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why only 2 sexes

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why only 2 sexes

Postby bmaleus » Sat May 01, 2010 8:41 pm

can anyone shed light on why most life forms have only 2 sexes? some simple forms have 1, most have 2, none have 3 or more. why? surely it would spread genes better with more sexes
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Postby Jillo725 » Sat May 01, 2010 9:11 pm

because most animals happen by the opposite sex very randomly and sparsely. to require them to meet ANOTHER sex would be inefficient for the individual to the benefit of the species. Not enough bonus for the negatives.
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Postby kolean » Sun May 02, 2010 3:11 am

Agreed. More than 2, would be very hard to get together to procreate. Within an evolutionary time period, perhaps, they all died out.

Then again, maybe we are not there yet, evolutionary speaking that is. We are still spread out over the Earth, and do not have the need to have more than 2 to be the 'fittest' of the species and procreate.
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Postby JackBean » Sun May 02, 2010 7:02 am

http://www.biology-online.org/search.ph ... um%2F#1421

As mentioned above, getting three animals together is quite improbable. Some lower animals have problems to get even two animals together and thus they have overcome that in some way.
Other thing is, that we are diploid, thus there is no need to have three sexes. If we were triploid, that would be something else ;)
http://www.biolib.cz/en/main/

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Postby Darby » Sun May 02, 2010 4:54 pm

A lot of Fungi have many genders.

The two basic roles - one specialized for passing food to the offspring, the other for getting a gamete to the other - are a pretty simple system, and it's not unusual for a simple, efficient system to persist.
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Re: why only 2 sexes

Postby skeptic » Wed May 05, 2010 4:34 am

I suspect the basic reason for only two sexes is probably just that going to more would be too complex a change. Evolution generally proceeds in very small steps. Each intermediate step must have, in itself, an adaptive advantage, or else be lost.

As has already been pointed out, having 3 sexes would require the organism be triploid. Not impossible. But to combine a triploid genome with all the other characteristics needed to get 3 sexes to mate? Multiple behavioural and genetic changes. I cannot see any simple way to get there by small steps, with each small step having an adaptive advantage.

It is for exactly the same reason that no 6 limbed terrestrial vertebrate lives. We cannot have a pegasus since there is no way to evolve the requisite extra limbs in small steps, with each stage being an advantage.
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Postby SelfishGene » Mon May 31, 2010 7:44 pm

A proper explanation to explain the benefit of sexual reproduction (between 2 partners) would be a gene-centered interpretation of natural selection not a group-centered one. A very simple one would be "the gene for sexual reproduction benefits its own propagation because chances increase of it being accompanied by other genes that'll help increase its chances for propagation."

(i got this explanation from The Selfish Gene, Dawkins recommends some further reading which also postulates a 3 sex model. if you're interested, i can look up what book he recommends)

3 partners may increase these chances but I can't see it having a substantial benefit since the 3rd parent can simply mate with the offspring of the first 2 and still have his genes passed down. However, I wonder if a bacterium has the potential to have received its genes from more than 2 bacterium in the previous generation. sexual reproduction between 2 bacterium is rare enough so 3 is probably very rare if it happens at all.
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Postby JackBean » Tue Jun 01, 2010 8:35 am

I think that bacteria are just exchanging their DNA but not mating, aren't they?
http://www.biolib.cz/en/main/

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Postby canalon » Tue Jun 01, 2010 11:59 am

Well, bacteria do have sex and they mate, at least that is how we bacteriologist are describing conjugation. However, mating is not linked with reproduction, it is just one way to exchange DNA.
And they mate between different species. Bacterial sexuality can be quite kinky :D
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Postby SelfishGene » Wed Jun 02, 2010 1:16 pm

canalon wrote:However, mating is not linked with reproduction, it is just one way to exchange DNA.



canalon, I'm a bit confused of why you would accept that bacteria can be sexual, but not sexual reproducers.

if we limit ourselves to the actual bacterial cell division , we can consider all instances as mitotic asexual reproduction. However, if we include conjugation into the picture we can say that the daughter generation inherited its genetic makeup from more than one "parent". this is obviously not the traditional sense of sexual reproduction, but i dont understand why it cant be considered as a special unique case of sexual reproduction.
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Postby canalon » Wed Jun 02, 2010 7:16 pm

Bacteria do not need sex to reproduce, and sexual activity is not linked with reproduction.
The 2 aspects are independant: the recipient of a conjugation will of course pass it to its daughter cells, but the daughter will have been made without the conjugation event. On the other hand this was not the case for you. Without the mating of your parents you would not be there. And DNA recombination is not possible outside of the reproduction event (i.e. you, your parents genetic make-up did not change when they conceived you, unlike the bacteria)
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Re: why only 2 sexes

Postby skeptic » Wed Jun 02, 2010 8:18 pm

There is a major difference with bacterial 'sex'. Bacteria swap tiny fragments of DNA called plasmids. Most of the DNA for a bacterium, though, is in a large central DNA - not the plasmids. In normal sexual reproduction, chromosomes divide by meiosis, and each gamete gets one copy of each chromosome. Fusion of gametes gives two chromosomes of each type. So all the genome gets divided. In bacteria that does not happen, with only fragments swapped around.
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