This question isn't as obvious as it first seems. Think about it a bit: Why do we have 2 sexes?
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.
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.
Why should be more necessary? 2 is enough and it does the job
Gene swapping is not so easy and efficient for many metazoan.
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.
It's not necessarily a good thing to have both partners tied up with a pregnancy, which consumes more resources, often makes one more vulnerable, etc. (I also don't agree that "twice as efficient" is "astronomical" in evolutionary terms, even accepting that it is "more efficient" at all.) And sexual dimorphism can be useful: it in effect turns one species into two specialized, cooperating subspecies.
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.)
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 lets 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 any one 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.
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.
Bacterial and archea still exist and are present in some extreme enviroment. Talk about evolution problem.
Is the advantage outweigthing the cost in term of evolution. I think so.
I will answer by this
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'm not thinking in terms of humans.
I didn't say anything about requiring care from parents, either.
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?)
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.
That doesn't follow. If it did, then a species would always wipe itself out before predators could. Moreover, it ignores the existence of cooperation within species.
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.
And we need to shuffle to keep up with the bacteria and viruses that infect us in speed of hereitary change.
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.
I just noticed I overlooked the following comment by Another God:
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).
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?
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.
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.
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