

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.

 

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.

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.

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.

 

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?